Our read command targeting system is a core portion of our network client infrastructure. The vast majority of relevant code lives in src/mongo/client.

Notable classes include (but are not limited to):

• ReadPreference
• RemoteCommandTargetter
• ReplicaSetMonitor interface and implementations

The ReplicaSetMonitor was already documented in src/mongo/client/readme.md here. We should add additional sections to that document and cross reference markdown headers.

This is intended to be abstract documentation, describe relationships and state transitions not code in common language.