Editing Consistency model (section)

=== Causal consistency ===

[[Causal consistency]]<ref name="Replica"/> defined by Hutto and Ahamad, 1990,<ref name="slow"/> 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:<ref name="Replica"/>

* 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. 
{| class="wikitable"
|-
! Sequence
! {{abbr|P|Processor}}1
! {{abbr|P|Processor}}2
! {{abbr|P|Processor}}3
! {{abbr|P|Processor}}4
|-
! 1
| W(x)1
| R(x)1
| R(x)1
| R(x)1
|-
! 2
|
| W(x)2
|
|
|-
! 3
| W(x)3
|
| R(x)3
| R(x)2
|-
! 4
|
|
| R(x)2
| R(x)3
|}

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''.<ref name=":0" />