Two basic approaches to the snoopy protocol have been explored: Write invalidates or write-update (write-broadcast).

(i) Write invalidates:

With a write-invalidate protocol, there can be multiple readers but only one write at a time.

Initially, a line may be shared among several caches for reading purposes.

When one of the caches wants to perform a write to the line it first issues a notice which invalidates the line in the other caches, making the line exclusive to the writing cache.

Once the line is exclusive, the owning processor can make local writes until some other processor requires the same line.

(ii) Write update (write broadcast):

With a write update protocol, there can be multiple writers as well as multiple readers. When a processors wishes to update a shared line, the word to be updated is distributed to all others, and caches containing that line can update it.