Currently the isEnabled() method in VectorClock is public, because various external places need to know whether or not the vector clock is enabled. It would be better (from a code coupling/cohesion POV) if iEnabled() was only used from within VectorClock class (and closely related classes).

Current uses:

• Minor optimisation (that could probably just be removed) in LogicalTimeMetadataHook::readReplyMetadata().
• Minor optimisation (that could probably just be removed) in appendClusterAndOperationTime().
• Debug assertion in ReplicationCoordinatorImpl::_setMyLastAppliedOpTimeAndWallTime(). This could possibly be moved into a VectorClock::ensureTimeIsNotAheadOfClusterTime(Timestamp) method. (The difficulty is that ReplicationCoordinatorExternalStateMock::getGlobalTimestamp() returns a mocked value.)
• Ensuring that arbiters don't attempt to load $clusterTime signing keys in LogicalTimeValidator::signLogicalTime(). This could be done if the LogicalTimeValidator class was to be internalised inside the VectorClock class (possibly along with other classes that it relies on — in the worst case, the entire $clusterTime signing and key management machinery).