In the write update protocol, the updates made by a process are made locally and multicast to all other replica managers possessing a copy of the data item, which immediately modify the data read by local processes. Processes read the local copies of data items, without the need for communication. In addition to allowing multiple readers, several processes may write the same data item at the same time; this is known as multiple-reader-multiple-writer sharing.

The memory consistency model that is implemented with write-update depends on several factors, mainly the multicast ordering property. Sequential consistency can be achieved by using multicasts that are totally ordered which do not return until the update message has been delivered locally. All processes then agree on the order of updates. The set of reads that take place between any two consecutive updates is well defined, and there ordering is immaterial to sequential consistency.

Reads are cheap in the write-update option. However, ordered multicast protocols are relatively expensive to implement in software.

                
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