[DOCS-15223] [Server] Confusing Flow Control Description Created: 08/Apr/22  Updated: 22/Jan/24

Status: Backlog
Project: Documentation
Component/s: manual, Server
Affects Version/s: None
Fix Version/s: None

Type: Task Priority: Major - P3
Reporter: Przemek Malkowski Assignee: Unassigned
Resolution: Unresolved Votes: 0
Labels: backlog, proactive, replication, sharding
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Participants:
Days since reply: 1 year, 43 weeks, 5 days ago
Epic Link: DOCSP-11702

 Description   

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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