[DOCS-16342] [SERVER] Investigate changes in SERVER-78657: Get rid of `getSplitCandidatesForSessionsCollection` and `minNumChunksForSessionsCollection` Created: 24/Aug/23  Updated: 13/Nov/23  Resolved: 10/Oct/23

Status: Closed
Project: Documentation
Component/s: manual, Server
Affects Version/s: None
Fix Version/s: 7.1.0-rc0, 7.0.2, Server_Docs_20231030, Server_Docs_20231106, Server_Docs_20231105, Server_Docs_20231113

Type: Task Priority: Minor - P4
Reporter: Backlog - Core Eng Program Management Team Assignee: Joseph Dougherty
Resolution: Fixed Votes: 0
Labels: sharding
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
Documented
documents SERVER-78657 Get rid of `getSplitCandidatesForSess... Closed
Participants:
Days since reply: 20 weeks, 2 days ago

 Description   
Original Downstream Change Summary

We are marking as deprecated the server parameter `minNumChunksForSessionsCollection`. It will be ignored internally.

It should be worth documenting this for the user, so in the future this server parameter could be internally removed. The problem is that `minNumChunksForSessionsCollection` has never been documented externally, and it makes no sense to do it from now.

I propose introducing a section called "parameter ignored/DEPRECATED" or similar in the docs and expose the `minNumChunksForSessionsCollection` there, with the possibility to extend this list in the future.

Description of Linked Ticket

Starting from v6.0, the balancer is distributing data based on the data size on shards and not merely on the number of chunks. Because of that, it is totally useless to presplit the session collection during balancing rounds and it's also counterproductive because (1) chunks will not be moved and (2) starting from v7.0 the auto merger will squash them back together.



 Comments   
Comment by Joseph Dougherty [ 18/Sep/23 ]

Thank you for the explanation, pol.pinol@mongodb.com – this all makes sense to me. I put a PR together documenting this deprecation.

Comment by Pol Pinol [ 18/Sep/23 ]

Hi joseph.dougherty@mongodb.com,

In most cases, a user will not be affected by this change. A server parameter (that was not documented) will now be deprecated. But, for those users that were aware of this server parameter and had a script to initiate the cluster using `minNumChunksForSessionsCollection` as a server parameter, they need to be aware that it will be ignored, i.e., the session collection will no longer presplit during balancer rounds based on the server parameter `minNumChunksForSessionsCollection`.

The point here is that, `minNumChunksForSessionsCollection` was never documented (https://www.mongodb.com/docs/manual/reference/parameters), and now we don't want to introduce it, just announce its deprecation.

Thanks!

Comment by Joseph Dougherty [ 14/Sep/23 ]

Hello pol.pinol@mongodb.com!

Thank you for raising this issue. Generally speaking we do not document ignored params. I think I understand the desire to mention that this had existed previously, but I'd prefer to state what a use should do in its absence to make any relevant updates.

That is, can we express this as what the user can do instead of modifying the value of `getSplitCandidatesForSessionsCollection`?

Thanks for your help!
Joe Dougherty

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