public Map<String, ReplicationPeerImpl> getPeerCache() {
    return Collections.unmodifiableMap(peerCache);
  }
Returns the set of peerIds of the clusters that have been connected and have an underlying
ReplicationPeer.
@return a Set of Strings for peerIds
hbase-replication/src/main/java/org/apache/hadoop/hbase/replication/ReplicationPeers.java