As part of the proposed design for PM-1939 we intend to generate System Change Stream events in correspondence with Sharding DDL operations that alter the subset of Shards containing at lest one chunk of the namespace targeted by a nss.watch() command.

Some of the desired change stream events (such as migrateChunkToNewShard and migrateLastChunkFromShard) are already being generated by the Server code to support C2C replication.

The objectives of this ticket are:

1. collect information on

• where and how this events are being generated
• where and how this events are being consumed
• which is the logic supporting the propagation of the events

2. confirm that the existing code can be easily leveraged/extended to other DDL operations

3. update the "Following the execution" section of the Design document based on the outcome of the investigation