Broker Server uses a portion of memory to cache guaranteed documents and other objects (for example, Broker metadata and explicit-destroy client queues and subscriptions) that Broker Server keeps in queue storage.

The max_cache_size setting restricts the amount of memory that Broker Server can use to cache data for queue storage. This parameter ensures that a certain amount of memory is available to retain queue storage objects in memory, but prevents the cache from depriving the Broker Server of the memory it needs to store volatile objects such as volatile documents and destroy-on-disconnect client queues and subscriptions.

For separate storage sessions, there are two cache settings, one for each storage session. For combined storage session, there is a single cache setting.