[SERVER-31015] Investigate if GetMoreRequest must include readConcern for tailable cursors Created: 08/Sep/17  Updated: 27/Oct/23  Resolved: 14/Sep/17

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

Type: Bug Priority: Major - P3
Reporter: Misha Tyulenev Assignee: Backlog - Query Team (Inactive)
Resolution: Works as Designed Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
Depends
Assigned Teams:
Query
Operating System: ALL
Participants:
Linked BF Score: 0

 Description   

The scenario that fails is described in BF-6485,
the core symptom is that the getMore sent to a secondary returns operation time that is less than the afterClusterTime set in the client request.
The reason of this is mongos parses the getMore cmdObj into the GetMoreRequest which does not include readConcern and therefore can not satisfy the request to wait.

the cursors are consistent because because the cursor is the find and getMore is an implementation detail. However the tailable cursors may need the support of readConcern



 Comments   
Comment by Andy Schwerin [ 11/Sep/17 ]

getmore operations don't need read concern because they inherit the concern from the cursor. All we promised was that the cursor contents are from after the time sent on the find or aggregate.

This is normally fine because getmore is an implement detail, invisible below the driver. Maybe get more should reject rather than ignore read concern.

I'd welcome argument on why tailable cursors are special.

Comment by Charlie Swanson [ 11/Sep/17 ]

This sounds like a legit bug to me. Is this also a bug with readConcern "majority" where a getMore might see operations that aren't majority committed? That would be a problem...

cc david.storch - this looks like another bug where we fail to serialize some of the options sent to mongos, not sure if this should move over to the query team.

Comment by Misha Tyulenev [ 08/Sep/17 ]

charlie.swanson , schwerin what do you think?

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