There is currently no general mechanism for code outside the replication subsystem to be notified when lastAppliedOpTime advances. The closest existing patterns are:

• OplogApplier::Observer::onBatchEnd(): secondaries only, not called on primaries
• getMyLastAppliedTimestamp(): lock-free read of a shadow atomic, but pull-based

This ticket propose to add a push-based observer mechanism that works on both primaries and secondaries, fires outside ReplicationCoordinatorImpl::_mutex, and supports multiple persistent observers.