Magali Toriti
email: magalitoriti@gmail.com

Aline Durand

Pierre Excoffon

Fabien Fohrer

Xylophagous insects of the wooden floor of Camelin block (Fréjus, France): an interdisciplinary approach combining archaeology and anthraco-entomology

Keywords: Roman excavation, Timber, Wood condition, Woodborers, Reference database, Southern France

Preventive excavation in the ancient Forum Iulii city (Fréjus, southern France) in 2013 revealed a housing block where a very well preserved charred wooden floor was discovered. It was laid in a house dating back to the first half of the 1st century AD and used as the floor of a room decorated with painted plaster. It was destroyed by fire around 70 AD. The ligneous species used for its construction (Aleppo pine, Scots pine, beech and fir) were characterized by anthracological analysis. Numerous traces of xylophagous insects and lignivorous fungi were observed. Their identification, their number and their distribution in the floor open on to very interesting new interpretations for both anthracologists and archaeologists as regards the condition of timber, the original source of the infestation, and the historical record of the building. This study is a direct and unprecedented application of a key for identifying traces of wood-boring insects in charred remains from the Roman period. Field samples were tested during the excavation of Camelin block, and microscope observation protocols were adapted to entomo-archaeology during anthraco-analysis.

Introduction

The stone constructions of the Roman world area wellknown, long investigated field because the remains discovered during archaeological excavations are abundant, and the monuments that are still standing remain as emblems of urbanity and imperial power. The same is not true of wooden construction, whose remains outside wetlands are farless abundant and consequently less studied [START_REF] Lamouille | Bois et architecture dans la Protohistoire et l'Antiquité[END_REF]. The anthracological approach greatly developed in the 1970s. It is based on the study of charcoals from domestic and craft combustion structures [START_REF] Western | Wood and charcoal in Archeology[END_REF][START_REF] Western | The ecological interpretation of ancient charcoals from Jericho[END_REF][START_REF] Fancelli-Galletti | I «carboni della grotta delle arene candide e l'evoluzione forestale in Liguri dopo l'ultima glaciazione[END_REF][START_REF] Vernet | Contribution à l'histoire de la végétation du Sud-Est de la France au Quaternaire, étude de macroflores de charbons de bois principalement[END_REF][START_REF] Willcox | A history of deforestation as indicated by charcoal analysis of four sites in Eastern Anatolia[END_REF][START_REF] Castelletti | I carboni della vetreria di Monte Lecco[END_REF]. The aim was to document the historical record of vegetation stands, of the local environment, of landscapes, and to reconstruct the morphology of the woodlands from which Roman society took its wood supplies, and thus get a view of their forest management if any. Apart from marine woods and the dendroarchaeological approach, anthracological studies have rarely targeted the carbonized remains of wooden construction, although the wooden structure is very often preserved by fire [START_REF] Buxó | Toiture et restes incendiés dans l'habitat de Lattes au IVe s. av. n. è[END_REF][START_REF] Chabal | Quatre puits de l'agglomération routière gallo-romaine d'Ambrussum (Villetelle-Hérault), Montpellier, Éditions[END_REF][START_REF] Liottier | Bois et forêts en France méditerranéenne durant la Protohistoire : une approche des pratiques et des usages. Études anthracologique, dendroécologique et spatiale[END_REF]. Moreover, these artifacts constitute additional and complementary information to xylological and anthracological studies.

Our objective was to better evaluate the use of wood in Roman construction and its links with architecture. But in addition to the potential contribution to the understanding of the construction site and the building process, the de-construction processes also need to be documented. Anthraco-entomological analysis (Toriti et al., 2020a(Toriti et al., , 2020b) ) characterizes and evaluates the condition of the timber used for construction by identifying the presence of woodborers within the wooden building. Based on this expertise, it is then possible to explain the sustainability of a building or, on the contrary, to understand the mechanisms that have weakened it.

Up to now, entomo-archaeology has been based on the study of insects preserved in archaeological strata [START_REF] Buckland | The use of insect remains in the interpretation of archaeological environments[END_REF][START_REF] Buckland | A Bibliography and Literature Review of Quaternary Entomology[END_REF][START_REF] Moret | L'archéo-entomologie, ou les insectes au service de l'histoire[END_REF][START_REF] Ph | Analyses entomologiques (coléoptères), Chapitre 5[END_REF], while paleoentomology has been based on other domains than archaeological stratigraphy [START_REF] Kenward | Further Archaeological Records of Aglenus Brunneus (Gylll.) in Britain and Ireland. Including Confirmation of its Present in the Roman Period[END_REF][START_REF] Ph | La Tène and Gallo-Roman Natural Environments and Human Impact at the Touffréville Rural Settlement, Reconstructed from Coleoptera and Plant Macroremains (Calvados, France)[END_REF][START_REF] Hall | The Environmental archaeology of Urban Landscape[END_REF][START_REF] Huchet | L'Archéoentomologie funéraire : une approche originale dans l'interprétation des sépultures[END_REF][START_REF] Huchet | Insectes et archéologie[END_REF]. However, entire insects or even anatomical parts such as elytra or antenna are only exceptionally observed in carbonized remains of wood because the adults fly away once the larval stage is over. On the other hand, traces of their passage through the anatomy of the wood are very frequently observed in the form of wormholes and galleries.

In order to write the history of the construction and deconstruction of buildings, we decided to analyze only the charred woody remains from Roman constructions, with a marked preference for the study of sites in the provinces of Narbonnaise and Cisalpine Gaul. The study area had to be chronologically and geographically circumscribed to build a corpus of sites from comparable periods and implanted in vegetation zones under a marked Mediterranean influence to assess the timber supply channels. We also decided to attempt to evaluate the condition of timber. We had to build new analysis tools (an identification key and a reference atlas) to study the xylophagous fauna traces left in timber, and then apply them to the study of the remains of carbonized timber from archaeological sites. The town of Fréjus, where preventive archaeology has been developing for several decades, presented a significant potential for the study of urban carbonized timber, which has been enriched over time.

Study area

Biogeography and vegetation context

The city of Fréjus is now located in the lower to middle meso-Mediterranean vegetation zone [START_REF] Ozenda | La végétation de la chaîne alpine dans l'espace montagnard européen[END_REF]. Initially dominated by the evergreen oak (Quercus ilex) forest, which is almost absent today because it was largely destroyed, the surrounding woodlands present a degraded facies of pine woods where Aleppo pine, a ubiquitous and pyrophytic species, has acted as a real stop gap along with stone pine and, more sporadically, mesogean pine [START_REF] Barbero | Carte écologique des Alpes au 1/100000 Cannes Q. 22[END_REF]Ozenda, 1985, 132-133).

Towards the southern coast, between Saint-Raphaël and Saint-Aigulf, scrublands harboring Rhuscotinus and Spartium junceum and grasslands harboring Brachypodium pinnatum dominate. Higher up, the eastern crystalline hills of the Estérel are occupied by stands of cork oak, myrtle and lentisk, interspersed with maritime pine woods and open formations of cistus (C. monspeliensis and C. ladaniferus), myrtle, narrow-leaved phillyrea and stecade lavender, and maquis-like formations harboring strawberry tree and tree heath [START_REF] Braun-Blanquet | Les groupements végétaux de la France méditerranéenne[END_REF]. A thermophilic cork oak forest harboring myrtle and lentisk and a less thermophilic one harboring Genista candicans and Cistus triflorus, interspersed with mesogean pine, cover the western reliefs that dominate the Argens valley. Scrublands harboring Cistus monspeliensis or Lavandula staechas, Myrtus communis and Phillyrea angustifolia, and those harboring Arbutus unedo and Erica arborea provide evidence of the opening of the forest [START_REF] Barbero | Carte écologique des Alpes au 1/100000 Cannes Q. 22[END_REF].

The upper meso-Mediterranean zone is represented by evergreen oak woods and grasslands harboring Brachypodium ramosum that are all very scarce. They are very quickly intertwined with the supra-Mediterranean zone, which is well developed in the whole northern zone of Fréjus and far beyond. Indeed, the pubescent oak and the Scots pine cap the highest points of the Estérel massif, as on the top of Mont Vinaigre (618 m a.s.l.). They are little developed in the eastern hills dominating the Argens valley, where the chestnut tree is sporadically present on the eastern flank of the Roucas des Lauquières [START_REF] Barbero | Carte écologique des Alpes au 1/100000 Cannes Q. 22[END_REF]. But beyond this, these two species form wide forests (Bargemon, Seillans, Fayence...) above 500 m above sea level [START_REF] Ozenda | La végétation de la chaîne alpine dans l'espace montagnard européen[END_REF], with facies becoming more and more collinean and supra-Mediterranean as one goes northwards. Thus, the deciduous oak forests harboring Aleppo pine and mesogean pine and the grasslands harboring Brachypodium pennatum are gradually being re-placed by a deciduous oak forest with Scots pine beyond 15 km from Fréjus or so. The boxwood formation, the garrigue harboring Buxus sempervirens, Spartium junceum, Coriaria myrtifolia and the grasslands harboring Bromus erectus provide evidence of the opening of the marcescent and supra-Mediterranean oak forests.

The first calcicolous beech woods harboring androsaces of Chaix and presenting decay facies of boxwood at the Glacière pass and in the wood of Malay are located between 1069 and 1397 m in altitude, and are the southernmost digitation of the Pre-Alps of Grasse-Castellane. They are located 27 km from Fréjus as the crow flies. The first fir trees are more northernly, at the Collet and Cabannes at the top of the Fil valley (900-1,300 m above sea level). These massifs typically belong to the mountain zone, with Scots pine, beech and fir stands, and even have an asylvatic summit area.

Woodlands harboring white poplars, willows, ash and glutinous alder border the banks of the Argens. sub-Mediterranean and collinean formations where hornbeam can be found along the tributaries, particularly upstream of the Reyran, Riou and Méaulx.

The wooded environment around Fréjus is very rich. Beyond 5-8 km around the city, numerous and diverse forest formations may potentially have supplied the ancient city with timber, given the socio-environmental dynamics in this area during and since antiquity [START_REF] Dubar | L'édification de la plaine deltaïque du Bas Argens (Var, France) durant la Protohistoire et l'Antiqui-té. Application d'un modèle numérique 2D à l'archéologie[END_REF][START_REF] Chabal | Environnement végétal de La Galère (Porquerolles, Var) étude anthracologique[END_REF].

Archaeological context

Forum Iulii is located on the Mediterranean coast. It was established by Julius Caesar in the middle of the 1st century B.C. Today, as in many other ancient and medieval ports (Lattara, Maguelone, Aigues-Mortes...), clogging and aggradation have caused the shoreline to move 1.5 km seaward [START_REF] Dubar | L'édification de la plaine deltaïque du Bas Argens (Var, France) durant la Protohistoire et l'Antiqui-té. Application d'un modèle numérique 2D à l'archéologie[END_REF][START_REF] Excoffon | Les ports dans l'espace méditerranéen antique. Narbonne et les systèmes portuaires fluvio-lagunaires[END_REF]. The Archaeology and Heritage Department of Fréjus city was created in 1982 one of the oldest ones in France. It strives to update, record and understand the traces of the town's ancient and medieval past.

In 2013, this service carried out a preventive archaeological excavation of 2,000 m² on Camelin urban block, located on the south-western flank of the natural hill on which the city is located, under the supervision of P. Excoffon (Figure 1). It cleared the full width of an insula, i.e. 34.23 m, and the two lanes parallel to the main N-S axis bordering it. The excavated remains covered more than four centuries of occupation between the end of the 1st century BC and the 4th century AD (Figure 2) [START_REF] Excoffon | Fréjus, îlot Camelin[END_REF]. It was during the diagnosis of the parcel in 2012 [START_REF] Excoffon | Fréjus îlot Camelin[END_REF], that sizeable rubified and burnt down areas were identified. The excavation campaign lasted eight months, and an anthracology specialist, M. Toriti, took part to it to carry out the necessary sampling and packaging of the charred woody material on site.

Materials and methods

The wooden floor

A set of five sandpits and at least 16 boards was discovered dating back to the first half of the 1st century AD. The sandpits formed a rectangular frame measuring 2.50 x 2.10 m. The boards perpendicular to the three east-west joists were mainly in the north-south direction. They were 1 to 3 cm thick on average at the time of excavation. The structure had few nails, and the main structural elements was slightly disorganized during demolition (Figure 3). Plaster and a partition wall were found above this wooden structure. These collapsed wall and roof elements preserved the wooden structure perfectly after the fire and almost kept it in its original condition. This floor came from a room belonging to a house designated as «house 1A» (Excoffon, Vauxion, 2016). It was dated back to the first half of the 1st century AD, and destroyed before the end of that same century. Situated near a room interpreted as being a kitchen (Excoffon et al., 2016) and probably opening back to an outdoor space, this room, with its elegantly painted wall [START_REF] Excoffon | Les enduits peints de l'îlot Camelin à Fréjus[END_REF], probably corresponded to a reception area. The installation of a drainage channel, a sort of gutter, under the floor also provides evidence of the need to sanitize this surface. Three areas containing fabrics were also located. This floor was used as a test bench for the previously mentioned questions.

Sampling and wood identification

During excavation, each piece of wood was identified, drawn, photographed, measured, and subjected to an accurate and complete architectural and archaeological observation in situ. The five long pieces of wood were entirely removed by section. Each of them was sampled in three sections conditioned in plastic boxes to make laboratory analyses easier. All the pieces of carbonized wood were studied by a classical anthracological approach [START_REF] Chabal | Forêts et sociétés en Languedoc (Néolithique final, Antiquité tardive) : l'anthracologie, méthode et paléoécologie[END_REF].

The anthracological analysis of the wooden floor followed a classical method of observation of wood under an optical microscope. Species identification was based on wood anatomy atlases [START_REF] Greguss | Identification of living Gymnosperms on the basis of xylotomy[END_REF][START_REF] Greguss | Holzanatomie der europäischen Laubhöl-zerund Sträucher[END_REF][START_REF] Jacquiot | Atlas d'anatomie des bois des Conifères, Paris[END_REF][START_REF] Jacquiot | Atlas d'anatomie des bois, les Angiospermes[END_REF][START_REF] Schweingruber | Anatomie europäischer Hölzer; anatomy of European woods[END_REF] and on the wood reference collection of the CReAAH CNRS UMR 6566 laboratory. The analysis effort focused on measurements (diameter, length), and the detection of anomalies (bark, knots, departure of branches, compression wood...), especially the identification of dry rot.

Sampling and identification of xylophagous insects

Entomo-archaeological analysis consists in a systematic survey and observation of the galleries left by xylophagous insects in each piece of wood, and then measuring their diameter to provide indications on the insects that bore through the wood. For example, the galleries of longhorn beetles, (Cerambycidae family), are recognizable by their oval shape and a fairly large size, greater than 3 mm in diameter. But gallery size is not a sufficient criterion to identify the pest involved because it depends on larval development. Therefore, the diagnosis more specifically focused on the observation of faecal pellets (larval excrement): size, appearance, shape, texture. The faecal pellet is indeed a discriminating element, as each xylophagous insect secretes their own pellets, which differ in size, shape and composition from those of other xylophagous insects. Faecal pellets were sampled by patting the wood and scratching the edges of the inner galleries with a pin.

To better characterize the presence of archaeological woodborers based on their excrement, a reference collection based on the galleries and faecal pellets of living insects bred in the CICRP (Interdisciplinary Center for the Conservation and Restoration of Heritage) laboratory in Marseille was elaborated. A specific determination key in the form of an illustrated atlas based on an anatomical description is being finalized [START_REF] Toriti | Les bois ouvragés en Gaule romaine : approches croisées archéologiques, anthraco-xylologiques et entomologiques[END_REF]. The methodology used to build this identification key was a classical approach to morphometry: the faecal pellets of each xylophagous species were collected from the collections of living insects of the CICRP and then systematically measured. Groups of statistically different sizes and morphologies were characterized to distinguish not only families but also genera. This was the case within the Ptinidae family [START_REF] Fohrer | Analyse des vermoulures pour la détermination de quelques espèces d'Insectes xylophages de la famille des Ptinidae (Coleoptera)[END_REF].

The interweaving of all these observations made it possible to identify the woodborers. But the precision of these identifications ranged from the family level to the genus and species levels: cerambycids gather 250 species (Berger, 2012) that cannot be discriminated to the present day; on the other hand, the most common insects among Ptinids are determined at the species level (Anobium punctatum (De Geer, 1774), Nicobium castaneum (Olivier, 1790), Oligomerus ptilinoides (Wollaston, 1854)) [START_REF] Ctba | Centre Technique du Bois et de l'Ameublement), Le traitement curatif des bois dans la construction[END_REF][START_REF] Fohrer | Analyse des vermoulures pour la détermination de quelques espèces d'Insectes xylophages de la famille des Ptinidae (Coleoptera)[END_REF][START_REF] Toriti | Les bois ouvragés en Gaule romaine : approches croisées archéologiques, anthraco-xylologiques et entomologiques[END_REF].

Sampling archaeological artefacts or ecofacts determines the quality and reliability of the results of the study. Therefore, special attention was paid to the sampling of wood material throughout excavation.

Results

Anthracological results

A total of 1,572 charcoals from the floor was analyzed (Figures 4 and 5). Beech (Fagus) came first with 34%, i.e. 535 charcoals identified in all wooden elements, corresponding to the two preserved sandpits (CH 20 and 27), two of the three joists as well as more scattered elements, probably belonging to the sandpits or joists (CH 4,6,14,15 and 17), which came off when the walls and roof collapsed. Aleppo pine/stone pine (Pinus halepensis/P. pinea) was the second most represented taxon with 26.59 %, to which must be added Aleppo pine (Pinus halepensis) per se at 14.06%. These charcoals corresponded to remains of boards from which a thin (2-3 cm) layer was preserved ( CH 1,2,3,5,7,8,10,11,12,13, 18, 24, 25 and 26). With 11.96% of presence, fir (Abies) appeared distinctly in one joist (CH 21) and one plank (CH 9). Plank 19 was exclusively made of Scots pine (Pinus sylvestris type: 1.9%). Finally, deciduous oak (Quercus: 0.25%; Quercus D. F.: 0.25%) was used for piece No 16.

Compression wood was observed on all coniferous pieces: 63% of Aleppo pine/stone pine fragments exhibited compression wood, including 6.7% corresponding to moderate to strong compression; 26.7% of the Scots pine fragments and 37.8% of the fir fragments were affected by weak compression. The vertical tracheids, visible mainly in radial longitudinal section, showed helical cracking of the walls giving them an airy twisted appearance (low compression) except for parts 2, 8 and 25, which had a tighter appearance (moderate compression).

On the northern part of floor where the carbonized woody remains were next to the remains of fabric and bread, the analysis revealed various other species such as cork oak or evergreen oak (Quercus suber: 3.43%; Quercus sempervirens: 0.64%), alder (Alnus: 3.12% and cf. Alnus: 0.19%), ash (Fraxinus: 0.51%), maple (Acer: 0.45%), and boxwood (Buxus: 0.13%). Bark was still visible on some of these fragments, and others were very small in size. This supports the hypothesis that they were part of to sets of small woods, such as bundles, stored on the floor at the time of the fire.

Entomological results

A total of 753 galleries was observed on 553 charcoals, corresponding to 35.22% of the whole structure. The iden-tified traces of infestation were of the beetle type: Anobium punctatum (Figure 6; Table 1) at 37.99%, Oligomerus cf. ptilinoides at 1.59%, and Ptinidae at 41.43%. A significant number of galleries ̶ 18.99% ̶ , remained undetermined because of the excessive degradation of the pellets (melted, agglomerated) due to carbonization.

Only CH 16 (oak) and CH 19 (Scots pine type) showed no evidence of woodworm infestation. On the other hand, the other 25 carbonized elements contained two to several dozen galleries; CH 27 was the most impacted item with 104 galleries. This proportion corresponds to an infestation rate (number of infested charcoals over the number of observed charcoals) of nearly 74%. Infestation was greatest in the eastern part and in the center of the structure. Elements in the eastern part of the structure (CH 1, 2, 3, 4, 5, 6, 22, 24 and 27) kept the stigma of a dry rot fungus (Figure 7). Among all the data collected on infestations, beech, which corresponded to the larger sizes as in CH 20, was the species with the highest infestation rate (58%), followed by Aleppo pine/stone pine (28%), and fir (11%). The remaining 3% concerned all infestations of species previously considered as residual and potentially of wood stored in the room (alder, maple, boxwood, etc.).

An irregularly shaped gallery on piece CH 24 did not provide any pellet and was therefore not determined. On the other hand, two potential fumitory (Fumaria spp.) seeds were found inside the gallery. Their presence suggests that ants might have stored these seeds in the galleries.

Discussion

Roman Timber: Supply Area and Environmental Availability

During the first half of the 1st century A.D., the construction of the insula discovered in Camelin block required a supply of building materials for its various buildings, i.e. stone, wood, but also clay, since many elements of unbacked clay architecture (bricks) were preserved thanks to the fire. The city, founded more than one century before, was a mineral universe where woodlands had no longer their place; consequently, the timber used for construction was exogenous to the city. The question that now arises is whether this timber supply was proximate or distant, and in turn what degree of economic and commercial, and even political influence Fréjus had on its territory and its proximate or more distant environment. Taking into account the socio-environmental dynamics known in this geographical area at the scale of eastern Provence during ancient times [START_REF] Borréani | Peuplement et histoire de l'environnement sur les îles d'Hyères (Var)[END_REF][START_REF] Dubar | Étude palynologique du carottage de Pont d'Argens (Roquebrune-sur-Argens, Var) : histoire holocène de la végétation en Provence cristalline ; facteurs naturels et anthropiques[END_REF][START_REF] Dubar | Évolution de la végétation littorale de la Côte d'Azur (Var et Alpes-Maritimes, France) en relation avec les derniers stades de la remontée du niveau marin à l'Holocène. Esquisse d'un modèle Quaternaire[END_REF][START_REF] Chabal | Étude anthracologique de la place 123 (Lattes IVe s. av. J.-C.) : approche spatiale diachronique[END_REF][START_REF] Chabal | Le bois dans l'architecture et le mobilier (500-475 av. J.-C.) : les boisements aux origines de Lattara[END_REF], the biogeographical limits of the vegetation zones have not significantly changed since antiquity: it is therefore possible to approach the question of provenance through this lens.

The species identified in the manufacturing of the floor were of the Pinus halepensis/P. pinea, Pinus sylvestris types, Quercus deciduous type, Fagus and Abies. The Aleppo pine and stone pine woods currently occupy the hills bordering the Argens river and extend up into the Maures and Estérel massifs. The builders of the insula had barely 5 km to travel to reach these hills. According to the Camargue and Lattois models [START_REF] Puertas | Palynologie dans le delta du Lez. Contribution à l'étude du paysage de Lattes[END_REF][START_REF] Chabal | Environnement végétal de La Galère (Porquerolles, Var) étude anthracologique[END_REF][START_REF] Chabal | Forêts et sociétés en Languedoc (Néolithique final, Antiquité tardive) : l'anthracologie, méthode et paléoécologie[END_REF][START_REF] Chabal | Étude anthracologique de la Capelière et synthèse sur l'histoire des boisements de Camargue, de l'âge du Fer au début du Moyen Âge[END_REF], the stone pine can also cover the coastal areas, which were even closer at that time since the shoreline was closer by 1.5 km [START_REF] Dubar | L'édification de la plaine deltaïque du Bas Argens (Var, France) durant la Protohistoire et l'Antiqui-té. Application d'un modèle numérique 2D à l'archéologie[END_REF][START_REF] Excoffon | Les ports dans l'espace méditerranéen antique. Narbonne et les systèmes portuaires fluvio-lagunaires[END_REF]. Palynological data from the Argens Valley showed an increase in pine and tree heather pollen rates (30% to 60% in some diagrams) towards 3000 BP, which indicates forest opening [START_REF] Dubar | Étude palynologique du carottage de Pont d'Argens (Roquebrune-sur-Argens, Var) : histoire holocène de la végétation en Provence cristalline ; facteurs naturels et anthropiques[END_REF][START_REF] Dubar | Évolution de la végétation littorale de la Côte d'Azur (Var et Alpes-Maritimes, France) en relation avec les derniers stades de la remontée du niveau marin à l'Holocène. Esquisse d'un modèle Quaternaire[END_REF]. Although pine pollen cannot be discriminated at the species level, its association with tree heather, whose pollen diffuses little, and the joint evolution of the two curves, which is reminiscent of the Ericetosum association [START_REF] Braun-Blanquet | La forêt d'yeuse languedocienne, (Quercion ilicis). Monographie phytosociologique[END_REF][START_REF] Braun-Blanquet | Les groupements végétaux de la France méditerranéenne[END_REF], argues in favor of Pinus halepensis and thus of a very local pollen composition. These data validate the hypothesis of a first immediate, very local supply area, less than 4 -5 km around Forum Iulii.

The second supply area was further away. In fact, the marcescent oak fully develops in the upper meso-Mediterranean and supra-Mediterranean zones, and the Scots pine in the supra-Mediterranean and mountainous vegetation areas. More rustic and less thermophilic than Quercus pubescens, it can infiltrate the lower part of the pubescent oak forest and even go down to 200 m [START_REF] Quézel | Écologie et biogéographie des forêts du bassin méditerranéen[END_REF]. It only becomes dominant in the upper parts of the series as in the Maures and Estérel. The pubescent oak is more calcicolous and therefore very little present on the siliceous soils of the Maures and Estérel, and prefers the valleys with deeper and more humid soils [START_REF] Quézel | Écologie et biogéographie des forêts du bassin méditerranéen[END_REF]. Palynological and anthracological data on this point are of no help. Based on the distribution of a few decades ago (Barbero and Loizel, 1974), the potential supply area would have been more than ten kilometers away as the crow flies, drawing a second probable supply area around Fréjus.

The characterization of beech and fir refers to montane ecosystems and to altitudes above 800-1,000 m. The first beech woods are now 27 km from Fréjus as the crow flies, and the first fir woods 35 km away, near the aqueduct that supplied water to the town of Fréjus during the Roman period. This situation would draw a third, very distant supply area ̶ between 25 and 35 km ̶ , which raises the question of the means of transport (water, cart...). But another hypothesis seems much more certain. Until around 3000 BP, mesophilic taxa (Quercus D. F., Corylus, Fagus) dominated the pollen spectra of the Argens valley cores and strongly decreased afterwards [START_REF] Dubar | Étude palynologique du carottage de Pont d'Argens (Roquebrune-sur-Argens, Var) : histoire holocène de la végétation en Provence cristalline ; facteurs naturels et anthropiques[END_REF][START_REF] Dubar | Évolution de la végétation littorale de la Côte d'Azur (Var et Alpes-Maritimes, France) en relation avec les derniers stades de la remontée du niveau marin à l'Holocène. Esquisse d'un modèle Quaternaire[END_REF]. According to Bui-Thi-Mai, beech and especially fir forests are established in the neighboring massifs of the Argens basin following a vegetation dynamic well characterized in Provence [START_REF] Beaulieu | Contribution pollen analytique à l'histoire tardi-et post-glaciaire de la végétation des Alpes méridionales françaises[END_REF][START_REF] Triat-Laval | Contribution pollen analytique à l'histoire tardi-et post-glaciaire de la végétation de la Basse Vallée du Rhône[END_REF][START_REF] Nichol-Pichard | Analyse pollinique d'une séquence à Tourves (Var-France)[END_REF][START_REF] Violino | Environnement et occupation humaine Bas-Lauvert (Antibes)[END_REF]Nicol-Pichard and Dubar, 1998;Andrieu-Ponel et al., 2000a, 2000b;[START_REF] Delhon | The migration of beech (Fagus sylvatica) up the Rhône: the history of a «mountain» species[END_REF] and in Languedoc [START_REF] Planchais | Palynologie lagunaire de l'étang de Mauguio. Paléoenvironnement végétal et action anthropique[END_REF][START_REF] Planchais | Impact de l'homme lors du remplissage de l'estuaire du Lez (Palavas, Hérault)[END_REF][START_REF] Durand | Anthracologie et paysages forestiers médiévaux : à propos de quatre sites languedociens[END_REF][START_REF] Durand | Dynamique biogéographique des boisements Languedoc : l'impact de l'an Mil[END_REF][START_REF] Chabal | Forêts et sociétés en Languedoc (Néolithique final, Antiquité tardive) : l'anthracologie, méthode et paléoécologie[END_REF], 2012;[START_REF] Britton Ch | Approche interdisciplinaire d'un bois méditerranéen entre la fin de l'antiquité et la fin du Moyen Âge, Saugras et Aniane, Valène et Montpellier[END_REF][START_REF] Liottier | Bois et forêts en France méditerranéenne durant la Protohistoire : une approche des pratiques et des usages. Études anthracologique, dendroécologique et spatiale[END_REF]. As fir pollen is poorly dispersive, its presence implies the presence of producing trees in the close vicinity. At Fréjus, rare identifications of fir have been noted in the anthracological and palynological diagrams from the Villa Romana site (Bouby et al., 2011, 225;Guillon, 2011, 241) as in Augustonemetum (Clermont-Ferrand) surrounding [START_REF] Cabanis | Les ressources en bois dans le bassin de Clermont-Ferrand du Néolithique à la période romaine d'après l'analyse dendro--anthracologique[END_REF]. In addition, numerous antique and medieval unpublished anthracological data (Cenzon, 2014; Venot-Tisserand, 2016), weakly characterize beech, or even fir, in archaeological sites located lower than their usual biogeographical limits in Provence. L. [START_REF] Chabal | Quatre puits de l'agglomération routière gallo-romaine d'Ambrussum (Villetelle-Hérault), Montpellier, Éditions[END_REF] showed that the current distribution of fir could not be transposed to the ancient period and, based on ecological models, postulated for its presence at low altitude in Languedoc. P. [START_REF] Quézel | Écologie et biogéographie des forêts du bassin méditerranéen[END_REF] indicate that it can infiltrate mixed oak forests. In Languedoc, beech definitively disappeared from the low-altitude Mediterranean complex during the Middle Ages [START_REF] Durand | Dynamique biogéographique des boisements Languedoc : l'impact de l'an Mil[END_REF](Durand, , 1998)). We can therefore envisage that beech and fir tree formations still existed in the valleys of the Argens and its tributaries during the ancient period thanks to privileged edaphic conditions. This very mesophilic third timber supply area would have been less than 25-35 km away from Fréjus, and closer to the second area that developed more than 10 km away from Fréjus. Even if the distance was short, the question of the transport of pubescent oak, Scots pine, fir and beech timber remains entirely open.

The environmental availability determined the localization of Fréjus timber supply areas. The analysis of a floor showed that during the 1st century AD, a period of full economic and political development of the city, these areas delineated a proximate or local territory within a radius of no more than 25-30 km around the city. As in Lattara, in Augustonemetum (Clermont-Ferrand) or in Roman cities from Belgica Gaul [START_REF] Bernard | Stratégie d'approvisionnement en bois en Gaule du nord-ouest (du Ier siècle avant au IVe siècle après J.-C.)[END_REF][START_REF] Cabanis | Les ressources en bois dans le bassin de Clermont-Ferrand du Néolithique à la période romaine d'après l'analyse dendro--anthracologique[END_REF][START_REF] Blondel | Approche dendroarchéologique de l'approvisionnement de la ville antique d'Augustonemetum (Clermont-Ferrand -Puy-de-Dôme) en bois d'oeuvre et exploitation forestière[END_REF][START_REF] Liottier | Bois et forêts en France méditerranéenne durant la Protohistoire : une approche des pratiques et des usages. Études anthracologique, dendroécologique et spatiale[END_REF] or even in Herculaneum [START_REF] Moser | Timber economy in the Roman Age: charcoal data from the key site of Herculaneum (Naples, Italy)[END_REF], they did not reveal inter-regional or international trade over very long distances as recent dendrochronological analysis of documents did for Rome [START_REF] Bernabei | Dendrochronological evidence for long-distance timber trading in the Roman Empire[END_REF].The builders chose the closest possible available natural resources, whose location and properties they knew perfectly well.

Roman timber: between technical choices and mechanical properties

The technical and mechanical properties required from timber are another key to reading the data from the floor of Camelin block. It was therefore necessary to investigate them through the lens of the relevance of the chosen wood in relation to its morphological, mechanical and physical properties with a view to a precise use of the piece of wood (in this case the slats, joists and sandpits supporting the floor).

As regards Gymnosperms, the regular characterization of compression wood (63% in total) means that the subjects grew under sufficiently difficult conditions for these stresses to be reflected in the anatomy of the wood. Given the geographical context, these were probably either trees rooted on fairly gentle slopes that corresponded well to the mounds and reliefs surrounding Fréjus since compression was generally moderate, or the lower part of large branches where growth had to be rebalanced. Today, compression wood is considered as a defect by the industry because of the loss of elasticity and tensile strength, which makes it more difficult to work, especially with a saw, and unsuitable for use in logs and beams, a fortiori in parquetry, because the knots stand out on the surface of the boards. However, it is 10 to 40% denser than normal wood and, as its name suggests, is more resistant to compression. The craftsmen of Fréjus clearly did not stop at these difficulties visible to the naked eye (color, shape, decentring pith...): they did not hesitate to cut coniferous trees, whether from Mediterranean or mountain areas, into planks; it can be assumed that this work was not done with a saw. Usually, these species were commonly used during protohistory and antiquity as beam timber or marine wood because they had a long span and straight trunks (Küster, 1993;Veal, 2017: 328-329;[START_REF] Moser | Timber economy in the Roman Age: charcoal data from the key site of Herculaneum (Naples, Italy)[END_REF]Liottier, 2019: 117, 153). But they also made good shelf and flooring wood or were used to make little furniture such as buckets, chests... [START_REF] Bernard | Stratégie d'approvisionnement en bois en Gaule du nord-ouest (du Ier siècle avant au IVe siècle après J.-C.)[END_REF]. Fir was especially used to make boards (floors or roofs) as early as protohistory, especially for shelving and mezzanines, and was probably selected for this purpose [START_REF] Bernard | Stratégie d'approvisionnement en bois en Gaule du nord-ouest (du Ier siècle avant au IVe siècle après J.-C.)[END_REF][START_REF] Cabanis | Les ressources en bois dans le bassin de Clermont-Ferrand du Néolithique à la période romaine d'après l'analyse dendro--anthracologique[END_REF]Veal, 2017, 340;[START_REF] Blondel | Approche dendroarchéologique de l'approvisionnement de la ville antique d'Augustonemetum (Clermont-Ferrand -Puy-de-Dôme) en bois d'oeuvre et exploitation forestière[END_REF]Liottier, 2019: 192, 202-207, 256, 268-281). This was also the case in Fréjus. Because of this specific use, fir timber would have been cut on site in mobile workshops to avoid the long-distance transport of heavy logs [START_REF] Bernard | Stratégie d'approvisionnement en bois en Gaule du nord-ouest (du Ier siècle avant au IVe siècle après J.-C.)[END_REF][START_REF] Blondel | Approche dendroarchéologique de l'approvisionnement de la ville antique d'Augustonemetum (Clermont-Ferrand -Puy-de-Dôme) en bois d'oeuvre et exploitation forestière[END_REF][START_REF] Liottier | Bois et forêts en France méditerranéenne durant la Protohistoire : une approche des pratiques et des usages. Études anthracologique, dendroécologique et spatiale[END_REF].

The construction of the structure combined coniferous and deciduous trees. Beech was the most frequently used spe-cies in the structure, especially for sandpits, as it is a longspanned, hard, dense wood, well suited for the frames. L. Liottier (2019: 204, 226) characterized this species for the manufacturing of door frames, but not for doors themselves; in Fréjus, similarly, the structure was made of beech, but the planks were not. Oak of the pubescent type, very hard, resistant, able to provide long spans when treated in stand, was used for a single piece of sandpit (CH 16). The set revealed a combination of complementary species combining hardwoods to strengthen its resistance and lighter ones. This phenomenon was common in the manufacturing of doors [START_REF] Chabal | Le bois dans l'architecture et le mobilier (500-475 av. J.-C.) : les boisements aux origines de Lattara[END_REF].

The craftsmen of Fréjus were able to combine the availability of species found in their environment and the qualities required to build the floor: sometimes solidity and density for the structure, sometimes lightness and the ability to be cut into planks.

Life and Death of a Wooden Construction: The Example of a Floor

Anthraco-entomological analysis only revealed the presence of xylophagous beetles belonging to the Ptinidae family in the floor of Camelin block. However, 35.2% of the wood used to build the structure was infested, which is a very high percentage compared to other studies of this type on timber [START_REF] Toriti | Les bois ouvragés en Gaule romaine : approches croisées archéologiques, anthraco-xylologiques et entomologiques[END_REF]Toriti et al., 2020aToriti et al., , 2020b)). This underlines an ongoing process of weakening of the wood of this structure brought to a halt by the fire. The details of the stratigraphy and the good conservation of the wood floor helped us to characterize this process.

Ptinidae are considered as sinanthropic and specialized in attacking dry wood, so their presence in worked wood is not surprising. The furniture beetle is ubiquitous in all woodwork, whether used for architecture or furniture [START_REF] Lepesme | Les Coléoptères des denrées alimentaires et des produits industriels entreposés[END_REF][START_REF] Delobel | Les Coléoptères des denrées alimentaires entreposées dans les régions chaudes[END_REF][START_REF] Laclos | La vrillette sans peine : première note (Coleoptera Anobiidae)[END_REF]. The genus Anobium was previously identified in the ancient site of the Villa Romana in Fréjus (Ponel, 2011, 254). Nevertheless, infestation can be favored by prior weakening of the structure, as in the present case: some time after the construction of the room, the gutter located under the sandpit to evacuate the general humidity of the room was actually a regular vector of humidification of the floor wood. As a consequence, a dry-rot type fungus developed, which was clearly observed during the anthraco-entomological diagnosis. The presence of this fungus accentuated the attractiveness of the wood for xylophagous insects, especially Anobium punctatum in sapwood (Bletchy, 1953), because it made it more digestible. Subsequently, the fungus gradually spread to the boards of the eastern part of the structure. The wood was humidified by the presence of the gutter, which favored the development of the fungus counter to its initial draining function, and this was the initial cause of the generalized infestation of the floor by the woodworm. The high concentration of galleries in the boards (CH 27, 24, 1 and 22; Figure 7) of the eastern part of the structure, also contaminated by the fungus, supports this hypothesis.

Common furniture beetles damage both hardwood and softwood without any distinction between the two. Nevertheless, they infested proportionally more beech sandpits and joists in this floor than they did for pine boards, which they probably only attacked secondarily. Over the generations, woodborers probably left the lower wooden pieces ̶ which had grown less attractive because they had already been eaten ̶ to turn to the upper ones. Given the minimum number of galleries recorded, A. punctatum infested the floor over at least two generations, and each generation lived in the wood for one to four years depending on the immediate environmental conditions in and around the building. Infestation therefore began at least two to eight years back. This raises questions about the perception of the phenomenon by the users of the building, and of this room in particular, since the flight holes of the common furniture beetle can be seen by uninformed eyes. This remark is all the more interesting as the room where the floor was laid was an important room intended for reception within a refined house of this insula.

The question of the oak piece (CH 16), located between the beech sandpit CH 23 and the beech piece CH 17, remains unanswered. Although all three pieces appear to belong to a continuum, the switch from heavily infested beech to completely healthy oak and back to infested beech raises the hypothesis of the repair of a beam of the floor that may have been too damaged by the infestation.

Anthraco-entomological analysis identified two species of Ptinidae, i.e. A. punctatum and O. cf. ptilinoides, which can coexist without any real competition. Timber colonization by two different insect species reinforces the idea of very attractive conditions for woodborers in this structure and suggests that generations may have followed one another quite rapidly. By characterizing two species from the same family, we can better measure the full impact of their attacks on the mechanical properties of the wood. Indeed, O. cf. ptilinoides larvae dig slightly larger galleries than A. punctatum larvae do, and cause more serious damage to the structure. Only the central CH 20 sandpit of the wooden structure kept traces of O. cf. ptilinoides activity. This may have been the beginning of infestation by a new species (Oligomerus), which failed due to the general fire of the building. The same may be true for the potential ant tracks in wood piece number 24, which were deposited shortly before the fire, but ants had no opportunity to colonize other parts of the structure.

Conclusion

The innovative use of anthraco-entomology opens up a new field of research interweaving the historical record of construction and that of pests. In the case of the excavation of Camelin block in Fréjus, it helped to approximate the location of timber supply areas and to reconstruct the entire implementation and destruction of the flooring of one of the houses of this insula.

The specific characterization of the wood used in the structure highlighted three potential supply areas. The first was the closest and most proximate one, less than 4-5 km around Fréjus: this was the area where the loggers, carpenters and joiners cut Aleppo pine/stone pine mainly into boards; the second one was more than 10 km away and concerned afforestation of the Scot pine type and marcescent oak type. The third one was a priori about 30 km away, but the beech and fir forests were likely closer to the ancient city than their current biogeographical distribution seems to indicate, so that this last circle was finally quite similar to the previous one. The woodcutters sought for long, straight, resistant timber to obtain good wood for beams (beech and pubescent oak) and floors (fir and pine). This is a first approach to the practices and uses of timber and the links that wood craftsmen had with their ligneous environment. In addition to environmental availability, they were guided in their choice by the mechanical and intrinsic qualities of the woods chosen to manufacture the structure, which they thus fortified meticulously, while avoiding pine trees that they favored for cutting the flooring slats. This search for resistant woods offering long spans and interesting durability for the sandpits and floor beams is perhaps also to be related with the ceremonial function of this reception room.

The floor of the house discovered on the site of Camelin block was heavily infested by woodborers (35% infestation rate). This finding constitutes an interesting key to understanding the different stages of the deterioration of the structure by fungal colonization owing to humidity from the gutter followed by a second woodborer attack. The history of a wooden structure is here particularly well documented from its genesis to its near collapse stopped by fire.

For the time being, this study remains very isolated for the Mediterranean area. Other points of comparison will undoubtedly help to write a history of wood pests, for these insects are extremely sensitive to climatic conditions. The accumulation of long-term studies should help to enrich the knowledge of environmental conditions in Fréjus and around its territory.

The observation and recording of the traces left by wood-boring insects was systematically carried out on archaeological charcoals. It is a time-consuming approach compared to conventional identification and anatomical observation methods, but it documents the archaeology of the construction and deconstruction of a Roman urban building in an unprecedented way.

Figure 1 .Figure 2 .

 12 Figure 1. Map of Fréjus city and location of Camelin block (by SAPVF : Archeology and Heritage departement of Fréjus city

Figure 3 .

 3 Figure 3. View of the floor at the time of its discovery, photography by SAPVF.

Figure 4 .

 4 Figure 4. Distribution of woody floor species, CAD, M.Toriti, according to SAPVF.

Figure 5 .

 5 Figure 5. Example of a beech wood sample infested with numerous galleries A.punctatum, M. Toriti. b. Archaeological faecal pellets of Anobium, M. Toriti. c. Carbonized A. punctatum faecal pellets from the floor, CMEBA-Scanmat. d. Example of a beech wood sample infested by Oligomerus, M. Toriti. e. Archaeological faecal pellets of Oligomerus, M. Toriti. f. Carbonized Oligomerus faecal pellets from the floor, CMEBA-Scanmat.

Figure 6 .

 6 Figure 6. Cubic-rot on carbonized beech (CH 27), M. Toriti. b. Cubic-rot on carbonized beech (CH 4), M. Toriti. c. General view of cubic-rot, CMEBA-Scanmat. d. Detail of damage caused by cubic-rot on wood, CMEBA-Scanmat.

Figure 7 .

 7 Figure 7. Distribution of A. punctatum galleries, CAD, M. Toriti.

Wood piece Dominated taxa Other taxa (bun- dles with bark) Nb of analyzed fragments Structure ele- ment

						Insect traces	Cube rot
							traces
	CH 1	Pinus halepensis	Alnus	60	Board	x	x
	CH 2	Pinus halepensis	-	60	Board	x	x
	CH 3	Pinus halepensis	-	30	Board	x	x
	CH 4	Fagus	Fraxinus	100	Plate element	x	x
	CH 5	Pinus halepensis	-	40	Board	x	x
	CH 6	Fagus	-	40	Plate element	x	x
	CH 7	Pinus halepensis	-	30	Board	x	-
	CH 8	Pinus halepensis	-	60	Board	x	-
	CH 9	Abies	-	60	Board	x	-
	CH 10	Pinus halepensis	-	30	Board	x	x
	CH 11	Pinus halepensis	-	30	Board	x	-
	CH 12	Pinus halepensis	Acer, Alnus	40	Board	x	-
	CH 13	Pinus halepensis	-	27	Board	x	-
	CH 14	Fagus	-	50	Plate element	x	-
	CH 15	Fagus	Fraxinus	60	Plate element	x	-
	CH 16	Quercus	-	53	Plate element	-	-
	CH 17	Fagus	-	100	Plate element	x	-
	CH 18	Pinus halepensis	-	23	Board	x	-
	CH 19 CH 20 CH 21	Pinus type sylvestris Fagus Abies	-Bruxus, Quercus suber Quercus suber	30 120 100	Joist element Plate Joist	-x x	---
	CH 22	Fagus	Alnus	100	Joist	x	x
	CH 23	Fagus	-	59	Joist	x	-
	CH 24	Pinus halepensis	-	60	Board	x	-
	CH 25	Pinus halepensis	-	30	Board	x	-
	CH 26	Pinus halepensis	-	40	Board	x	-
	CH 27	Fagus	Alnus, Quercus	100	Plate	x	x

Table 1 .

 1 Anthracological identification and characterization of woody floor species.

	Wood piece	Anobium punctatum Oligomerus ptili-	Ptinidae	Undetermined Total galle-	Proportion of the
			noïdes			ries	infestation (%)
	CH 1	1	-	9	5	15	21,7
	CH 2	3	-	5	6	14	20
	CH 3	4	-	13	-	17	50
	CH 4	12	-	36	10	58	52
	CH 5	2	-	5	1	8	20
	CH 6	2	-	2	1	5	10
	CH 7	1	-	2	3	4	13,3
	CH 8	3	-	2	-	5	8,3
	CH 9	9	-	7	3	19	18
	CH 10	-	-	8	2	10	30
	CH 11	1	-	-	1	2	6,7
	CH 12	3	-	-	-	3	7,5
	CH 13	3	-	1	-	4	14,8
	CH 14	4	-	-	3	7	12
	CH 15	20	-	7	7	34	50
	CH 16	-	-	-	-	-	-
	CH 17	53	-	5	1	59	55
	CH 18	12	-	16	10	38	52,2
	CH 19	-	-	-	-	-	-
	CH 20	33	12	15	3	63	45,8
	CH 21	1	-	46	1	48	28
	CH 22	20	-	34	6	60	44
	CH 23	34	-	54	-	88	21,3
	CH 24	24	-	23	9	56	78,3
	CH 25	7	-	1	2	10	33,3
	CH 26	14	-	7	1	22	50
	CH 27	21	-	16	68	104	74
	TOTAL	286	12	312	143	753	35,22
	%	37,99	1,6	41,43	18,99	100	-

Table 2 .

 2 Identification of insects according to the floor room.

Funding

This work was supported by the French Region Pays de la Loire (PhD grant), OSUR Rennes I University (Photographs MEB by CMEBA-SCANMAT laboratory) and the Archaeological and Heritage Service of Fréjus.


Alif Akbar Pranata

Olivier Barais

Johann Bourcier

Ludovic Noirie

Misconfiguration Discovery with Principal Component Analysis for Cloud-Native Services

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Miscon guration Discovery with Principal Component Analysis for Cloud-Native Services

Alif Akbar Pranata, Olivier Barais, Johann Bourcier Univ Rennes 1, Inria, Irisa Rennes, France alif-akbar.pranata@inria.fr, olivier.barais@irisa.fr, johann.bourcier@inria.fr Ludovic Noirie Nokia Bell Labs Nozay, France ludovic.noirie@nokia-bell-labs.com Abstract-Cloud applications and services have significantly increased the importance of system and service con guration activities.

ese activities include updating (i) these services, (ii) their dependencies on third parties, (iii) their con gurations, (iv) the con guration of the execution environment, (v) network con gurations.

e high frequency of updates results in signi cant con guration complexity that can lead to failures or performance drops. To mitigate these risks, service providers extensively rely on testing techniques, such as metamorphic testing, to detect these failures before moving to production. However, the development and maintenance of these tests are costly, especially the oracle, which must determine whether a system's performance remains within acceptable boundaries.

is paper explores the use of a learning method called Principal Component Analysis (PCA) to learn about acceptable performance metrics on cloudnative services and identify a metamorphic relationship between the nominal service behavior and the value of these metrics. We investigate the following research question: Is it possible to combine the metamorphic testing technique with learning methods on service monitoring data to detect error-prone recon gurations before moving to production? We remove the developers' burden to de ne a speci c oracle in detecting these con guration issues. For validation, we applied this proposal on a distributed media streaming application whose authentication was managed by an external identity and access management services.

is application illustrates both the heterogeneity of the technologies used to build this type of service and its large con guration space. Our proposal demonstrated the ability to identify error-prone recon gurations using PCA. Keywords-Recon gurations; Metamorphic testing; Principal component analysis; Cloud-native services I. I A growing number of network services and applications are now being deployed in the form of so-called "cloud native" applications or services. [START_REF] Kratzke | Understanding cloud-native applications a er 10 years of cloud computing-a systematic mapping study[END_REF].

ese services are characterized by the following features:

• A service-based architecture. e style can be any architectural model that is modular and loosely coupled. • A common packaging model and a self-contained execution environment that provides portability as well as isolation using container, VM and/or uni-kernel solutions.

• A continuous and automated process to develop, build and deploy these services. As a side-e ect of this trend, the frequency of services recon gurations is signi cantly increased [START_REF] Wang | Research on dynamic recon guration technology of cloud computing virtual services[END_REF], [START_REF] Assuncao | Dynamic Work ow Recon gurations for Recovering from Faulty Cloud Services[END_REF], [START_REF] Durán | Robust and reliable recon guration of cloud applications[END_REF].

ese recon gurations are related to: changes in the source code of services or third-party libraries of these services, a change in the parameters of one of these services, or a change in the underlying network parameters. e so ware complexity is now overwhelming and it becomes di cult to guarantee, a priori, that a new service con guration will be correct or less error-prone [START_REF] Sayagh | So ware Con guration Engineering in Practice Interviews, Survey, and Systematic Literature Review[END_REF]. Each of these new, modi ed con gurations has the potential to introduce errors at the service level. While some recon gurations or updates can signi cantly change the service behavior, most of these recon gurations do not fundamentally change the business logic. Still, business outputs are a ected by this complexity due to unexpected errors.

Each recon guration is seen as a network state over time. Given that errors and failures are caused by recon guration management, therefore, from a high abstraction level, the expected service behavior should remain "close" to the previous con gurations, meaning that for each particular network state should have relations to the state -1 con guration as the oracle de nition to embrace failures. So ware testing techniques are well-known techniques to address recon guration management issues and to ensure performance validity [START_REF] Danglot | A snowballing literature study on test ampli cation[END_REF]. In such techniques, each test de nes an oracle, characterized by input-output de nitions to check whether a test has passed or failed. However, de ning oracle and maintaining the testing code base is burdensome and costly.

A metamorphic testing method has been proposed [START_REF] Chen | Metamorphic testing: A new approach for generating next test cases[END_REF] to test programs removing the need for oracle. It employs properties of the target function, known as metamorphic relations, to generate follow-up test cases and verify the outputs automatically. e intuition behind using metamorphic testing to alleviate the oracle problem is as follows: Even if we cannot determine the correctness of the actual outputs for individual input, it may still be possible to use relations among the expected outputs of multiple related inputs (and the inputs themselves). Following the principle of metamorphic testing, it has been suggested that indi-cating a service recon guration validity is possible if there are metamorphic relations between the previous behaviors of the service and the recent behavior of the service with the new con guration [START_REF] Boussaa | Leveraging metamorphic testing to automatically detect inconsistencies in code generator families. So ware Testing[END_REF]. Yet, since it is nearly impossible to have an exact characterization of the behavior of a service, the main challenge is to nd an e cient method to characterize the service behavior and compare several executions of a particular system.

In this paper we study the discovery of error-prone recon gurations to understand the complex nature of native cloud services and identify the actions that may lead to erroneous behavior due to miscon gurations. Motivated by the ability of metamorphic testing to avoid oracle de nition for testing so ware systems, we rely on performance metrics as an input to this analysis to identify potential miscon gurations. Two types of metrics are introduced: generic metrics and business metrics that must be kept within acceptable limits to avoid user experience degradation and maintain continuous delivery

We applied a learning method called principal component analysis (PCA) [START_REF] Wold | Principal component analysis[END_REF], [START_REF] Jollife | Principal component analysis: A review and recent developments[END_REF] on cloud-native applications metrics. As a proof of concept, we tested our approach in a media streaming application, simulating a real-world use case scenario of cloud-native systems. Given the advantage of no oracle de nition and exible metrics selection, our experiment shows that the use of generic and business metrics, plo ed as the metamorphic relations of multiple system executions, as the inputs of the PCA are e ective to discover potential valid and invalid system recon gurations in the cloud environment. We also con rm such validity of each recon guration by measuring the proximity of metamorphic relations of each system execution.

e remainder of the paper is as follows. Section II explores real-world implementation examples that motivate this paper, discusses the concept of PCA, and directs a research question, followed by the proposed approach in section III. We show our implementation scenario and the experiment setup in IV, then we evaluate and validate our approach in section V. Section VI discusses the related works. Finally, section VII concludes the paper and foresees our future works.

II. M B

In this section, we provides examples of today's network system complexity from companies and our speci c implementation. en, we discuss principal component analysis (PCA) method for our evaluation technique. To understand the purposes and constraints of our work, we develop a research question and raise the hypothesis.

A. Motivating Examples: So ware Complexity e so ware is complex [START_REF] Sayagh | So ware Con guration Engineering in Practice Interviews, Survey, and Systematic Literature Review[END_REF], [START_REF] Lew | So ware Complexity and Its Impact on So ware Reliability[END_REF]. It has the inherent characteristic of having several degrees of dependencies that exist and work together to form a whole system to meet customer needs. A so ware system is therefore the result of the spontaneous composition of a set of modules.

e trend towards the "so warization" of everything allows IT to move gradually to so ware-based implementation, usually with API technology for service dependencies and communication. An example of this evolution is the current trend of so ware-de ned networks/network functions virtualization (SDN/NFV) in a native cloud service environment [START_REF] Bruschi | An SDN/NFV Platform for Personal Cloud Services[END_REF]. A case of Amazon networks is formed by so ware-based microservices, creating full connectivity through which API communicates information data over network protocols in the cloud environment 1 .

e complexity and dynamic of so ware module interaction makes system management di cult. Failures are inevitable, especially in the production environment, when engineers and experts are not always available to maintain and manage the systems. As a motivating example, consider a large-scale distributed system with microservices deployed in cloud servers to provide seamless data exchange and processing around the world. We take a more concrete faulty event by Google Clout Platform (GCP) 2 . GCP has Memcache, an app engine service that speeds up response to data queries by storing them in cache memory. Google utilizes well-built, automatic failover to ensure a seamlessly switch from one failed data center to the other to prevent failure a er se ing up Memcache for a consistent view of data center serving every application. Yet, utilizing a wellmaintained failover mechanism does not mean that systems are invulnerable. Failures can occur due to the unforeseen circumstances at any time during runtime due to a single failure of a module.

In this paper, we take an example of a simple media streaming application (built for our use case implementation in section IV) where some instances of video server deliver video data to their clients. e implementation complexity increases when an access management server and a reversed proxy server sit between the video servers and clients to provide authentication and authorization mechanisms as well as a load balancing scheme. Considering only the video server application, we can have at least 174 application modules. From those modules, we have eleven core dependencies with the so ware version of each dependency as listed in table I. When we deploy and run our media streaming application system in a cloud environment, the system may evolve during the runtime and it becomes hard to manage background events in the execution. If, at any moment, we upgrade the version of one of the dependencies, other services that depend on it may have to adapt their behavior and may fail.

B. Principal Component Analysis (PCA)

Principal component analysis (PCA) [START_REF] Wold | Principal component analysis[END_REF], [START_REF] Jollife | Principal component analysis: A review and recent developments[END_REF] is an unsupervised learning method, multivariate data analysis of large data sets. It reduces the dimensionality to increase interpretability without losing essential information at the same time. e core idea behind PCA is that it creates from the original data using orthogonal transformation new uncorrelated variables that successively maximize variance.

is new variable is called the principal components (PCs). Finding PCs reduces solving an eigenvalue/eigenvector problem. It accomplishes such task at hand, not a priori, making PCA an adaptive tool for data analysis. e adaptivity increases as a result of its capability to be tailored to various data types and structures.

e main objective of PCA is to reduce the dimensionality of the original data a er obtaining the maximum amount of variance with the selected minimum number of PCs. e process of PCA is explained as follows.

1) Given the original 2 x n (n is the number of metrics), PCA computes the eigenvectors and the eigenvalues of the covariance matrix. e eigenvectors are used to project the data from n dimensions to decrease representation, and the eigenvalues provide the data variance in the eigenvector direction. 2) PCA iterates the calculation of nding PCs using the number of eigenvector. e rst eigenvector determines the direction of the highest data variance and the rst PC is obtained from the data with the greatest possible variance in the data set.

3)

e subsequent PCs (second, third, and so on) are obtained in the same manner using the respective eigenvector value inside the iteration. However, there is a condition for each PC: it is uncorrelated with the previous one and it accounts for the next highest variance. 4) Each of the eigenvector and the eigenvalue is associated with the direction of each PC. e eigenvector associated with the largest eigenvalue has the same direction as the rst PC. e subsequent PCs have the same association rule. PCA plots data points a er its calculation into a PCA space. PCA can show data that have extreme points, called outliers, which do not follow the model of the majority of the data at the boundaries of the space.

C. Research estion

We formulated the following research question: is it possible to combine metamorphic testing technique with unsupervised learning methods on service monitoring data to detect error-prone recon gurations before moving to production? Our hypothesis arises for the answer.

Metamorphic testing is a comparison testing technique of two programs that have a relational pa ern called meta- morphic relations (MRs) for checking and comparing the correctness of a ributes in programs. In common cases, the MRs are known, for example when we perform metamorphic testing in functional programming (such as NumPy library 3 in Python) which relies mostly on mathematical functions computations and thus are comparable. But there may be other cases, such as in our implementation context, which is di cult to nd relations and pa erns as we avoid oracle de nitions. is leads to insu cient tooling for comparison and analysis. e combination of metamorphic testing with unsupervised learning method is foreseen as the solution. Given neither the metamorphic relations nor the oracle de nitions, our proposed approach automatically creates relations among di erent sets of input and output.

III. T P A

We propose a technique to discover miscon guration in cloud-native services. Our approach was motivated by the metamorphic testing technique that removes the need to de ne oracle in advance. As a replacement, we de ne our oracle with an unsupervised learning method called principal component analysis (PCA) method. We choose PCA among similar machine learning and dimensional reduction technique such as ICA, LDA, and SVD as it suit our requirement: measuring uncorrelated and unsupervised metrics. We monitored and analyzed our approach in our experiment using monitoring agents and statistical distance measurements.

A. General Approach

Our approach was motivated by the metamorphic testing technique, which compares and analyze multiple program executions with each of them may have di erent performance behavior due to adjusted con gurations, or recon gurations, during the runtime. We de ne a recon guration as an action trigger that a ects the execution behavior yet does not change the application business outputs. Reconguration aimed to cause intentional system disruption. e examples of recon gurations are: changing parameter, revoking access, shu ing down, or restarting a service ( e exact recon gurations in section V-B). Our approach tested recon gurations and discovered miscon gurations that may fail cloud-native services execution. Metamorphic testing also o ers exibility in removing a priori oracle de nition and choosing any metrics for evaluation. e metrics are useful for the PCA to create relations among di erent program executions. In the metamorphic testing technique, these relations are called metamorphic relations (MRs). Our approach used the MRs as our oracle de nition for PCA. We compared and analyzed the MRs to check the correctness of the executions under testing.

Figure 1 illustrates the general approach of our proposal. We had two testing steps, denoted by (A) and (B), which di er in the context of execution. Step (A), called baseline execution, is de ned as the initial execution on which we had experts knowledge about the normal expected execution. To avoid manual and repetitive e ort for generating baseline execution, we automate the process, starting from creating nodes, connecting links, con guring the networks a ributes (ip, gateway, etc.) per nodes, launching services in the nodes, until destroying the nodes. We tested our system using the knowledge for the (A) input in a system under test (SUT) consisting of network architecture that runs our use case scenario with running services in a cloud environment (detailed description of use case scenario in section IV). During the execution, we monitored the execution behavior, extracted and obtained the performance metrics data for building the model of execution behavior using PCA. Our model builder run the PCA method for each baseline execution to obtain this testing output, namely the baseline model.

With the baseline model in (A), we then executed another set of executions (B) in the SUT. Here in (B), we applied recon gurations to the SUT to change the execution be-havior. e subsequent processes were similar to (A) until we built the (B)'s PCA model. From here, we combined the PCA model from (A) and (B) to obtain our oracle by evaluating and comparing the PCA model. We obtained the PCA space, consisting of each data point representing each execution behavior by the PCA calculation. We then evaluated and compared each data point and validated their relations using euclidean distance (ED). More speci cally, we measured the euclidean distance between each data point and the centroid value of the major cluster in the PCA space. e euclidean distance also served as the MRs in the perspective of the metamorphic testing approach. Finally, we decided the correctness of each execution, whether it is valid and invalid recon gurations based on the ED relations of each execution in the PCA space. is decision is the nal output of our proposed approach. Further validation can be made by the experts, whether a particular execution has the correct decision of valid or invalid recon gurations.

By the euclidean distance (or MRs), Our approach categorized and de ned two sets of output representing the system execution correctness as follows:

1) valid recon guration, which did not a ect the normal delivery of data a er the trigger was applied. 2) invalid recon guration, which produced error messages and disrupted the data delivery a er the trigger was applied. We de ne error messages as the report of error obtained by monitoring agent in the form of bad requests/responses in data exchange. Here, we obtained the number of HTTP error code messages.

B. Monitoring & Analysis

We deployed monitoring agents in our network architecture for monitoring purposes. Our monitoring agents collected the metrics data and exported the data into a comma-separated values le format. For analysis, we obtained and compared a set of two scenario outputs of baseline execution and recon gurations. Speci cally, we selected metrics for observation, extracted the metrics data from each scenario, and constructed our data set from the data for PCA method processing.

We obtained the result abstraction of valid and invalid recon gurations with the PCA method. We a rmed the valid recon gurations as the correct system behavior. At the same time, we also discovered system misco gurations caused by invalid recon gurations. In order to validate the classi cation of recon guration validity, we calculated the euclidean distance (ED) of each data points to the centroid of the baseline execution scenario in the PCA space. Data points representing each execution behavior may be sca ered in the PCA space in far proximity that we considered outliers. To determine the outliers, we computed the 97.5%-antile Q of the Chi-square distribution as a cuto value of the ED ( X 2 a,0.975 ). According to the table of the Chi-square distribution 4 , the value is equal to √ 7.38 = 2.71. Any ED value bigger than that is identi ed as the outlier, which means that the recon guration is invalid. Otherwise, the system running behavior is valid a er the recon gurations.

IV. I

We implemented our proposed approach in a cloudnative system running a real-world business application.

e application in our experiment had a use case scenario with a system under test (SUT), which is de ned as the set of particular services as the testing targets. e implementation was a media streaming system with services that integrated processing and communicated media data.

A. Use Case Scenario

Application domains range widely in cloud environments, such as data stream processing. Such processing operates a series of continuous data delivery to achieve outputs.

ese domains include media/video streaming application, delivering video information and playing it ondemand as per user needs [START_REF] Alasaad | Innovative Schemes for Resource Allocation in the Cloud for Media Streaming Applications[END_REF]. e common protocol in this application is HTTP live streaming (HLS), which has gained popularity over the past years due to its ne rate adaptation and workload sharing [START_REF] Le | An Evaluation of Bitrate Adaptation Methods for HTTP Live Streaming[END_REF].

Media streaming in a large-scale system can create service and delivery quality issues to clients, mostly when recon gurations occur during the runtime. For example, when a service is down or changes the parameter value, other dependant services may lose its states to continue functioning correctly. Due to the complex nature of such application systems, we devised an experiment technique in a media streaming application to discover dynamic system behavior and detect miscon gurations resulting from recon gurations.

We applied our approach to a real-world video serverclients communication with an identity and access management service. ere were four objects in our implementation: video server, client nodes (end users), identity and access management server (or auth server for brevity), and database server. Figure 2 illustrates the work ow of these objects. Initially, the video server executed an identi cation mechanism by asking the auth server to obtain a secure token [START_REF] Kratzke | Understanding cloud-native applications a er 10 years of cloud computing-a systematic mapping study[END_REF].

en, clients requested media stream to the video server [START_REF] Wang | Research on dynamic recon guration technology of cloud computing virtual services[END_REF]. Before holding its secure token from the auth server, the video server could not directly grant valid access to the clients for the requested video data. Instead, it redirected the request to an authorization endpoint so the clients can log in to the video server for authorized access (3). Once logged in, the auth server communicated with its database server to ask for client identi cation (4). In our implementation, the database server sat in the same machine with the auth server. If not identi ed, the video servers access was blocked; otherwise, the auth server identi ed the request (5) and redirected the client responses with auth code [START_REF] Danglot | A snowballing literature study on test ampli cation[END_REF]. is code contains an access token that has a limited lifespan. e detail of short lifespan bene t is out of the scope in our discussion (in short, it is bene cial for enhanced security: to keep short validity duration for frequent checking of a ackers in the system). Due to the short lifespan of access token, the clients had to refresh the auth code with a refresh token when the access token reached the timeout. Upon obtaining the auth code, the clients requested the video data with its code to the video server [START_REF] Chen | Metamorphic testing: A new approach for generating next test cases[END_REF]. e Video server checked once again for the code validity and requested a valid session with an ID token to the auth server [START_REF] Boussaa | Leveraging metamorphic testing to automatically detect inconsistencies in code generator families. So ware Testing[END_REF]. e auth server returned the ID token (9), and nally, the valid connection between clients and video server was established.

To evaluate the implementation, we chose two kinds of native system metrics: generic and business. e generic metrics were a quantitative measurement that dealt with values inherent from the general system running performance, such as tra c rate, CPU usage, and memory usage.

e business metrics were the application-speci c metric that became the key parameter in running such an application in real-world execution. In our implementation, the business metric was the number of error messages caused by recon guration. We elaborate our metrics selection in section V-A.

B. Experiment Setup

We set up and run our network architecture consisting of nodes that run services in container applications. Our services in the architecture communicated and constructed our media streaming application. We experimented and evaluated our approach in GNS3 network emulation environment.

e GNS3 server runs in our private cloud lab with computing resources that can accommodate load consumption with as many as thousands of nodes: Intel(R) Xeon(R) Gold 6238 CPU with 2.10GHz 88 Core, 187GB memory, and 11 TB disk storage size.

We show the network architecture of media streaming application in our experiment in gure 3. We had four objects in the experiment as described in section IV-A. For video server and clients, we deployed multiple instances of each of them. We also deployed a reversed proxy server to serve load balancing for requests sent to each video server instance. We separated service nodes based on service layering and function into the system under test (SUT) cluster, testing cluster, and monitoring cluster. Following this setup, we implemented our approach to the use case scenario, evaluated our experiments and validated the results.

V. E V

We evaluated our proposed approach in the experiments for various system recon gurations on system under test (SUT) and discovered miscon gurations in cloud-native systems. Our experiments compared and analyzed a series of initial execution, which was labeled as the baseline by experts, and executions with recon guration schemes that changed the system runtime behavior. e baseline is de ned as a test scenario with expected correct behavior due to the absence of execution changes during runtime. In monitoring, we obtained execution metrics data and collected them into a data set for the evaluation with principal component analysis (PCA).

A. Metrics and Data Collection

For the evaluation, the PCA allows us to select arbitrary metrics. We introduced metrics selection from generic metrics in various application domains, and business metrics, which are systems-speci c metrics. With PCA, we processed the metrics data and produced data points in the PCA space. e correlations of each data point were regarded as the metamorphic relations (MRs), which help to determine the system performance correctness. We chose the generic and business metrics as follows (the bold names inside parentheses refer to the column names in table II).

1) Number of HTTP error codes (err). We obtained and counted the total number of 400 error codes for showing which recon gurations produce invalid con gurations. e number of error codes is one of the business metrics, representing the failures in a media streaming execution. 2) CPU usage. In container application, each service has CPU usage which denotes its workload. We measured the average (cpu avg) and standard deviation (cpu std) of the CPU usage in percentage. 3) Sent tra c. Requests from clients were sent to the video server and subsequently were routed to the auth server for access management mechanism. e sent tra c also included video data. We measured the average (sent avg) and standard deviation (sent std) of the sent tra c in megabyte per thirty seconds. 4) Received tra c. Being authorized, clients received the responses of valid access from the auth server. e received tra c also included video data. We measured the average (rcvd avg) and standard deviation (rcvd std) of the received tra c in megabyte per thirty seconds. 5) Memory usage. Memory denotes the memory consumption of the container application used by the service. We measured the average (mem avg) and standard deviation (mem std) of the memory usage in megabyte (MB). 6) Uptime (uptime). We counted the uptime from the rst client requested media streaming until the last client closed the streaming. During the uptime, media streaming processing and communication occur, such as token exchange, media data delivery, and error message exchange. We observed the uptime per minute. e uptime is another business metric in our evaluation. We collected data from the above metrics into a twodimensional matrix, which then formed our data set, with features indicating the metrics in column, and components denoting our test cases in row. is matrix is shown in table II.

We tested multiple execution times in test cases, which is de ned as a unique system execution and can be performed multiple times per execution scenario (forming a series of test cases). e purpose of multiple test cases is to obtain data consistency, which formed a historical data series of each execution scenario.

e historical data builds a behavioral execution trend for the comparison with other execution scenarios. Table III shows the number of test cases in each execution scenario. In the table, we executed especially the baseline executions more in quantity to build data con dence for the basis of comparison with execution with recon guration schemes.

B. Recon guration Schemes

Recon gurations aimed to cause intentional system disruption during runtime. To achieve this goal, we triggered each recon guration in the SUT at the time the last client has started the streaming until the system runtime has ended. e following explains the types of recon guration in our experiment and their behavior.

1) restarting auth server (kcRestart). We restarted the server when all clients had run the streaming and observed that all clients could not manage token expiration when the access token must be refreshed at a particular interval. e clients failed to receive streaming data from the video server. e number of error messages was equal to the number of clients. 2) misredirecting URI (UriRedirect). A valid redirect URI is a scheme in the auth server that allows the server to redirect responses to the requested application a er successful login. In baseline execution, we set this value with a wildcard (*) and intentionally recongured it with a false URI at runtime to disrupt the redirection scheme. Although valid responses from the video server were missing, the clients retrieved the video information from its cache, allowing normal streaming.

3) removing web origin value (webOrigin). We removed the web origin wildcard and obtained error messages by this recon guration. We observed this since the video server failed to refresh the access token in the auth server. We also noticed this produced a great number of error messages compared to other recon gurations, although it only stopped the media streaming and did not crash the video player instance. 4) revoking token (revokeToken). e auth server periodically sent two kinds of token: access and refresh token to maintain valid authentication once clients were authorized. We destroyed this valid authentication by revoking the token and observed failures in the token exchange mechanism.

Table IV summarizes our observation in executing recon guration scheme, including the categorization of valid or invalid recon guration referring to section III-A.

C. Results & Discussion

1) Results: We evaluated our implementation using the PCA method. e PCA result is shown in gure 4. Data points sca er and have distance among each of them in a PCA space in the gure. In most experimentations, any execution contexts share similar output if they have similar actions, triggers applied to them in the experiment. Speci c to our experimentation, they follow a trend in performance behavior in the PCA space and form a cluster.

e di erentiator can be seen among di erent execution scenarios; ey indicate di erent trends, thus classifying themselves in the space and constituting a cluster. If they are far from the trend of the dominant execution due to di erent recon gurations, they are considered as an outlier.

e baseline execution is the major trend and the data points from its test cases form a cluster ( gure 4, green). Other execution scenarios in the experiment with recon gurations, in this case kcRestart (blue), webOrigin (purple), and revokeToken (magenta), are sca ered and some are seen as outliers in the PCA space. We call these outliers as invalid recon gurations. ese outliers may a ect the system performance due to its deviation found in their metrics value. With this information of outliers, we argue that obtaining the decision of invalid recon gurations can help to notify the system developers and managers for potential weaknesses of the systems as discovered by our proposed approach with PCA analysis. However, we also nd a particular recon guration, UriRedirect (brown), that does not have errors during execution yet has similar behavior to the baseline execution.

is kind of recon guration produces a small principal component correlation output (by the PCA) and locates close to the baseline. We categorize this phenomenon as valid recon gurations (along with the baseline execution). Yet, this may become a seemingly-ne execution with subtle potential to fail the system as it may have hidden, suspected deviations in the performance. For system developers and managers, we report this phenomenon as a warning for potential system weaknesses. Having our PCA result, we proceeded to measure each data point correlation in the PCA space by calculating the distances.

To validate the determination of valid and invalid recongurations, we calculated the euclidean distance between each data point in the PCA space and the baseline cluster centroid. In the view of metamorphic testing, this distance measurement identi es the MRs for our built oracle ( gure 1). Any data point that does not follow the trend of the baseline execution was suspected as the outliers. We show the results of our outliers detection method in gure 5. We set the threshold of 2.71 in the y-axis, following the 97.5%-antile Q of the Chi-square distribution commonly used in the literature [START_REF] Boussaa | Leveraging metamorphic testing to automatically detect inconsistencies in code generator families. So ware Testing[END_REF], [START_REF] Enot | Preprocessing, classi cation modeling and feature selection using ow injection electrospray mass spectrometry metabolite ngerprint data[END_REF], [START_REF] Hubert | Robust PCA for skewed data and its outlier map[END_REF]. Modi cations in this threshold value may change the determination of the recon gurations validity; e lower the value, the more data points will be detected as the outliers. In gure 1, data points number 31-33 and 37-42 is considered as the outliers. ey con rm that such points belong to the execution scenarios of the baseline execution with kcRestart, webOrigin, and revokeToken recon gurations ( gure 4). By this calculation, we also con rm that the baseline execution with UriRedirect recon guration (data points 34-36 in gure 1) has the correct behavior, being close to the baseline scenario ( gure 4), thus regarded as valid recon guration.

2) Discussion: e phenomena of data points sca eration a er PCA show that each system execution assembles into clusters and di ers based on the execution scenario applied to them. ere were observable pa erns in which we remarked four types of relational pa erns (as symbolized by the MRs in metamorphic testing) among di erent phenomena:

1) baseline and all recon gurations. In this relational pa ern, the baseline serves as the guideline and comparator of the system execution to each recon guration. In this pa ern, we disregarded the decision of valid and invalid recon gurations. 2) baseline and invalid recon gurations. In this pa ern, invalid recon gurations were identi ed as the system miscon gurations. ey were seen as outliers and informed an explicit warning for the experts and developers about system weaknesses that can fail the system execution.

3) baseline and valid recon gurations. Valid recon gurations showed delicate performance and can be reported to the system developers for potential failures that can be hard to manage in the execution and may fail the system. 4) valid and invalid recon gurations. Both phenomena should be marked as potential system weaknesses, regardless of the severity of the system failures.

e combination of metamorphic testing and multivariate data learning method e ectively detects potential weaknesses of service recon gurations due to its absences of a priori oracle requirement and exible metrics data selection. By our approach, the four relational pa erns are adequate to discover miscon gurations for further reports and analyses by cloud business experts.

VI. R W Recon guration Veri cation

Large-scale data centers and cloud computing have turned system con guration into a challenging problem. Several widely-publicized outages have been blamed not only on so ware bugs but also on con guration bugs. To face this challenge, Shambaugh et al. [START_REF] Shambaugh | Rehearsal: A con guration veri cation tool for puppet[END_REF] provide a veri cation tool for Puppet con gurations (DSL to describe cloud con gurations). In [START_REF] Durán | Robust and reliable recon guration of cloud applications[END_REF], Duran et al. propose a decentralized and robust protocol for runtime recon guration of cloud applications with failures. e proposed protocol focuses on VM instantiation and destruction as well as component start-up and shutdown. In the same trend, Jarraya et al. [START_REF] Jarraya | Cloud calculus: Security veri cation in elastic cloud computing platform[END_REF] provide a formal framework for the speci cation of virtual machine migration and its security policy updates.

ese three approaches represent so-called white-box techniques [START_REF] Nidhra | Black box and white box testing techniques-a literature review[END_REF] for which the evolution of a con guration has a precise semantics in the meaning of programming language and logic. In this paper, we address recon gurations with a ner granularity (for example, version changes of a third party library) of which the semantics are not de ned. erefore, we use so-called black-box testing techniques to validate the correctness of a recon guration.

Closer to our proposal, ConfAdvisor [START_REF] Chiba | Confadvisor: A performance-centric con guration tuning framework for containers on kubernetes[END_REF] provides a performance-centric con guration tuning framework for containers on Kubernetes. Parts of this framework can be used to implement automatic con guration exploration using our approach to pre-detect invalid con guration. Yet, as our proposed approach does not match with the implementation scenario in Kubernetes environment where oracles can be found in advanced, we opt for our approach with metamorphic testing and PCA in GNS3 cloud environment.

Metamorphic Testing

As discussed in the introduction, the core of our approach is to investigate the possibility of building a metamorphic relationship following a recon guration that does not change the business logic of a cloud-native service by looking at a set set of generic and business metrics. A recent survey on metamorphic testing [START_REF] Segura | A survey on metamorphic testing[END_REF] highlights that this approach is used in lots of domains [START_REF] Segura | Metamorphic testing of restful web apis[END_REF], [START_REF] Donaldson | Metamorphic testing for (graphics) compilers[END_REF] and can be applied when we can de ne a clear metamorphic relation between inputs and outputs [START_REF] Liu | A new method for constructing metamorphic relations[END_REF].

In [START_REF] Chen | Metamorphic testing: A review of challenges and opportunities[END_REF], Chen et al. discuss the next challenges for metamorphic testing, which is to identify and select systematic metamorphic relations. e authors identi ed three opportunities that match the context of our approaches (cloud, big data, and agile development). Based on their initial idea, this paper proposes a mechanism to bene t from learning metamorphic relations between the correct behavior of services and these monitoring data a er recon gurations, from agile development and a vast volume of monitoring data production services. In this paper, the metamorphic testing approach combines the PCA to provide metamorphic relations that are useful for determining the correctness or validity of system executions.

VII. C

F W is paper proposes a miscon guration discovery technique of cloud-native services using PCA on monitored service data. Our approach was motivated by metamorphic testing that removes a priori oracle de nition when passing functional tests of cloud-native services execution for discovering the service miscon guration. Instead, we created our oracle using data point correlations in the PCA space from the arbitrary selection of generic and business metrics. Applying the proposed approach to a distributed media streaming application shows that the approach effectively detects error-prone recon gurations through scenario execution without any associated oracle de nition. A er the PCA method and the euclidean distance analysis, business expert judgments help to validate the correctness of particular recon guration and understand the complex nature of cloud-native systems.

For future works, we keep working on implementing several e cient heuristics to detect miscon gurations that do not impact a priori the business logic of the application.

e simple heuristic proposed in this paper is relevant, but it is necessary to compare several heuristics to detect these speci c changes. Finding a useful heuristic is a way to include the approach in a continuous integration pipeline. We are also interested to not only change the system execution with recon gurations, but also with network failure injections, for example pu ing link latency, adding packet losses, or changing link bandwidth. Finally, we are keen to obtain the ability to re-execute work ows on updates of services in a test environment to directly exploit the monitored metrics in production.

A is work was done in the framework of the joint research lab between Inria and Nokia Bell Labs on Smart, Automated Programmable Infrastructures for End-to-end Network Services (SAPIENS). R

Figure 1 .

 1 Figure 1.

Figure 2 .

 2 Figure 2. e work ow of identity and access management mechanism by the video server instances and clients.

Figure 3 .

 3 Figure 3.

Figure 4 .

 4 Figure 4.e data points (from test cases) of each execution scenario in a PCA space: baseline execution (the green points) and various recon guration scheme.

Figure 5 .

 5 Figure 5. e euclidean distance plot of each data point to the centroid of baseline execution and the threshold which separates valid and invalid recon guration.

Table I M

 I

	Names of dependency	Version
	body-parser	1.19.0
	cors	2.8.5
	express	4.17.1
	express-h p-proxy	1.6.0
	express-session	1.17.0
	keycloak	1.2.0
	keycloak-connect	9.0.3
	keycloak-js	9.0.3
	request	2.88.2
	videojs-resolution-switcher	0.4.2
	xmlh prequest	1.8.0

  We prune this table, only showing one entry/test case per execution scenario.

						Table II
				PCA
	Execution scenario	err	cpu avg	cpu std	sent avg sent std	rcvd avg	rcvd std	mem avg	mem std	uptime
	baseline	0	6.216	2.291	8.173	3.308	7.998	3.233	126.923	6.191	13
	baseline, kcRestart	50	2.683	3.383	2.723	4.573	2.657	4.496	124.980	4.033	13
	baseline, wrongUri	0	7.194	2.259	9.377	3.472	9.182	3.400	130.037	5.355	13
	baseline, webOrigin	2152	4.111	2.123	3.405	3.443	3.330	3.383	128.050	5.683	13
	baseline, revokeToken	50	3.585	3.720	3.893	5.059	3.807	4.977	128.929	5.578	12
	Note: Table III
	T
	Execution scenario	Number of test cases
	baseline				30
	baseline, kcRestart			3
	baseline, UriRedirect			3
	baseline, webOrigin			3
	baseline, revokeToken			3

Table IV T

 IV

	Execution scenario	Description	Observation	Output category (section III-A)
	Baseline	Running normal exection	No impact, normal streaming delivery	Valid recon guration
	baseline, kcRestart	Restarting auth server	-Access token is expired at clients node -Application crashes, re-login is required	Invalid recon guration
			-Normal streaming delivery
	baseline, UriRedirect	Misredirecting URI from auth server to video server	-e clients retrieve the video information	Valid recon guration
			from its cache
			-Response from valid URI redirection
	baseline, webOrigin	Removing webOrigin wildcard value	is not permi ed	Invalid recon guration
			-Failed CORS request
			-Access and refresh token are missing
	baseline, revokeToken	Revoking access and refresh token	-Authentication is no longer	Invalid recon guration
			valid for the clients

A presentation by Chris Munns at Amazon.com: I love APIs

2015:Microservices at Amazon. Reference: h ps://www.slideshare.net/apigee/ilove-apis-2015-microservices-at-amazon-54487258.[START_REF] Wang | Research on dynamic recon guration technology of cloud computing virtual services[END_REF] An incident happened in 06/11/2017 for 1 hour 50 minutes. Reference: h p://status.cloud.google.com/incident/appengine/17007.

NumPy library. Reference: h ps://numpy.org/

Chi-Square distribution table. Reference: h ps://store.fmi.uniso a.bg/fmi/statist/education/Virtual Labs/tables/tables3.html