[SERVER-5525] mongos should send inline mapreduce with slaveOk to secondaries Created: 05/Apr/12  Updated: 11/Jan/21  Resolved: 11/Jan/21

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

Type: New Feature Priority: Major - P3
Reporter: Brett Goldstein Assignee: Charlie Swanson
Resolution: Duplicate Votes: 3
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
Depends
Duplicate
duplicates SERVER-45974 Enable $merge/$out on secondaries in ... Closed
is duplicated by SERVER-27312 Mapreduce does not run on Secondary i... Closed
Related
related to SERVER-7423 Support commands for read preference Closed
Sprint: Query 2021-01-25
Participants:

 Description   

This will allow for more efficient processing of ongoing M/R jobs when there are available secondaries



 Comments   
Comment by Charlie Swanson [ 11/Jan/21 ]

Hi all. In the 4.4 release, mapReduce was re-implemented to use the aggregation framework. Under the hood, it is implemented using the machinery for $out/$merge. As a result, this project has allowed non-inline mapReduce operations to run on secondaries. As such, I am resolving this ticket as I believe it has been completed.

However, since we want to encourage applications to use aggregation instead of mapReduce, we do not intend to add non-inline mapReduce on secondaries as a supported feature. Drivers and the shell should continue to ignore non-primary read preference for non-inline mapReduce operations. Mongos will always send non-inline mapReduce to shard primaries regardless of the read preference. Also, causal consistency for mapReduce on secondaries remains explicitly banned.

 

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