Description
Description:
We will no longer support 'atClusterTime' with enableMajorityReadConcern:false. The external-facing implication of this is that:
- in 4.2 with enableMajorityReadConcern:false, we will not support transactions in sharded clusters with readConcern level 'snapshot'.
Done as part of earlier ticket. but need to update error message
- (Note that 4.0 does not support transactions in sharded clusters at all, and 4.2 with enableMajorityReadConcern:false only supports single-shard transactions.)
Done as part of earlier ticket.
- 4.0 users should be cautioned that if they are using enableMajorityReadConcern:false and single replica set transactions, then they may wish to avoid readConcern level 'snapshot' if they may transition to using a sharded cluster in 4.2, since this will break their application.
- In 4.2 with enableMajorityReadConcern:false, single-shard transactions will support readConcern levels 'local' and 'majority'.

Engineering Ticket Description:
This means that single-shard transactions on enableMajorityReadConcern:false shards cannot be run with readConcern level snapshot.
Scope of changes
Impact to Other Docs
MVP (Work and Date)
Resources (Scope or Design Docs, Invision, etc.)
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