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Consistency model

In computer science, a consistency model specifies a contract between the programmer and a system, wherein the system guarantees that if the programmer follows the rules for operations on memory, memory will be consistent and the results of reading, writing, or updating memory will be predictable. Consistency models are used in distributed systems like distributed shared memory systems or distributed data stores (such as filesystems, databases, optimistic replication systems or web caching). Consistency is different from coherence, which occurs in systems that are cached or cache-less, and is consistency of data with respect to all processors. Coherence deals with maintaining a global order in which writes to a single location or single variable are seen by all processors. Consistency deals with the ordering of operations to multiple locations with respect to all processors.

High level languages, such as C++ and Java, maintain the consistency contract by translating memory operations into low-level operations in a way that preserves memory semantics, reordering some memory instructions, and encapsulating required synchronization with library calls such as.

Example
Assume that the following case occurs:
 * The row X is replicated on nodes M and N
 * The client A writes row X to node M
 * After a period of time t, client B reads row X from node N

The consistency model determines whether client B will definitely see the write performed by client A, will definitely not, or cannot depend on seeing the write.

Types
Consistency models define rules for the apparent order and visibility of updates, and are on a continuum with tradeoffs. There are two methods to define and categorize consistency models; issue and view.
 * Issue: Issue method describes the restrictions that define how a process can issue operations.
 * View: View method which defines the order of operations visible to processes.

For example, a consistency model can define that a process is not allowed to issue an operation until all previously issued operations are completed. Different consistency models enforce different conditions. One consistency model can be considered stronger than another if it requires all conditions of that model and more. In other words, a model with fewer constraints is considered a weaker consistency model.

These models define how the hardware needs to be laid out and at a high-level, how the programmer must code. The chosen model also affects how the compiler can re-order instructions. Generally, if control dependencies between instructions and if writes to same location are ordered, then the compiler can reorder as required. However, with the models described below, some may allow writes before loads to be reordered while some may not.

Strict consistency
Strict consistency is the strongest consistency model. Under this model, a write to a variable by any processor needs to be seen instantaneously by all processors.

The strict model diagram and non-strict model diagrams describe the time constraint – instantaneous. It can be better understood as though a global clock is present in which every write should be reflected in all processor caches by the end of that clock period. The next operation must happen only in the next clock period.

This is the most rigid model. In this model, the programmer's expected result will be received every time. It is deterministic. Its practical relevance is restricted to a thought experiment and formalism, because instantaneous message exchange is impossible. It doesn't help in answering the question of conflict resolution in concurrent writes to the same data item, because it assumes concurrent writes to be impossible.

Sequential consistency
The sequential consistency model was proposed by Lamport(1979). It is a weaker memory model than strict consistency model. A write to a variable does not have to be seen instantaneously, however, writes to variables by different processors have to be seen in the same order by all processors. Sequential consistency is met if "the result of any execution is the same as if the (read and write) operations of all processes on the data store were executed in some sequential order, and the operations of each individual processor appear in this sequence in the order specified by its program." Adve and Gharachorloo, 1996 define two requirements to implement the sequential consistency; program order and write atomicity.
 * Program order: Program order guarantees that each process issues a memory request ordered by its program.
 * Write atomicity: Write atomicity defines that memory requests are serviced based on the order of a single FIFO queue.

In sequential consistency, there is no notion of time or most recent write operations. There are some operations interleaving that is the same for all processes. A process can see the write operations of all processes but it can just see its own read operations. Program order within each processor and sequential ordering of operations between processors should be maintained. In order to preserve sequential order of execution between processors, all operations must appear to execute instantaneously or atomically with respect to every other processor.

These operations need only "appear" to be completed because it is physically impossible to send information instantaneously. For instance, in a system utilizing a single globally shared bus, once a bus line is posted with information, it is guaranteed that all processors will see the information at the same instant. Thus, passing the information to the bus line completes the execution with respect to all processors and has appeared to have been executed. Cache-less architectures or cached architectures with interconnect networks that are not instantaneous can contain a slow path between processors and memories. These slow paths can result in sequential inconsistency, because some memories receive the broadcast data faster than others.

Sequential consistency can produce non-deterministic results. This is because the sequence of sequential operations between processors can be different during different runs of the program. All memory operations need to happen in the program order.

Linearizability (also known as atomic consistency or atomic memory) can be defined as sequential consistency with a real-time constraint, by considering a begin time and end time for each operation. An execution is linearizable if each operation taking place in linearizable order by placing a point between its begin time and its end time and guarantees sequential consistency.

Verifying sequential consistency through model checking is undecidable in general, even for finite-state cache coherence protocols.

Causal consistency
Causal consistency defined by Hutto and Ahamad, 1990, is a weakening of the sequential consistency model by categorizing events into those causally related and those that are not. It defines that only write operations that are causally related, need to be seen in the same order by all processes. For example, if an event b takes effect from an earlier event a, the causal consistency guarantees that all processes see event b after event a. Tanenbaum et al., 2007 provide a stricter definition, that a data store is considered causally consistent under the following conditions:

 * Writes that are potentially causally related must be seen by all processes in the same order.
 * Concurrent writes may be seen in a different order on different machines.

This model relaxes sequential consistency on concurrent writes by a processor and on writes that are not causally related. Two writes can become causally related if one write to a variable is dependent on a previous write to any variable if the processor doing the second write has just read the first write. The two writes could have been done by the same processor or by different processors.

As in sequential consistency, reads do not need to reflect changes instantaneously, however, they need to reflect all changes to a variable sequentially.

W(x)2 happens after W(x)1 due to the read made by P2 to x before W(x)2, hence this example is causally consistent under Hutto and Ahamad's definition (although not under Tanenbaum et al.'s, because W(x)2 and W(x)3 are not seen in the same order for all processes). However R(x)2 and R(x)3 happen in a different order on P3 and P4, hence this example is sequentially inconsistent.

Processor consistency
In order for consistency in data to be maintained and to attain scalable processor systems where every processor has its own memory, the processor consistency model was derived. All processors need to be consistent in the order in which they see writes done by one processor and in the way they see writes by different processors to the same location (coherence is maintained). However, they do not need to be consistent when the writes are by different processors to different locations.

Every write operation can be divided into several sub-writes to all memories. A read from one such memory can happen before the write to this memory completes. Therefore, the data read can be stale. Thus, a processor under PC can execute a younger load when an older store needs to be stalled. Read before write, read after read and write before write ordering is still preserved in this model.

The processor consistency model is similar to the PRAM consistency model with a stronger condition that defines all writes to the same memory location must be seen in the same sequential order by all other processes. Processor consistency is weaker than sequential consistency but stronger than the PRAM consistency model.

The Stanford DASH multiprocessor system implements a variation of processor consistency which is incomparable (neither weaker nor stronger) to Goodman's definitions. All processors need to be consistent in the order in which they see writes by one processor and in the way they see writes by different processors to the same location. However, they do not need to be consistent when the writes are by different processors to different locations.

Pipelined RAM consistency, or FIFO consistency
Pipelined RAM consistency (PRAM consistency) was presented by Lipton and Sandberg in 1988 as one of the first described consistency models. Due to its informal definition, there are in fact at least two subtly different implementations, one by Ahamad et al. and one by Mosberger.

In PRAM consistency, all processes view the operations of a single process in the same order that they were issued by that process, while operations issued by different processes can be viewed in different order from different processes. PRAM consistency is weaker than processor consistency. PRAM relaxes the need to maintain coherence to a location across all its processors. Here, reads to any variable can be executed before writes in a processor. Read before write, read after read and write before write ordering is still preserved in this model.

Cache consistency
Cache consistency requires that all write operations to the same memory location are performed in some sequential order. Cache consistency is weaker than processor consistency and incomparable with PRAM consistency.

Slow consistency

In slow consistency, if a process reads a value previously written to a memory location, it cannot subsequently read any earlier value from that location. Writes performed by a process are immediately visible to that process. Slow consistency is a weaker model than PRAM and cache consistency.

Example: Slow memory diagram depicts a slow consistency example. The first process writes 1 to the memory location X and then it writes 1 to the memory location Y. The second process reads 1 from Y and it then reads 0 from X even though X was written before Y.

Hutto, Phillip W., and Mustaque Ahamad (1990) illustrate that by appropriate programming, slow memory (consistency) can be expressive and efficient. They mention that slow memory has two valuable properties; locality and supporting reduction from atomic memory. They propose two algorithms to present the expressiveness of slow memory.

Session guarantees
These 4 consistency models were proposed in a 1994 paper. They focus on guarantees in the situation where only a single user or application is making data modifications.

Monotonic read consistency
Tanenbaum et al., 2007 defines monotonic read consistency as follows:

"If a process reads the value of a data item x, any successive read operation on x by that process will always return that same value or a more recent value."

Monotonic read consistency guarantees that after a process reads a value of data item x at time t, it will never see the older value of that data item.

Monotonic write consistency
Monotonic write consistency condition is defined by Tanenbaum et al., 2007 as follows:

"A write operation by a process on a data item X is completed before any successive write operation on X by the same process."

Read-your-writes consistency
A value written by a process on a data item X will be always available to a successive read operation performed by the same process on data item X.

Writes-follows-reads consistency
In writes-follow-reads consistency, updates are propagated after performing the previous read operations. Tanenbaum et al., 2007 defines the following condition for writes-follow-reads consistency:

"A write operation by a process on a data item x following a previous read operation on x by the same process is guaranteed to take place on the same or a more recent value of x that was read."

Weak memory consistency models
The following models require specific synchronization by programmers.

Weak ordering
Weak ordering classifies memory operations into two categories: data operations and synchronization operations. To enforce program order, a programmer needs to find at least one synchronisation operation in a program. Synchronization operations signal the processor to make sure it has completed and seen all previous operations done by all processors. Program order and atomicity is maintained only on synchronisation operations and not on all reads and writes. This was derived from the understanding that certain memory operations – such as those conducted in a critical section - need not be seen by all processors until after all operations in the critical section are completed. It assumes reordering memory operations to data regions between synchronisation operations does not affect the outcome of the program. This exploits the fact that programs written to be executed on a multi-processor system contain the required synchronization to make sure that data races do not occur and SC outcomes are produced always.

Coherence is not relaxed in this model. Once these requirements are met, all other "data" operations can be reordered. The way this works is that a counter tracks the number of data operations and until this counter becomes zero, the synchronisation operation isn't issued. Furthermore, no more data operations are issued unless all the previous synchronisations are completed. Memory operations in between two synchronisation variables can be overlapped and reordered without affecting the correctness of the program. This model ensures that write atomicity is always maintained, therefore no additional safety net is required for weak ordering.

In order to maintain weak ordering, write operations prior to a synchronization operation must be globally performed before the synchronization operation. Operations present after a synchronization operation should also be performed only after the synchronization operation completes. Therefore, accesses to synchronization variables is sequentially consistent and any read or write should be performed only after previous synchronization operations have completed.

There is high reliance on explicit synchronization in the program. For weak ordering models, the programmer must use atomic locking instructions such as test-and-set, fetch-and-op, store conditional, load linked or must label synchronization variables or use fences.

Release consistency
The release consistency model relaxes the weak consistency model by distinguishing the entrance synchronization operation from the exit synchronization operation. Under weak ordering, when a synchronization operation is to be seen, all operations in all processors need to be visible before the synchronization operation is done and the processor proceeds. However, under the release consistency model, during the entry to a critical section, termed as "acquire", all operations with respect to the local memory variables need to be completed. During the exit, termed as "release", all changes made by the local processor should be propagated to all other processors. Coherence is still maintained.

The acquire operation is a load/read that is performed to access the critical section. A release operation is a store/write performed to allow other processors to use the shared variables.

Among synchronization variables, sequential consistency or processor consistency can be maintained. Using SC, all competing synchronization variables should be processed in order. However, with PC, a pair of competing variables need to only follow this order. Younger acquires can be allowed to happen before older releases.

RCsc and RCpc
There are two types of release consistency, release consistency with sequential consistency (RCsc) and release consistency with processor consistency (RCpc). The latter type denotes which type of consistency is applied to those operations nominated below as special.

There are special (cf. ordinary) memory operations, themselves consisting of two classes of operations: sync or nsync operations. The latter are operations not used for synchronisation; the former are, and consist of acquire and release operations. An acquire is effectively a read memory operation used to obtain access to a certain set of shared locations. Release, on the other hand, is a write operation that is performed for granting permission to access the shared locations.

For sequential consistency (RCsc), the constraints are:
 * acquire → all,
 * all → release,
 * special → special.

For processor consistency (RCpc) the write to read program order is relaxed, having constraints:
 * acquire → all,
 * all → release,
 * special → special (except when special write is followed by special read).

Note: the above notation A → B, implies that if the operation A precedes B in the program order, then program order is enforced.

Entry consistency
This is a variant of the release consistency model. It also requires the use of acquire and release instructions to explicitly state an entry or exit to a critical section. However, under entry consistency, every shared variable is assigned a synchronization variable specific to it. This way, only when the acquire is to variable x, all operations related to x need to be completed with respect to that processor. This allows concurrent operations of different critical sections of different shared variables to occur. Concurrency cannot be seen for critical operations on the same shared variable. Such a consistency model will be useful when different matrix elements can be processed at the same time.

Local consistency
In local consistency, each process performs its own operations in the order defined by its program. There is no constraint on the ordering in which the write operations of other processes appear to be performed. Local consistency is the weakest consistency model in shared memory systems.

General consistency
In general consistency, all the copies of a memory location are eventually identical after all processes' writes are completed.

Eventual consistency
An eventual consistency is a weak consistency model in a system with the lack of simultaneous updates. It defines that if no update takes a very long time, all replicas eventually become consistent.

Most shared decentralized databases have an eventual consistency model, either BASE: basically available; soft state; eventually consistent, or a combination of ACID and BASE sometimes called SALT: sequential; agreed; ledgered; tamper-resistant, and also symmetric; admin-free; ledgered; and time-consensual.

Relaxed memory consistency models
Some different consistency models can be defined by relaxing one or more requirements in sequential consistency called relaxed consistency models. These consistency models do not provide memory consistency at the hardware level. In fact, the programmers are responsible for implementing the memory consistency by applying synchronization techniques. The above models are classified based on four criteria and are detailed further.

There are four comparisons to define the relaxed consistency:
 * Relaxation: One way to categorize the relaxed consistency is to define which sequential consistency requirements are relaxed. We can have less strict models by relaxing either program order or write atomicity requirements defined by Adve and Gharachorloo, 1996. Program order guarantees that each process issues a memory request ordered by its program and write atomicity defines that memory requests are serviced based on the order of a single FIFO queue. In relaxing program order, any or all the ordering of operation pairs, write-after-write, read-after-write, or read/write-after-read, can be relaxed. In the relaxed write atomicity model, a process can view its own writes before any other processors.
 * Synchronizing vs. non-synchronizing: A synchronizing model can be defined by dividing the memory accesses into two groups and assigning different consistency restrictions to each group considering that one group can have a weak consistency model while the other one needs a more restrictive consistency model. In contrast, a non-synchronizing model assigns the same consistency model to the memory access types.
 * Issue vs. view-based: Issue method provides sequential consistency simulation by defining the restrictions for processes to issue memory operations. Whereas, view method describes the visibility restrictions on the events order for processes.
 * Relative model strength: Some consistency models are more restrictive than others. In other words, strict consistency models enforce more constraints as consistency requirements. The strength of a model can be defined by the program order or atomicity relaxations and the strength of models can also be compared. Some models are directly related if they apply the same relaxations or more. On the other hand, the models that relax different requirements are not directly related.

Sequential consistency has two requirements, program order and write atomicity. Different relaxed consistency models can be obtained by relaxing these requirements. This is done so that, along with relaxed constraints, the performance increases, but the programmer is responsible for implementing the memory consistency by applying synchronisation techniques and must have a good understanding of the hardware.

Potential relaxations:
 * Write to read program order
 * Write to write program order
 * Read to read and read to write program orders

Relaxed write to read
An approach to improving the performance at the hardware level is by relaxing the PO of a write followed by a read which effectively hides the latency of write operations. The optimisation this type of relaxation relies on is that it allows the subsequent reads to be in a relaxed order with respect to the previous writes from the processor. Because of this relaxation some programs like XXX may fail to give SC results because of this relaxation. Whereas, programs like YYY are still expected to give consistent results because of the enforcement of the remaining program order constraints.

Three models fall under this category. The IBM 370 model is the strictest model. A read can be complete before an earlier write to a different address, but it is prohibited from returning the value of the write unless all the processors have seen the write. The SPARC V8 total store ordering model (TSO) model partially relaxes the IBM 370 Model, it allows a read to return the value of its own processor's write with respect to other writes to the same location i.e. it returns the value of its own write before others see it. Similar to the previous model, this cannot return the value of write unless all the processors have seen the write. The processor consistency model (PC) is the most relaxed of the three models and relaxes both the constraints such that a read can complete before an earlier write even before it is made visible to other processors.

In Example A, the result is possible only in IBM 370 because read(A) is not issued until the write(A) in that processor is completed. On the other hand, this result is possible in TSO and PC because they allow the reads of the flags before the writes of the flags in a single processor.

In Example B the result is possible only with PC as it allows P2 to return the value of a write even before it is visible to P3. This won't be possible in the other two models.

To ensure sequential consistency in the above models, safety nets or fences are used to manually enforce the constraint. The IBM370 model has some specialised serialisation instructions which are manually placed between operations. These instructions can consist of memory instructions or non-memory instructions such as branches. On the other hand, the TSO and PC models do not provide safety nets, but the programmers can still use read-modify-write operations to make it appear like the program order is still maintained between a write and a following read. In case of TSO, PO appears to be maintained if the R or W which is already a part of a R-modify-W is replaced by a R-modify-W, this requires the W in the R-modify-W is a ‘dummy’ that returns the read value. Similarly for PC, PO seems to be maintained if the read is replaced by a write or is already a part of R-modify-W.

However, compiler optimisations cannot be done after exercising this relaxation alone. Compiler optimisations require the full flexibility of reordering any two operations in the PO, so the ability to reorder a write with respect to a read is not sufficiently helpful in this case.

Relaxed write to read and write to write
Some models relax the program order even further by relaxing even the ordering constraints between writes to different locations. The SPARC V8 partial store ordering model (PSO) is the only example of such a model. The ability to pipeline and overlap writes to different locations from the same processor is the key hardware optimisation enabled by PSO. PSO is similar to TSO in terms of atomicity requirements, in that it allows a processor to read the value of its own write and prevents other processors from reading another processor's write before the write is visible to all other processors. Program order between two writes is maintained by PSO using an explicit STBAR instruction. The STBAR is inserted in a write buffer in implementations with FIFO write buffers. A counter is used to determine when all the writes before the STBAR instruction have been completed, which triggers a write to the memory system to increment the counter. A write acknowledgement decrements the counter, and when the counter becomes 0, it signifies that all the previous writes are completed.

In the examples A and B, PSO allows both these non-sequentially consistent results. The safety net that PSO provides is similar to TSO's, it imposes program order from a write to a read and enforces write atomicity.

Similar to the previous models, the relaxations allowed by PSO are not sufficiently flexible to be useful for compiler optimisation, which requires a much more flexible optimisation.

Relaxing read and read to write program orders: Alpha, RMO, and PowerPC
In some models, all operations to different locations are relaxed. A read or write may be reordered with respect to a different read or write in a different location. The weak ordering may be classified under this category and two types of release consistency models (RCsc and RCpc) also come under this model. Three commercial architectures are also proposed under this category of relaxation: the Digital Alpha, SPARC V9 relaxed memory order (RMO), and IBM PowerPC models.

These three commercial architectures exhibit explicit fence instructions as their safety nets. The Alpha model provides two types of fence instructions, memory barrier (MB) and write memory barrier (WMB). The MB operation can be used to maintain program order of any memory operation before the MB with a memory operation after the barrier. Similarly, the WMB maintains program order only among writes. The SPARC V9 RMO model provides a MEMBAR instruction which can be customised to order previous reads and writes with respect to future read and write operations. There is no need for using read-modify-writes to achieve this order because the MEMBAR instruction can be used to order a write with respect to a succeeding read. The PowerPC model uses a single fence instruction called the SYNC instruction. It is similar to the MB instruction, but with a little exception that reads can occur out of program order even if a SYNC is placed between two reads to the same location. This model also differs from Alpha and RMO in terms of atomicity. It allows a write to be seen earlier than a read's completion. A combination of read modify write operations may be required to make an illusion of write atomicity.

RMO and PowerPC allow reordering of reads to the same location. These models violate sequential order in examples A and B. An additional relaxation allowed in these models is that memory operations following a read operation can be overlapped and reordered with respect to the read. Alpha and RMO allow a read to return the value of another processor's early write. From a programmer's perspective these models must maintain the illusion of write atomicity even though they allow the processor to read its own write early.

Transactional memory models
Transactional memory model is the combination of cache coherency and memory consistency models as a communication model for shared memory systems supported by software or hardware; a transactional memory model provides both memory consistency and cache coherency. A transaction is a sequence of operations executed by a process that transforms data from one consistent state to another. A transaction either commits when there is no conflict or aborts. In commits, all changes are visible to all other processes when a transaction is completed, while aborts discard all changes. Compared to relaxed consistency models, a transactional model is easier to use and can provide higher performance than a sequential consistency model.

Other consistency models
Some other consistency models are as follows:
 * Causal+ consistency
 * Cross-Client Monotonicity
 * Delta consistency
 * Fork consistency
 * One-copy serializability
 * Serializability
 * Vector-field consistency
 * Weak consistency
 * Strong consistency

Several other consistency models have been conceived to express restrictions with respect to ordering or visibility of operations, or to deal with specific fault assumptions.

Consistency and replication
Tanenbaum et al., 2007 defines two main reasons for replicating; reliability and performance. Reliability can be achieved in a replicated file system by switching to another replica in the case of the current replica failure. The replication also protects data from being corrupted by providing multiple copies of data on different replicas. It also improves the performance by dividing the work. While replication can improve performance and reliability, it can cause consistency problems between multiple copies of data. The multiple copies are consistent if a read operation returns the same value from all copies and a write operation as a single atomic operation (transaction) updates all copies before any other operation takes place. Tanenbaum, Andrew, & Maarten Van Steen, 2007 refer to this type of consistency as tight consistency provided by synchronous replication. However, applying global synchronizations to keep all copies consistent is costly. One way to decrease the cost of global synchronization and improve the performance can be weakening the consistency restrictions.

Data-centric consistency models
Tanenbaum et al., 2007 defines the consistency model as a contract between the software (processes) and memory implementation (data store). This model guarantees that if the software follows certain rules, the memory works correctly. Since, in a system without a global clock, defining the last operation among writes is difficult, some restrictions can be applied on the values that can be returned by a read operation. The goal of data-centric consistency models is to provide a consistent view on a data store where processes may carry out concurrent updates.

Consistent ordering of operations
Some consistency models such as sequential and also causal consistency models deal with the order of operations on shared replicated data in order to provide consistency. In these models, all replicas must agree on a consistent global ordering of updates.

Grouping operations
In grouping operation, accesses to the synchronization variables are sequentially consistent. A process is allowed to access a synchronization variable that all previous writes have been completed. In other words, accesses to synchronization variables are not permitted until all operations on the synchronization variables are completely performed.

Client-centric consistency models
In distributed systems, maintaining sequential consistency in order to control the concurrent operations is essential. In some special data stores without simultaneous updates, client-centric consistency models can deal with inconsistencies in a less costly way. The following models are some client-centric consistency models:

Consistency protocols
The implementation of a consistency model is defined by a consistency protocol. Tanenbaum et al., 2007 illustrates some consistency protocols for data-centric models.

Continuous consistency
Continuous consistency introduced by Yu and Vahdat (2000). In this model, the consistency semantics of an application is described by using conits in the application. Since the consistency requirements can differ based on application semantics, Yu and Vahdat (2000) believe that a predefined uniform consistency model may not be an appropriate approach. The application should specify the consistency requirements that satisfy the application semantics. In this model, an application specifies each consistency requirement as a conit (abbreviation of consistency units). A conit can be a physical or logical consistency and is used to measure the consistency. Tanenbaum et al., 2007 describes the notion of a conit by giving an example.

There are three inconsistencies that can be tolerated by applications.

 * Deviation in numerical values: Numerical deviation bounds the difference between the conit value and the relative value of the last update. A weight can be assigned to the writes which defines the importance of the writes in a specific application. The total weights of unseen writes for a conit can be defined as a numerical deviation in an application. There are two different types of numerical deviation; absolute and relative numerical deviation.
 * Deviation in ordering: Ordering deviation is the discrepancy between the local order of writes in a replica and their relative ordering in the eventual final image.
 * Deviation in staleness between replicas: Staleness deviation defines the validity of the oldest write by bounding the difference between the current time and the time of the oldest write on a conit not seen locally. Each server has a local queue of uncertain write that is required an actual order to be determined and applied on a conit. The maximal length of uncertain writes queue is the bound of ordering deviation. When the number of writes exceeds the limit, instead of accepting new submitted write, the server will attempt to commit uncertain writes by communicating with other servers based on the order that writes should be executed.

If all three deviation bounds are set to zero, the continuous consistency model is the strong consistency.

Primary-based protocols

Primary-based protocols can be considered as a class of consistency protocols that are simpler to implement. For instance, sequential ordering is a popular consistency model when consistent ordering of operations is considered. The sequential ordering can be determined as primary-based protocol. In these protocols, there is an associated primary for each data item in a data store to coordinate write operations on that data item.

Remote-write protocols
In the simplest primary-based protocol that supports replication, also known as primary-backup protocol, write operations are forwarded to a single server and read operations can be performed locally.
 * Example: Tanenbaum et al., 2007 gives an example of a primary-backup protocol. The diagram of primary-backup protocol shows an example of this protocol. When a client requests a write, the write request is forwarded to a primary server. The primary server sends request to backups to perform the update. The server then receives the update acknowledgement from all backups and sends the acknowledgement of completion of writes to the client. Any client can read the last available update locally. The trade-off of this protocol is that a client who sends the update request might have to wait so long to get the acknowledgement in order to continue. This problem can be solved by performing the updates locally, and then asking other backups to perform their updates. The non-blocking primary-backup protocol does not guarantee the consistency of update on all backup servers. However, it improves the performance. In the primary-backup protocol, all processes will see the same order of write operations since this protocol orders all incoming writes based on a globally unique time. Blocking protocols guarantee that processes view the result of the last write operation.

Local-write protocols
In primary-based local-write protocols, primary copy moves between processes willing to perform an update. To update a data item, a process first moves it to its location. As a result, in this approach, successive write operations can be performed locally while each process can read their local copy of data items. After the primary finishes its update, the update is forwarded to other replicas and all perform the update locally. This non-blocking approach can lead to an improvement. The diagram of the local-write protocol depicts the local-write approach in primary-based protocols. A process requests a write operation in a data item x. The current server is considered as the new primary for a data item x. The write operation is performed and when the request is finished, the primary sends an update request to other backup servers. Each backup sends an acknowledgment to the primary after finishing the update operation.

Replicated-write protocols
In replicated-write protocols, unlike the primary-based protocol, all updates are carried out to all replicas.

Active replication
In active replication, there is a process associated with each replica to perform the write operation. In other words, updates are sent to each replica in the form of an operation in order to be executed. All updates need to be performed in the same order in all replicas. As a result, a totally-ordered multicast mechanism is required. There is a scalability issue in implementing such a multicasting mechanism in large distributed systems. There is another approach in which each operation is sent to a central coordinator (sequencer). The coordinator first assigns a sequence number to each operation and then forwards the operation to all replicas. Second approach cannot also solve the scalability problem.

Quorum-based protocols
Voting can be another approach in replicated-write protocols. In this approach, a client requests and receives permission from multiple servers in order to read and write a replicated data. As an example, suppose in a distributed file system, a file is replicated on N servers. To update a file, a client must send a request to at least N/2 + 1 in order to make their agreement to perform an update. After the agreement, changes are applied on the file and a new version number is assigned to the updated file. Similarly, for reading replicated file, a client sends a request to N/2 + 1 servers in order to receive the associated version number from those servers. Read operation is completed if all received version numbers are the most recent version.

Cache-coherence protocols
In a replicated file system, a cache-coherence protocol provides the cache consistency while caches are generally controlled by clients. In many approaches, cache consistency is provided by the underlying hardware. Some other approaches in middleware-based distributed systems apply software-based solutions to provide the cache consistency.

Cache consistency models can differ in their coherence detection strategies that define when inconsistencies occur. There are two approaches to detect the inconsistency; static and dynamic solutions. In the static solution, a compiler determines which variables can cause the cache inconsistency. So, the compiler enforces an instruction in order to avoid the inconsistency problem. In the dynamic solution, the server checks for inconsistencies at run time to control the consistency of the cached data that has changed after it was cached.

The coherence enforcement strategy is another cache-coherence protocol. It defines how to provide the consistency in caches by using the copies located on the server. One way to keep the data consistent is to never cache the shared data. A server can keep the data and apply some consistency protocol such as primary-based protocols to ensure the consistency of shared data. In this solution, only private data can be cached by clients. In the case that shared data is cached, there are two approaches in order to enforce the cache coherence.

In the first approach, when a shared data is updated, the server forwards invalidation to all caches. In the second approach, an update is propagated. Most caching systems apply these two approaches or dynamically choose between them.

title: Consistency model


text:
Salisbury Plain

Salisbury Plain is a chalk plateau in southern England covering 300 sqmi. It is part of a system of chalk downlands throughout eastern and southern England formed by the rocks of the Chalk Group and largely lies within the county of Wiltshire, but stretches into Hampshire.

The plain is famous for its rich archaeology, including Stonehenge, one of England's best known landmarks. Large areas are given over to military training; thus, the sparsely populated plain is the biggest remaining area of calcareous grassland in northwest Europe. Additionally, the plain has arable land, and a few small areas of beech trees and coniferous woodland. Its highest point is Easton Hill.

A large amount of land is set aside for military use as Salisbury Plain Training Area.

Physical geography
The boundaries of Salisbury Plain have never been truly defined, and there is some difference of opinion as to its exact area. The river valleys surrounding it, and other downs and plains beyond them loosely define its boundaries. To the north the scarp of the downs overlooks the Vale of Pewsey, and to the northwest the Bristol Avon. The River Wylye runs along the southwest, and the Bourne runs to the east.

The Hampshire Avon runs through the eastern half of the plain, and to the south the plain peters out as the river valleys close together before meeting at Salisbury. From here the Avon continues south to the English Channel at Christchurch. The Hampshire Downs and the Berkshire Downs are chalk downland to the east and north of Salisbury Plain, and the Dorset Downs and Cranborne Chase are to the southwest. In the west and north west the geology is mainly the clays and limestones of the Blackmore Vale, Avon Vale and Vale of Wardour. The Mendip Hills rise to the west of Salisbury Plain, and the Cotswolds to the north west.

Amesbury is considered the largest settlement on the plain (southern fringes), though there are a number of small villages, such as Tilshead, Chitterne and Shrewton in the middle of the plain, as well as various hamlets and army camps. The A303 road runs through the southern area of the plain, while the A345 and the A360 cut across the centre.

20,000 hectares are designated Sites of Special Scientific Interest and Special Areas of Conservation, and the entire Salisbury Plain is a Special Protection Area for birds.

History
Salisbury Plain is famous for its history and archaeology. In the Neolithic period, Stone Age people began to settle on the plain, most likely centred around the causewayed enclosure of Robin Hood's Ball. Large long barrows such as White Barrow and other earthworks were built across the plain. By 2500 BC areas around Durrington Walls and Stonehenge had become a focus for building, and the southern part of the plain continued to be settled into the Bronze Age.

Around 600 BC, Iron Age hillforts came to be constructed around the boundaries of the plain, including Scratchbury Camp and Battlesbury Camp to the southwest, Bratton Camp to the northwest, Casterley Camp to the north, Yarnbury and Vespasian's Camp to the south, and Sidbury Hill to the east.

Roman roads are visible features, probably serving a settlement near Old Sarum. Villas are sparse, however, and Anglo-Saxon place names suggest that the plain was mostly a grain-producing imperial estate.

In the 6th century, Anglo-Saxon incomers built planned settlements in the valleys surrounded by strip lynchets, with the downland left as sheep pasture. To the south is the city of Salisbury, whose medieval cathedral is famous for having the tallest spire in the country, and the building was, for many centuries, the tallest building in Britain. The cathedral is evidence of the prosperity the wool and cloth trade brought to the area. In the mid-19th century the wool and cloth industry began to decline, leading to a decline in the population and change in land use from sheep farming to agriculture and military use. Wiltshire became one of the poorest counties in England during this period of decline.

There are a number of chalk carvings on the plain, of which the most famous is the Westbury White Horse. The Kennet and Avon Canal was constructed to the north of the plain, through the Vale of Pewsey.

In September 1896, George Kemp and Guglielmo Marconi experimented with wireless telegraphy on Salisbury Plain, and achieved good results over a distance of 1.25 mi.

The British Army first conducted manoeuvres at what is now Salisbury Plain Training Area in 1898.

Ecology
Because of the large training areas inaccessible to the public, the plain is a wildlife haven, and home to two national nature reserves, but there is concern that the low level of grazing on the plain could allow scrub to encroach on the grassland. The plain supports the largest known expanse of unimproved chalk downland in north west Europe, and represents 41% of Britain's remaining area of this wildlife habitat. The plain supports 13 species of nationally rare and scarce plants, 67 species of rare and scarce invertebrates and forms a site of international importance for birds. In addition to chalk downland, the plain supports scrub and woodland habitats, temporary and permanent pools and the River Bourne.

Vegetation
A diversity of soil types, slope, aspect and past and present land-use has given rise to various grassland communities. Historical evidence suggests that large areas of grassland are of great antiquity, and areas which were cultivated at the beginning of the 20th century have experienced nearly 100 years of chalk grassland re-colonisation. Parts of East Salisbury Plain and the periphery of Central and West comprise areas of grassland currently managed for grazing pasture and hay-cutting, whilst the middle of Centre and West are ungrazed. A large proportion of Salisbury Plain supports upright brome (Bromus erectus) species-rich grassland, within which a continuous floristic variation is seen. A widespread type on the plain is characterised by an abundance of red fescue (Festuca rubra), crested hair-grass (Koeleria macrantha), salad burnet (Sanguisorba minor), lady's bedstraw (Galium verum), rough hawkbit (Leontodon hispidus), common rock-rose (Helianthemum nummularium) and dropwort (Filipendula vulgaris). The high constancy of this last species is a distinctive feature of the upright brome grasslands on Salisbury Plain and is otherwise only known from one other site in Hampshire. Where upright brome is less dominating, plants such as small scabiosa (Scabiosa columbaria), clustered bellflower (Campanula glomerata), dyer's greenweed (Genista tinctoria), kidney vetch (Anthyllis vulneraria), sainfoin (Onobrychis viciifolia) and horseshoe vetch (Hippocrepis comosa) are characteristic associates. The rare and notable plants which occur here include burnt-tip orchid (Neotinea ustulata), slender bedstraw (Galium pumilum), field fleawort (Senecio integrifolius) and the nationally scarce British endemic early gentian (Gentianella anglica). Particularly associated with long established turf on thin rendzina soils, and rabbit-grazed areas of the eastern and central ranges, are low-growing perennials including squinancy-wort (Asperula cynanchica), chalk milkwort (Polygala calcarea), dwarf thistle (Cirsium acaule), wild thyme (Thymus praecox), the nationally scarce bastard toadflax (Thesium humifusum) and purple milk-vetch (Astragalus danicus) in its most southerly British station.

Devil's-bit scabious (Succisa pratensis), saw-wort (Serratula tinctoria) and betony (Stachys officinalis) are all abundant and exemplify the oceanic character of the chalk grassland on the plain, a feature which is confined to South West England. Similarly restricted is a community in which dwarf sedge Carex humilis forms a conspicuous component. This type of grassland has its stronghold in Wiltshire and occurs on the less disturbed areas of the central ranges. Herb diversity is generally lower in the tall, upright brome-dominated swards, but wild parsnip (Pastinaca sativa), hogweed (Heracleum sphondylium) and greater knapweed (Centaurea scabiosa) are characteristic. Parasitic on the latter species is knapweed broomrape (Orobanche elatior), occurring in greater quantity on the plain than anywhere else in Britain. False-oat grass (Arrhenatherum elatius) grassland is also widespread, but is particularly a feature of the western ranges, often indicating areas of past cultivation.

On anthills, and in the more disturbed turf that is especially a feature of the impact area, mouse-ear hawkweed (Hieracium pilosella) is abundant together with sheep's fescue (Festuca ovina) and wild thyme. Annuals are also characteristic of this habitat, including common whitlowgrass (Erophila verna), rue-leaved saxifrage (Saxifraga tridactylites,) hairy rock-cress (Arabis hirsuta) and the nationally scarce dwarf mouse-ear (Cerastium pumilum) and fine-leaved sandwort (Minuartia hybrida). A very local community characterised by a lichen-rich turf (Cladonia species) and the broom moss (Dicranum scoparium) is found in some stabilised missile-impaction craters on the central ranges. This vegetation type is found elsewhere only on Porton Down SSSI and on the Brecklands.

Small areas of chalk-heath vegetation occur on superficial clay-with-flints deposits. Here chalk-loving plants such as salad burnet (Sanguisorba minor) and dropwort co-exist with plants typical of acid soils, including gorse (Ulex europaeus), heather (Calluna vulgaris) and the uncommon annual knawel (Scleranthus annuus). Two Red Data Book (RDB) plants occur on the plain. The largest population in Britain of tuberous thistle (Cirsium tuberosum) occurs on the western ranges and is notable for the low incidence of hybridisation with dwarf thistle, a contributory cause of its decline in other localities. Meadow clary (Salvia pratensis) persists as a small colony in tall upright brome grassland.

Salisbury Plain supports a diverse bryophyte flora with seven nationally scarce species which have seen a general decline in other chalk grassland sites, including Barbula acuta, Phascum curvicolle, Pleurochaete squarrosa, Thuidium abietinum and Weissia sterilis.

Although there is some scrub development on the plain, it is remarkable that large expanses of the chalk grassland remain open with very little invasion of woody species. Of particular interest are the large stands of juniper (Juniperus communis) on Bulford Downs and Beacon Hill. Both pyramidal and prostrate forms are present and this site, along with Porton Down SSSI to the south, supports the best remaining examples of the lowland type of juniper associated with chalk and mixed scrub in England.

Insects
The botanically and structurally diverse grasslands support a large range of rare and uncommon chalk downland invertebrates. Where abundance has been assessed strong populations of national and local importance are present, and the large area of habitat available to them is important in ensuring their survival.

Butterflies
The plain is an important stronghold for declining downland butterflies. A high concentration of colonies of three nationally scarce species, the Adonis blue (Polyommatus bellargus), Duke of Burgundy (Hamearis lucina), and the largest population of marsh fritillary (Euphydryas aurinia) on the chalk, occur. A colony of brown hairstreak (Thecla betulae) is present on East Salisbury Plain at one of its two Wiltshire localities. Strong populations of other downland species such as chalkhill blue (Polyommatus coridon) and dark green fritillary (Argynnis aglaja) are found, and of note here is the occurrence of grayling (Hipparchia semele), a butterfly rarely found away from the coast.

Moths
An outstanding assemblage of two rare (RDB), 36 nationally scarce and two regionally notable moths are present, most of which are either chalk grassland specialists or are partly dependent on chalk grassland. The RDB species scarce forester (Adscita globulariae) is present, and amongst many species of nationally scarce moths are the cistus forester (Adscita geryon), six-belted clearwing (Bembecia scopigera), oblique striped (Phibalapteryx virgata), pimpernel pug (Eupithecia pimpinellata), shaded pug (Eupithecia subumbrata) and narrow-bordered bee hawk moth (Hemaris tityus). Larvae of these moths feed on the chalk grassland plants which are widespread on the plain. Other nationally scarce moths such as orange-tailed clearwing (Synanthedon anthraciniformis) depend on the associated scrub habitats.

Bees
The bee fauna is particularly rich in species which depend on chalk grassland. One of only two British populations of the endangered (RDB) mining bee Melitta dimidiata is present on the plain, and two other RDB species which occur are Andrena hattorfiana and its nest parasite the cuckoo bee Nomada armata. This is a rare inland site for the nationally scarce brown-banded carder bee (Bombus humilis), and the only lowland English site for the broken-belted bumblebee (Bombus soroeensis).

Flies
The Diptera (flies) include four RDB species which depend on chalk grassland, the picture-wing flies Chaetorellia loricata, Urophora solstitialis and Terellia vectensis and the hover fly Volucella inflata.

Crustaceans
Recent observations have shown that Salisbury Plain is an important site for the RDB crustacean, the fairy shrimp Chirocephalus diaphanus which is dependent on temporary pools, a rare and declining habitat. On the plain this habitat requirement is met by numerous pools created by repeated tank movements along the earth tracks which cross the chalk grassland.

Others
Other nationally scarce invertebrates occur within the Orthoptera (grasshoppers and crickets), Heteroptera (bugs) and Coleoptera (beetles), the latter group including a RDB soldier beetle, Cantharis fusca.

Birds
The area as a whole is of national and international importance for breeding and wintering birds. It supports seven species listed on Annex 1 of the EC Directive on the Conservation of Wild Birds, populations of six species of Red Data bird and several species of candidate Red Data bird. Amongst the breeding birds three species are particularly noteworthy. Up to 20 pairs of stone-curlew representing 12% of the British population breed on the plain. The area accounts for approximately 20% of breeding records for quail in Britain each year, and numbers of breeding hobby are thought to exceed 1% of the British population on a regular basis. Other important breeding species include common buzzard, barn owl, long-eared owl, nightingale, stonechat, whinchat, wheatear, corn bunting and, on occasion, Montagu's harrier. The overall breeding assemblage is exceptionally diverse for a British dry grassland site. In winter the plain is an important area for foraging flocks of thrushes, finches and buntings. These, together with abundant small mammals are prey for wintering hen harrier, merlin and short-eared owl. Hen harriers occur in nationally significant numbers each winter, and the plain is an important winter roost for this species in southern England. In 2003 the great bustard was reintroduced into Britain on Salisbury Plain.

Snakes and amphibians
Other species of interest on Salisbury Plain include the great crested newt (Triturus cristatus). This newt occurs in dew ponds across the plain and in pools along the Bourne River, together with smooth newt (Triturus vulgaris), common frog (Rana temporia) and common toad (Bufo bufo). Barred grass snake (Natrix helvetica) are also often seen near pools, and common lizard (Lacerta vivipara), slowworm (Anguis fragilis) and adder (Vipera berus) are present.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Salisbury Plain


text:
Franco-Flemish School

The designation Franco-Flemish School, also called Netherlandish School, Burgundian School, Low Countries School, Flemish School, Dutch School, or Northern School, refers to the style of polyphonic vocal music composition originating from France and from the Burgundian Netherlands in the 15th and 16th centuries as well as to the composers who wrote it. The spread of their technique, especially after the revolutionary development of printing, produced the first true international style since the unification of Gregorian chant in the 9th century. Franco-Flemish composers mainly wrote sacred music, primarily masses, motets, and hymns.

Term and controversy
Several generations of Renaissance composers from the region loosely known as the Low Countries (Imperial and French fiefs ruled in personal union by the House of Valois-Burgundy in the period from 1384 to 1482)—i.e. present-day Northern France, Belgium and the Southern Netherlands—are grouped under "Franco-Flemish School", though a teacher-student-relationship between them rarely existed. Most of these musicians were born in the thriving Burgundian provinces of Artois, Flanders, Brabant, Hainaut, or Limburg. Others were born in Northern and Southern France, like Guillaume Faugues, Simone de Bonefont and Antoine Brumel who was one of the most influential composers of his generation. During periods of political and economic stability, the courts of the Burgundian dukes were a centre of cultural activity in Europe.

Franco-Flemish composers had their origins in ecclesiastical choir schools such as at the cathedrals and collegiate churches of Saint-Quentin, Arras, Valenciennes, Douai, Bourges, Liège, Tournai, Cambrai, Mons, Antwerp, Bruges, and Ghent, although they were famous for working elsewhere. Numerous musicians established themselves in French court or moved to the European courts in Italy where they were called "I fiamminghi" or Oltremontani ("those from over the Alps") and Spain—notably in the Flemish chapel (capilla flamenca) of the Habsburgs, or to towns in Germany, and other parts of Europe—Poland, the Czech lands, Austria, Hungary, England, Sweden, Denmark, Saxony—carrying their styles with them. The exact centres shifted during this time, and by the end of the sixteenth century the focal point of the Western musical world had moved from the Low Countries to Italy.

To conclude, let us recall that the expression "Franco-Flemish" and the more biased one of "Dutch school" are still controversial among musicologists. They were not in use at that time and seem to cover only part of the linguistic, political, territorial and historical reality.

Development
Following are five groups, or generations, that are sometimes distinguished in the Franco-Flemish/Netherlandish school. Development of this musical style was continuous, and these generations only provide useful reference points.

 * The First generation (1420–1450), dominated by Jean Tapissier, Guillaume Du Fay, Gilles Binchois and Antoine Busnois; this group of composers is most often known as the Burgundian School. The origins of the style of the first generation embraces both earlier Burgundian traditions and also Italian and English styles. For example, in 1442, the poet Martin le Franc praised Binchois and Dufay for following Dunstaple in adopting the contenance angloise ("English character").
 * The Second generation (1450–1485), with Ockeghem as its main exponent, others including Orto, Compère, Prioris, Agricola, Caron, Faugues, Regis and Tinctoris.
 * The Third generation (1480–1520): Jean Mouton, Obrecht, de la Rue, Isaac, Brumel, Févin, Pipelare, Richafort, Divitis, and most significantly Josquin des Prez.
 * The Fourth generation (1520–1560): Gombert, Phinot, Crecquillon, Manchicourt, Arcadelt, Rore, Willaert, Courtois, Clemens non Papa and Bonefont.
 * The Fifth generation (1560–1615/20): Lassus, de Monte, Vaet, Regnart, Luython, Wert, de Macque, Claude Goudimel and Rogier. By this time, many of the composers of polyphonic music were native to Italy and other countries:  the Netherlandish style had naturalized on foreign soil, and become a true European style.

The Franco-Flemish motet
Composed between 1450 and 1520, these motets were typically written for four voices, with all voices being equal. They often exhibit thick, dark textures, with an extended low range. The most notable composers of this style include Ockeghem and Josquin, whose De profundis clamavi ad te, composed between 1500 and 1521, provides a good example.

title: Franco-Flemish School


text:
Talk:Free jazz

Coltrane
I would want to check my memory of the dates here, but I've got a strong feeling that there's something funny about putting "John Coltrane" in that list. He's well known as a free player -- and justly so, arguably he was the best, and has never been equaled despite many attempts -- but he wasn't really in the first wave of innovation like the other three names in that list (Cecil Taylor, Ornette Coleman, Albert Ayler). I'm pretty sure Coltrane was still doing Bop when they started stepping outside the lines. And in fact, Coltrane was strongly influenced by John Gilmore's playing (Gilmore was one of the main guys in the Sun Ra Arkestra). So if you do include Coltrane in the list, then why not Gilmore? But then things start getting unwieldly if the list gets that long, and it could be you're going to get still more people coming by adding their favorites.

Anyway, I'm contemplating a re-write of the introduction there... if anyone cares, let me know if you think this reasoning sounds stupid. -- Doom

Styles, names, labels, lofts
Sure, rewrite it a bit: I think the description of the music itself is pretty good (though it conflates too many different styles, I think: for instance, it's important to distinguish between Ornette Coleman's quite regularly pulsed music of the 1950s & 1960s, with drummers like Blackwell & Higgins, & the more "wavelike" rhythms invented by Sunny Murray with Cecil Taylor's band). But there's no historical depth here. Coltrane, yes, was a slightly older player who came to free jazz under the influence of younger musicians like Ayler & Coleman.

 * Yeah, I think there should be more names mentioned here, perhaps: Don Cherry, the many important drummers & bassists, Pharoah Sanders, Sun Ra, Paul Bley, Archie Shepp, Sam Rivers, for starters. Something on Tristano's early experiments with freedom, maybe.  Something on the Jimmy Giuffre Trio, too (as the main branch-line for a "cool", more abstract free-jazz, in its way just as influential).  & then maybe something on the two key 1960s labels--ESP & Impulse!.  & then there's the loft scene of the 1970s, leading into the current scene--surely players like Charles Gayle, William Parker, Mat Maneri, Matt Shipp, David S Ware, &c deserve a reference.  I like thick description, as long as the entry doesn't just become a flabby list. --Ndorward

Eric Dolphy
Finally got around to moving John Coltrane out of the list of "pioneers", calling him the best known example instead. Simple enough change, I don't know why it didn't occur to me before.

Since I brought up the possible problem of people padding out that list with their favorites, someone has come by and added Eric Dolphy's name... and unless I'm missing some chapters of the Eric Dolphy story, I suspect he should be regarded as a member of the second wave, rather than a pioneer.

It probably would be good to try and do a more detailed historical time-line... And I agree that it's best to provide descriptions of an artists music rather than just a list of names. For example, the New Wave music entry seems a little too light weight with it's long, alphabetic list of band names. -- Doom 19:14, Mar 10, 2004 (UTC)

 * I agree that Dolphy is not the best choice for a list of pioneers of free jazz, and probably shouldn't stand as a representative of the genre at all. He played over chord changes for almost all of his recorded career.  While he did aspire to play with Cecil Taylor, and participated in free pieces by Coleman and Coltrane, Mingus would never have hired him if he was known for playing "free."  So, while he's worthy of mention, maybe since the article is so short right now, leave him out.

Racial/social aspect
Would a section on the racial politics of free jazz be welcome? It seems right now that the entry is pretty well-established in the directions of theory and personnel, but is lacking any mention of what bassist Richard Davis calls the "kiss my black ass" factor. Many of the important early free jazz collectives were explicitly afro-centric (AACM, for example) and while Coleman's quartet was multi-racial, titles like "This Is Our Music" make certain things clear. The 60's were, of course, the decade of the most heightened racial tension in the US, and many free jazz musicians found that they were treated better, and their music more appreciated, in Europe. This could be a lead-in point to discuss the differences (musical and social/historical) between American and European free jazz (not to mention Japanese, etc). I'll start working on this, comment here if you object or wish to redirect the section.

 * Could it be worth putting more on the social-collective aspects into this section? I'm thinking here of Jazz Composers Guild, AACM and 70s Loft movt in the US? This collective creation of the settings for playing seem important in their correspondence with the collective creation of the music itself? And it is a further factor in the European music: FMP, Spontaneous Music Ensemble, London Musicians Collective, etc. --AllyD 19:07, 30 May 2005

 * Agreed: the racial thing is what led free jazz to be created... Has anyone read Carles and Comolli's "Free Jazz/Black Power"? The 'Philosophies' section could be much bigger... Ciacchi 00:23, 16 November 2007 (UTC)

 * I only partially agree, and I cite John Szwed's "Space is the Place" biography of Sun Ra where he quotes Ra explaining that the Arkestra was (nearly) exclusively for black musicians because white musicians were already playing this modern music; Sonny was reputed to have played Messaen and other modern piano works from memory, Coltrane spent hours working through Slonimsky's Thesaurus of Scales, and iirc Cecil Taylor has also mentioned the influence of post-Schoenberg European composers as a major influence in his music. Not surprising that in a modern era of global communications, Stravinsky and Ravel should be interested in Gershwin's Jazz and Coltrane and Braxton interested in Stockhausen and Messaen!

 * In a way the comments from the Jazz camp echo those comments from the Scott Joplin era where 'serious' musicians found themselves excluded from serious consideration if their skin was the wrong colour. While it is true these pioneers of Free Jazz were carrying the European modern tradition into the bop idiom and making it their own, arguably carrying that tradition on when Europeans themselves had largely stagnated with them, there is still an undeniable influence of modern twentieth century compositional theory (cf "Tristan Chord" vs Coltrane's quaternary harmonies)

 * Another 'racial' angle can be found on YouTube in a segment where George Russell talks to Ornette Coleman and gets agreement with his observation that their 'New Thing' is not new American technology, but is in fact a repackaging of old Third World technology.
 * Teledyn (talk) 21:36, 8 January 2010 (UTC)

Ornette & "Free Jazz"
Didn't I read that the album "Free Jazz" was given its title by someone at Atlantic without Ornette knowing about it, and that Ornette himself didn't even like the name? It's a small point but if others concur then I think it's worth correcting it. Ornette 15:58, 30 September 2005 (UTC)

Opening sentence
Is anyone else bothered by the nebulous & rather misleading opening sentence: "Free jazz is a movement of jazz music characterized by diminished dependence on formal constraints." It seems to me neither accurate nor helpful. Free jazz replaced many of the traditional features of jazz (especially those established by bebop and hard bop) with new ones. (In some cases these "new" features were actually a resurrection of earlier jazz values in a new context--for instance, the collective polyphony of early jazz bands.) ND 14:36, 9 April 2006 (UTC)

 * That's a very good point!Gingermint (talk) 21:42, 23 April 2011 (UTC)

Meter
How free is meter in free jazz. does it resemble Metis folk music in having a steady rythm rather then a meter?--User: Knoss

 * I doubt it sounds much like Metis folk music, but get yourself a copy of Ascension & see what you think.... --ND 19:46, 1 March 2007 (UTC)

 * For what it's worth, the term pulse is frequently used to describe the time structures in free jazz. ---Sluzzelin 23:37, 1 March 2007 (UTC)

 * What I wanted to know was if free jazz generaly has bars or an irregular beat.--User: Knoss

 * Most of what you hear in free jazz isn't notated, yet composed themes or transcriptions of solos in free jazz are sometimes written with bar intersections, because partition makes it easier to read. But the bars don't necessarily represesent a fixed meter or time segment. Free jazz can make use of regular beats, irregular beats and no beat at all, which is when the pulse can become relevant. ---Sluzzelin talk  01:19, 2 March 2007 (UTC)

 * Yeah, but I feel that pulse is a bit too strong in a lot of free cases - look at the work of some of the recent New Orleans guys, for example, or the Euro scene, or some of Masada's works. Free Jazz is all about destroying existing modes of music and then moving beyond them through collective improvisation, so even overall pulse may be a bit strong. 129.81.180.29 (talk) 19:45, 18 March 2008 (UTC)

 * guess thats why it's free... lol --User: Knoss

Samples
Hey could someone get audio samples of some standard free jazz music? Zouavman Le Zouave (Talk to me! • O)))) 18:34, 5 March 2007 (UTC)

 * Unfortunately, it's recent enough as a movement that it's still under copyright. Though there are sites like Church Number Nine that post copyrighted free jazz recordings. --ND 12:49, 6 March 2007 (UTC)

 * However, there are samples of copyrighted material on Wikipedia, and it wouldn't hurt to have a minute or two of Cecil Taylor vs Ornette Coleman vs Sonny Sharrock, or something, just to show the progression. 129.81.180.29 (talk) 20:07, 18 March 2008 (UTC)

Instruments
Previously, the instrument list listed only sax, trombone, guitar and drums. i'm no free jazz expert but what free jazz i've heard has definitley had piano (Cecil Taylor, Paul Bley) and double bass. If anyone has any objection to this addition, change it but at least give a reason. Glassbreaker5791 21:47, 27 July 2007 (UTC)

"Out Jazz"
I've also heard free jazz called out jazz, and there's some mention of that here. Can someone make an out jazz redirect to here? 66.71.70.134 (talk) 05:18, 8 October 2008 (UTC)

This article and Avant-garde jazz article - same territory?
I'm seeking opinions on the roles of these articles at Wikipedia_talk:WikiProject_Jazz. Views welcome there. AllyD (talk) 19:56, 15 November 2009 (UTC)
 * Just a note that the discussion has since been archived, at Wikipedia_talk:WikiProject_Jazz/Archives/2009_1. -- Gyrofrog (talk) 18:34, 18 August 2010 (UTC)

Soul music?
I removed the following sentence from the definition section because, in context, it made no sense:

 * This eventually led to the concept of “soul” in the 1960s.

At the end of a long discussion of how free jazz developed, this sentence is, at best, a non sequitur, but it is also factually incorrect. Can anyone offer an explanation for how it got there? ---  RepublicanJacobite  The'FortyFive' 15:23, 7 October 2010 (UTC)

 * The only thing I can think of along these lines is an "Eddieism" that Eddie Harris wrote. His definition was something like, playing with maximum regard to feelings and minimum regard to technique. But this wasn't in regards to free jazz, which he defined as (something like) everyone playing independently but connecting via ESP. -- Gyrofrog (talk) 16:21, 7 October 2010 (UTC)
 * My thought was that an editor who expanded the section left in a stray sentence from an earlier version, perhaps? In context, as I said, I can make no sense of it.  Free jazz and soul jazz are rather different creatures, so I do not think that is what is being referred to. ---  RepublicanJacobite  The'FortyFive'  16:36, 7 October 2010 (UTC)

Harmony and Musical Examples
It would be great to see some musical examples and a little more technical stuff about the harmony and, in general, the theory found (or intended) in the music. Something that a musician interested in this subject could find useful. Gingermint (talk) 21:41, 23 April 2011 (UTC)

 * Not possible. I would point you first of all to Ekkehard Jost's eminent study Free Jazz (1974). It takes a whole book merely to describe the different approaches, so there is really nothing to be gained from generalizing about harmony and other theoretical aspects, because they varied so widely from musician to musician. 82.176.209.52 (talk) 22:01, 18 February 2012 (UTC)

Bill Dixon
It was brought up on the Reference desk/Entertainment that this article's brief mention of Bill Dixon is inadequate in comparison to his role in the movement. Although I am not very knowledgeable on the subject, it seems the New York Times might agree:,. However, I do not feel confident enough to edit this myself and decided to post this here instead. — 71.20.250.51 (talk) 08:20, 8 October 2014 (UTC)
 * I linked the single mention of Dixon in the current article, which is the least that can be done. As an article, it should probably extend to have more on the movement as well as prominent individuals, hence taking in the Jazz Composers Guild (organised by Dixon ), AACM, Globe Unity Orchestra, etc. AllyD (talk) 17:01, 8 October 2014 (UTC)
 * The 'History' section is quite large; it should be broken down into subsections (by somebody more familiar with the subject than me). — 71.20.250.51 (talk) 20:03, 8 October 2014 (UTC)

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AACM, etc.
There is nothing at all in this article about the work and influence of Chicago's Association for the Advancement of Creative Musicians (AACM), and of other similarly-minded musicians, except for a single mention of Anthony Braxton. To me, this seems to omit a major thread of free jazz, but I myself am not knowledgeable enough to address this. (AllyD has mentioned this same issue several times, above.) Acwilson9 (talk) 04:54, 10 June 2019 (UTC)

 * @Acwilson9 I agree entirely. The AACM is no doubt one of the most important and influential institutions in all of jazz. At the very least Muhal Richard Abrams should be mentioned for his contributions to the genre, along with his great influence(often times a direct and personal influence) to most free jazz practitioners post-early 60s. Tylerpob (talk) 23:04, 22 June 2024 (UTC)

Earlier free jazz than Lennie Tristano
Apparently the 1945 record "The Atomic Era" by Bud Freeman and Ray McKinley (Majestic Records #1031) is the earliest free jazz, even earlier than that recorded by Lennie Tristano later in the 1940s. Source 173.88.246.138 (talk) 23:16, 22 February 2022 (UTC)

 * @173.88.246.138
 * Frankly, that piece sounds absolutely nothing like free jazz; whereas Lennie Tristano's Intuition absolutely does. Tylerpob (talk) 23:00, 22 June 2024 (UTC)

title: Talk:Free jazz


text:
Talk:Richard C. Weaver

Untitled
Clever guy. Sigg3.net 19:39 21 Jul 2003 (UTC)

This article says that the only verified time that he met a President was President Clinton. Then it details a time that he met President Bush (Jr). I believe that it should state both initially.Matchand (talk) 07:02, 28 November 2009 (UTC)

the link at the bottom to his website is broken, or the site is nonexistent.Heatherfire 03:57, 12 January 2007 (UTC)
 * seems to be working now (the one link that's there anyway)Matchand (talk) 07:02, 28 November 2009 (UTC)

What is his crime?
"On February 6, 2003 Weaver attended the National Prayer Breakfast meeting and was able to hand Bush an eight page typed letter about Iraq 'from God'. In 2005, Weaver was arrested on an outstanding warrant related to this event." What was he arrested for? Was he ever charged? Since when is it illegal to hand a letter to the president in a democratic country, if you are not endangering him in any way? Credulity (talk) 19:57, 30 July 2012 (UTC)

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title: Talk:Richard C. Weaver


text:
Bill Bright

William R. Bright (October 19, 1921 – July 19, 2003) was an American evangelist. In 1951 at the University of California, Los Angeles, he founded Campus Crusade for Christ as a ministry for university students. In 1952 he wrote The Four Spiritual Laws. In 1979 he produced the film Jesus.

In 1996 Bill Bright was awarded the $1.1 million Templeton Prize for Progress in Religion, and donated the money to promote the spiritual benefits of fasting and prayer. In 2001 he stepped down as leader of the organization and Steve Douglass became president. He died in 2003.

Early life and education
Bill Bright was born in Coweta, Oklahoma, on October 19, 1921. He was the sixth child and fifth son of Forrest Dale and Mary Lee Rohl Bright. His father Forrest Dale was a cattle rancher while his mother Mary Lee was a school teacher prior to marrying Forrest. Bill's father Forrest was actively involved in the Oklahoma Republican Party with Bill remaining a staunch Republican throughout his life. Bill studied economics at Northeastern State University in Tahlequah, Oklahoma. As a student at Northeastern State University, he was initiated into the Zeta chapter of the Sigma Tau Gamma fraternity, and has subsequently been granted honorable alumnus status to the Alpha Gamma Omega Christ-Centered Fraternity. In 1942, Bill enlisted in the United States Navy Reserve but did not see combat service due to a burst eardrum from playing football during high school. While in his early 20s he moved to Los Angeles, California, and founded a company called "Bright's California Confections." During the 1940s, Bill attended the First Presbyterian Church, Hollywood where he became an evangelical Christian. Bright was influenced by Henrietta Mears, who served as the Director of Christian Education at First Presbyterian Church, and Billy Graham, who later became a prominent American evangelical leader.

In 1946, Bill Bright quit his candy business to pursue Biblical studies and theology at Princeton Theological Seminary and Fuller Theological Seminary. According to the historian John G. Turner, Bright struggled with his academic studies and did not complete his degree at either institution. While studying at Fuller seminary, Bright felt what he regarded as the call of God to reach out to university students and abandoned his academic studies. Before starting his campus ministry, Bright sold off his confections company and settled a financial dispute with his former business partners, the Taylor family.

Family
Bill Bright married Vonette Bright on December 30, 1948. The two had been engaged to marry since the spring of 1946. However, Bill at the prompting of Zachary's parents agreed to delay the wedding until her impending 1948 graduation from Texas State College for Women. During the 1950s, Bill and Vonette adopted two boys named Brad and Zachary Bright.

Writings
In 1965, Bright wrote The Four Spiritual Laws, an evangelistic Christian tract. In the booklet he outlines his view of the essentials of the Christian faith concerning salvation. It is summarized as four spiritual laws or principles that govern what he sees as human beings' relationship with God. The booklet ends with a prayer of repentance.

Creation of Campus Crusade for Christ, early 1950s
Bright had initially planned to produce an evangelical film called "The Great Adventure" but abandoned the project due to a lack of funding.

Though Bright had initially considered partnering with other churches, his disenchantment with their ability to mentor new Christian converts led him to start Campus Crusade for Christ as a parachurch organization. In 1951, after recruiting several volunteers from Fuller Seminary and Hollywood Presbyterian, Bright started Campus Crusade's first chapter at the University of California, Los Angeles (UCLA). According to Turner, Campus Crusade was also inspired by Bright's desire to combat Communist influence in US universities including UCLA, which was then regarded as a hotbed of student radicalism.

By 1952, Bright's Campus Crusade had reportedly converted 250 students at UCLA including the student body president, campus newspaper editor, and several athletes including African American decathlete and future Olympian Rafer Johnson. While Bill and his colleagues focused on the male students, Vonette focused on reaching out to the female students.

In 1953, Campus Crusade established its headquarters in Los Angeles' Westwood Boulevard. Bright's campus outreach was also aided by his Hollywood Presbyterian mentor Henrietta Mears, who allowed the Brights to share her Bel Air home and spoke at several Crusade functions.

Expansion of Campus Crusade beyond UCLA
Bright's success at UCLA led him to establish Campus Crusade branches at several other US universities.

Campus Crusade's expansion across several US campuses created friction with other Christian campus groups including InterVarsity Christian Fellowship and liberal campus chaplains, who disagreed with the evangelistic tone of Bright's ministry. In 1956, Bright wrote a 20–minute evangelistic presentation called "God's Plan for Your Life", which set the tone for Campus Crusade's evangelism and discipleship program. Bright also initially partnered with the fundamentalist Bob Jones University. However, the relationship deteriorated after Bright sided with Billy Graham, who had accepted the sponsorship of liberal Protestants for his 1957 New York crusade. In response, Bob Jones Sr. and his son Bob Jones Jr. severed relations with Bright's ministry. According to Turner, this split with Bob Jones University led Bright to gravitate towards the "new evangelical" wing of the Protestant movement, which was associated with Billy Graham's cooperative evangelism.

During the decades to follow, Bill Bright and his wife, Vonette Bright, continued this work, and the ministry expanded greatly. In 2011 Campus Crusade for Christ had 25,000 missionaries in 191 countries.

Later life
Bright held five honorary doctorate degrees: a Doctor of Laws from the Jeonbuk National University of Korea, a Doctor of Divinity from John Brown University, a Doctor of Letters from Houghton Seminary, a Doctor of Divinity from the Los Angeles Bible College and Seminary, and a Doctor of Laws from Pepperdine University.

In 1983, he chaired the National Committee for the National Year of the Bible. He was named the 1996 recipient of the $1.1 million Templeton Prize for Progress in Religion. He donated the prize money to causes promoting the spiritual benefits of fasting and prayer.

He wrote more than 100 books and booklets, and thousands of articles and pamphlets that have been distributed in most major languages by the millions. He endorsed the document Evangelicals and Catholics Together. Bright was a co-founder of the Alliance Defense Fund, which funds high-profile litigation cases on behalf of Christians' First Amendment rights, as well as litigation to end abortion access and to restrict LGBTQ rights. He was also a co-signatory of the Land letter of 2002 which outlined a just war rationale for the 2003 invasion of Iraq, providing a theological underpinning for the invasion being planned by President George W. Bush.

He produced the film Jesus in 1979, which was released by Warner Bros. in the United States. It was not a financial success, losing approximately $2 million. While praising its "meticulous attention to authenticity", critics panned the film for being "painfully monotonous". The Los Angeles Times called it a "...dull Sunday-School treatment of the life of Christ."

In 1988 he led the protest against the Martin Scorsese film The Last Temptation of Christ and he called the film "blasphemous". He offered to buy the film's negative from Universal in order to destroy it.

Death
Bright died on July 19, 2003, in Orlando, Florida. His wife died in 2015.

The Rev. Billy Graham released a statement on Bright's death: "He has carried a burden on his heart as few men that I've ever known - a burden for the evangelization of the world. He is a man whose sincerity and integrity and devotion to our Lord have been an inspiration and a blessing to me ever since the early days of my ministry."

Politics
Bill Bright's father Dale Bright was a staunch Republican who served as the chairman of the Wagoner Country Republican Party. Like his father, Bill remained a staunch Republican supporter and voter throughout his life. As an evangelical Christian, Bright was also anti-Communist and stated that one of his reasons for starting Campus Crusade was to combat Communist influence in US universities. During the 1970s, Bright defended the South Korean President Park Chung Hee, who imposed martial law and assumed dictatorial powers. He claimed that Park was fighting against Communism and supported religious freedom.

title: Bill Bright


text:
Talk:Betamax

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title: Talk:Betamax


text:
Carlos Morocho Hernández

Carlos Enrique Hernández Ramos (April 21, 1940 – July 2, 2016) was a Venezuelan world champion professional boxer. Known professionally as Carlos Morocho Hernandez (Carlos "Dark Haired" or, possibly, "Twin" Hernandez), he ended his career following a TKO by Scottish boxer Ken Buchanan.

History
Carlos Enrique Hernandez Ramos was born on 21 April 1940, in the populous parish of La Pastora, Caracas. For the peculiarity of being dark-haired, was identified for most of his career as "Morocho", but also was known as "Kid Helicoide". His first triumphs in boxing came at the age of 15 years old, earning the titles of Champion of the Federal District and National Featherweight monarch. In his first international tournament, he was included in the Second World Amateur Boxing Championships held in Mexico, where he earned the "Ring of Diamonds".

He was also champion of Central America and the Caribbean and was undefeated after 25 amateur fights. Following his amateur career, which he finished undefeated, he tried his hand at professional boxing, which seemed a more lucrative field to him.

The "Morocho" made the leap to professional boxing under the tutelage of Juancho Medina. His debut was on 25 January 1959 at Nuevo Circo de Caracas, against Felix Gil, at featherweight. On 18 January 1965, Carlos "Morocho" Hernández won the world light-welterweight championship, facing the American Eddie Perkins and giving Venezuela its first world champion.

After a career of successes and excesses, Carlos "Morocho" Hernández, met the end and defeat when he faced, in London, on 11 May 1971 Scotsman Ken Buchanan, to whom Hernandez lost by TKO in the eighth round.

Death
Carlos "Morocho" Hernandez" died in Caracas, Venezuela, on July 2, 2016, due to health complications.

title: Carlos Morocho Hernández


text:
Underworld (disambiguation)

The underworld is a place in religion and mythology to where the souls of the recently departed go.

Underworld may also refer to:
 * Greek underworld, the Greek version
 * Criminal underworld, another name for organized crime
 * Hell in Christianity, sometimes used more favorably than "hell"

Comics

 * Underworld (comics), a Marvel Comics character
 * Underworld (comic strip), an adult comic strip by Kaz

Computer games

 * Tomb Raider: Underworld, released in 2008, the eighth game in the Tomb Raider series
 * Ultima Underworld, series of computer games first released in 1992
 * Underworld: The Eternal War, a 2004 PlayStation 2 game based on the 2003 film
 * Underwurlde, 1984 computer game featuring Sabreman

Film and television

 * Underworld (1927 film), a silent film directed by Josef von Sternberg
 * Underworld (1937 film), an Oscar Micheaux film
 * Underworld (1985 film), a horror film written by Clive Barker
 * Underworld (1996 film), a comedy/thriller film starring Denis Leary and Joe Mantegna
 * Underworld (film series), a horror/action film series with five installments
 * Underworld (2003 film), the first movie, released in 2003
 * Underworld: Evolution, the 2006 sequel
 * Underworld: Rise of the Lycans, the 2009 prequel
 * Underworld: Awakening, 2012, fourth movie in the series
 * Underworld: Blood Wars, 2016, fifth movie in the series
 * Underworld (2003 film), a 2003 Bangladeshi film, featuring Shakib Khan
 * Underworld (2004 film), a 2004 Sri Lankan film
 * Underworld (Doctor Who), a serial in the British science fiction television series Doctor Who
 * Underworld Trilogy, 2010 drama/thriller television series
 * Cities of the Underworld, a documentary television series

Literature

 * Underworld (novel), a 1997 novel written by Don DeLillo
 * Underworld: The Mysterious Origins of Civilization, a non-fiction work of historical speculation written by Graham Hancock
 * Resident Evil: Underworld, the fourth entry in S.D. Perry's series of novels based on Capcom's franchise of videogames, Resident Evil
 * Underworld (Reginald Hill novel), a 1988 novel from the Dalziel and Pascoe series
 * Underworld (Doctor Who novel), novelization of the television episode
 * Underworld (film series novels), novelizations of the Underworld film series and an original novel based on the setting

Music

 * Underworld (band), a British electronic music band
 * Underworld (Adagio album), a 2003 album
 * Underworld (Divinyls album), a 1996 album
 * Underworld (Symphony X album), a 2015 album
 * Underworld (Vamps album), a 2017 album
 * Underworld (Tonight Alive album), a 2018 album
 * Camden Underworld, a music venue and nightclub located under The World's End pub in Camden Town, London

title: Underworld (disambiguation)