@Override
    public void leftCluster() {
        // The local cluster node left the cluster.
        if (XMPPServer.getInstance().isShuttingDown()) {
            // Do not put effort in restoring the correct state if we're shutting down anyway.
            return;
        }

        // Upon leaving a cluster, clustered caches are reset to their local equivalent (by the swap from the clustered
        // cache implementation to the default cache implementation that's done in the implementation of
        // org.jivesoftware.util.cache.CacheFactory.leftCluster). This means that they now hold no data (as a new cache
        // has been created). Data that's available on the local node needs to be added again.
        restoreCacheContent();

        incomingServerSessionInfoByClusterNode.clear();

        // It does not appear to be needed to invoke any kind of event listeners for the data that was lost by leaving
        // the cluster (eg: sessions connected to other cluster nodes, now unavailable to the local cluster node):
        // There are six caches in play here, but only the content of one of them goes accompanied by firing off event
        // listeners (sessionInfoCache). However, when already running in a clustered environment, those events are
        // never broadcasted over the cluster, so there shouldn't be a need to do so for all sessions that were
        // gained/lost when joining or leaving a cluster either.
    }
Returns the number of milliseconds to elapse between clearing of idle server sessions.

@return the number of milliseconds to elapse between clearing of idle server sessions.
xmppserver/src/main/java/org/jivesoftware/openfire/SessionManager.java