Buddy Replication allows you to suppress replicating your data to all instances in a cluster. Instead, each instance picks one or more 'buddies' in the cluster, and only replicates to these specific buddies. This greatly helps scalability as there is no longer a memory and network traffic impact every time another instance is added to a cluster.

One of the most common use cases of Buddy Replication is when a replicated cache is used by a servlet container to store HTTP session data. One of the pre-requisites to buddy replication working well and being a real benefit is the use of session affinity , more casually known as sticky sessions in HTTP session replication speak. What this means is that if certain data is frequently accessed, it is desirable that this is always accessed on one instance rather than in a round-robin fashion as this helps the cache cluster optimise how it chooses buddies, where it stores data, and minimises replication traffic.

If this is not possible, Buddy Replication may prove to be more of an overhead than a benefit.