Editing Consistency model (section)

{{short description|Rules that guarantee predictable computer memory operation}}
{{confusing|date=January 2015}}

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 [[data consistency|consistent]] and the results of reading, writing, or updating memory will be predictable. Consistency models are used in [[Distributed computing|distributed systems]] like [[distributed shared memory]] systems or distributed data stores (such as [[filesystem]]s, [[database]]s, [[optimistic replication]] systems or [[web caching]]). Consistency is different from coherence, which occurs in systems that are [[cache coherence|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 language]]s, such as [[C++]] and [[Java (programming language)|Java]], maintain the consistency contract by translating memory operations into low-level operations in a way that preserves [[memory semantics (computing)|memory semantics]], reordering some memory instructions, and encapsulating required synchronization with library calls such as <code>pthread_mutex_lock()</code>.<ref>{{cite journal
|  author = Mark D. Hill
|   title = Multiprocessors Should Support Simple Memory Consistency Models
|    date = August 1998
| journal = IEEE Computer
|  volume = 31
|   issue = 8
|   pages = 28–34
|     doi = 10.1109/2.707614
| url = http://digital.library.wisc.edu/1793/8706
}}</ref>