[SERVER-36659] Allow shard to choose global read timestamp when first txn statement targets one shard Created: 14/Aug/18  Updated: 06/Dec/22  Resolved: 06/May/20

Status: Closed
Project: Core Server
Component/s: Sharding
Affects Version/s: None
Fix Version/s: None

Type: Task Priority: Major - P3
Reporter: Jack Mulrow Assignee: [DO NOT USE] Backlog - Sharding Team
Resolution: Duplicate Votes: 0
Labels: ShardedTxn:FutureOptimizations, pm-564
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
Depends
is depended on by SERVER-47952 If a snapshot read targets one shard,... Closed
Related
Assigned Teams:
Sharding
Participants:

 Description   

To establish a global snapshot for snapshot level read concern, mongos currently selects a cluster time when executing the first statement in a transaction then sends it to each targeted shard as anĀ atClusterTime read concern option and will send it to each new shard contacted by subsequent statements. This guarantees all targeted shards in the transaction read from the same timestamp.

If the first statement targets only one shard, mongos can instead allow the shard to select a read timestamp itself as long as the shard returns to mongos which timestamp it read from, so mongos can store it and send it to all new shard participants that may be targeted by subsequent statements, if any.


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