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  1. Documentation
  2. DOCS-15223

[Server] Confusing Flow Control Description

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      In the documentation: https://www.mongodb.com/docs/manual/tutorial/troubleshoot-replica-sets/#flow-control

      we can read:

      For flow control to engage, the replica set/sharded cluster must have: featureCompatibilityVersion (FCV) of 4.2 and read concern majority enabled. That is, enabled flow control has no effect if FCV is not 4.2 or if read concern majority is disabled.

      However, flow control goal is to really throttle writes when there is certain replication lag. Also, in 

      https://www.mongodb.com/docs/manual/reference/parameters/#mongodb-parameter-param.flowControlTargetLagSeconds we can read:

      The target maximum majority committed lag when running with flow control. When flow control is enabled, the mechanism attempts to keep the majority committed lag under the specified seconds. 

      and in https://www.mongodb.com/docs/manual/reference/command/replSetGetStatus/#mongodb-data-replSetGetStatus.optimes.lastCommittedOpTime

      Information, from the viewpoint of this member, regarding the most recent operation that has been written to a majority of replica set members.

       

      So I think it is clear that it is about write concern and not read concern?

      Besides, before version 4.4 it was even impossible to set read concern globally:

      https://www.mongodb.com/docs/manual/reference/read-concern/

      Starting in MongoDB 4.4, replica sets and sharded clusters support setting a global default read concern.

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            przemek.malkowski@gmail.com Przemek Malkowski
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              Created:
              Updated:
              1 year, 43 weeks, 5 days ago