New parameters added to $changeStream aggregation stage.

Description of Linked Ticket

Summary

Change streams on sharded clusters should return correct events when shards are removed or added.

Motivation

This has been a user request and will allow us to offer change streams for more use cases while increasing client-side debugability. There are scenarios that may lead to incorrect events for change stream users on sharded clusters. 

For example: if a shard gets removed from a cluster before we are finished reading events from it and we resume a stream from a point before the shard was removed, we will silently skip every event that occurred on the removed shard between that time and the point where it was removed. This violates one of change streams’ fundamental guarantees (no skipped events).

In a serverless world, where we want to be able to add and remove shards at will, this will become an important issue to fix. 

Cast of Characters

• Product Owner: Katya Kamenieva
• Project Lead: Mindaugas Malinauskas
• Program Manager: Ana Meza
• Drivers Contact: TBD

Documentation

Scope Document
Technical Design Document
Product Description
Docs Update

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