In order to support targeted mirroring by tag, the MirrorMaestro needs to get a list of hosts that match a given tag. We can do this through the TopologyVersionObserver and its associated ReplicationCoordinator. We need an api on the TopologyVersionObserver where we supply a tag and get a list of hosts that have the tag. We also would like to cache the list of hosts to avoid recomputing in the event that the same tag is supplied multiple times. See more details here.