singletonStore element enables modifications to be stored by only one node in the cluster, the coordinator. Essentially, whenever any data comes in to some node it is always replicated so as to keep the caches' in-memory states in sync; the coordinator, though, has the sole responsibility of pushing that state to disk. This functionality can be activated setting the enabled subelement to true in all nodes, but again only the coordinator of the cluster will store the modifications in the underlying cache loader as defined in cacheloader element. You cannot define a cache loader as shared and with singletonStore enabled at the same time. Default value for enabled is false .

Optionally, within the singletonStore element, you can define a class element that specifies the implementation class that provides the singleton store functionality. This class must extend org.jboss.cache.loader.AbstractDelegatingCacheLoader , and if absent, it defaults to org.jboss.cache.loader.SingletonStoreCacheLoader .

The properties subelement defines properties that allow changing the behaivour of the class providing the singleton store functionality. By default, pushStateWhenCoordinator and pushStateWhenCoordinatorTimeout properties have been defined, but more could be added as required by the user-defined class providing singleton store functionality.

pushStateWhenCoordinator allows the in-memory state to be pushed to the cache store when a node becomes the coordinator, as a result of the new election of coordinator due to a cluster topology change. This can be very useful in situations where the coordinator crashes and there's a gap in time until the new coordinator is elected. During this time, if this property was set to false and the cache was updated, these changes would never be persisted. Setting this property to true would ensure that any changes during this process also get stored in the cache loader. You would also want to set this property to true if each node's cache loader is configured with a different location. Default value is true .

pushStateWhenCoordinatorTimeout is only relevant if pushStateWhenCoordinator is true in which case, sets the maximum number of milliseconds that the process of pushing the in-memory state to the underlying cache loader should take, reporting a PushStateException if exceeded. Default value is 20000.

Note on using the singletonStore element: setting up a cache loader as a singleton and using cache passivation (via evictions) can lead to undesired effects. If a node is to be passivated as a result of an eviction, while the cluster is in the process of electing a new coordinator, the data will be lost. This is because no coordinator is active at that time and therefore, none of the nodes in the cluster will store the passivated node. A new coordinator is elected in the cluster when either, the coordinator leaves the cluster, the coordinator crashes or stops responding.