Even with the write through policy, inconsistency can occur unless other cache monitor the memory traffic or receive some direct notification of the update.

For any cache coherence protocol, the objective is to let recently used local variables get into the appropriate cache and stay there through numerous reads and write, while using the protocol to maintain consistency of shared variables that might be in multiple caches at the same time.

Write through protocol:

A write through protocol can be implemented in two fundamental versions.
            (a)   Write through with update protocol
            (b)   Write through with invalidation of copies

Write through with update protocol:

When a processor writes a new value into its cache, the new value is also written into the memory module that holds the cache block being changed. Some copies of this block may exist in other caches, these copies must be updated to reflect the change caused by the write operation.

The simplest way of doing this is to broadcast the written data to all processor modules in the system.