SERVER-43779 Add section on Two Phase Drops to the Replication Architecture Guide

Author: lankas

src/mongo/db/repl/README.md

@@ -748,3 +748,36 @@ where we cannot assume a drop will come and fix them, we abort and retry initial
 The oplog application phase concludes when the node applies `minValid`. The node checks its sync
 source's Rollback ID to see if a rollback occurred and if so, restarts initial sync. Otherwise, the
 `DataReplicator` shuts down and the `ReplicationCoordinator` starts steady state replication.
+
+# Dropping Collections and Databases
+
+In 3.6, the Two Phase Drop Algorithm was added in the replication layer for supporting collection
+and database drops. It made it easy to support rollbacks for drop operations. In 4.2, the
+implementation for collection drops was moved to the storage engine. This section will cover the
+behavior for the implementation in the replication layer, which currently runs on nodes where
+<!-- TODO SERVER-43788: Link to the section describing enableMajorityReadConcern=false -->
+`enableMajorityReadConcern` is set to false.
+
+## Dropping Collections
+
+Dropping an unreplicated collection happens immediately. However, the process for dropping a
+replicated collection requires two phases.
+
+In the first phase, if the node is the primary, it will write a "dropCollection" oplog entry. The
+collection will be flagged as dropped by being added to a list in the `DropPendingCollectionReaper`
+(along with its OpTime), but the storage engine won't delete the collection data yet. Every time the
+`ReplicationCoordinator` advances the commit point, the node will check to see if any drop's OpTime
+is before or at the majority commit point. If any are, those drops will then move to phase 2 and
+the `DropPendingCollectionReaper` will tell the storage engine to drop the collection.
+
+By waiting until the "dropCollection" oplog entry is majority committed to drop the collection, it
+guarantees that only drops in phase 1 can be rolled back. This means that the storage engine will
+still have the collection's data and in the case of a rollback, it can then easily restore the
+collection.
+
+## Dropping Databases
+
+When a node receives a `dropDatabase` command, it will initiate a Two Phase Drop as described above
+for each collection in the relevant database. Once all collection drops are replicated to a majority
+of nodes, the node will drop the now empty database and a `dropDatabase` command oplog entry is
+written to the oplog.