The most popular protocol for maintaining cache coherency is called snooping. Cache controllers monitor or snoop on the bus to determine whether or not the cache contains a copy of a shared block.

For reads, multiple copies can reside in the cache of different processors, but because the processors need the most recent copy, all processors must get new values after a write. See also cache, competitive-caching, false sharing, write-invalidate, write-update.

For writes, a processor must have exclusive access to write to cache. Writes to unshared blocks do not cause bus traffic. The consequence of a write to shared data is either to invalidate all other copies or to update the shared copies with the value being written. See also cache, competitive-caching, false sharing, write-invalidate, write-update.