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Type: Task
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Resolution: Done
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Priority: Major - P3
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Affects Version/s: None
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Component/s: Aggregation Framework, Querying
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None
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Fully Compatible
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v3.4, v3.2
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Query 2016-12-12
Starting with MongoDB 3.2.0, aggregations which require a merging shard randomly select a merger amongst the shards participating in the query. This behavior was introduced under SERVER-18925 in order to help evenly distribute query load. In many situations this behavior is helpful, but it can have drawbacks. For example, when additional shards take on a merging role, more intra-cluster connections are required, which in turn requires additional resources. Therefore, we should introduce a setParameter on mongos called internalQueryAlwaysMergeOnPrimaryShard which can be used to disable random merger selection. The parameter's value is false by default. When configured to true, all aggregations which require a merging shard will merge on the primary shard. Pipelines which do not split into a shards part and merger part are not affected.
The parameter can be set on the command line when starting mongos or via a config file. To set it using the shell, connect to mongos and run the following:
db.adminCommand({setParameter: 1, internalQueryAlwaysMergeOnPrimaryShard: true});
Note that this parameter is for mongos only; attempting to set it against a mongod will result in an error:
> db.adminCommand({setParameter: 1, internalQueryAlwaysMergeOnPrimaryShard: true}); { "ok" : 0, "errmsg" : "attempted to set unrecognized parameter [internalQueryAlwaysMergeOnPrimaryShard], use help:true to see options " }
- is related to
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SERVER-24815 Merging aggregation pipeline strategy should be configurable
- Backlog
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SERVER-18925 Merging part of aggregation pipeline should be performed on a random shard to distribute the load
- Closed