Editing Consistency model (section)

===== Cache-coherence protocols =====

In a replicated file system, a cache-coherence protocol<ref name="Replica"/> 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.