[DOCS-15210] Add Warning for ReadConcern: Majority on PSA Replicasets Created: 04/Apr/22  Updated: 13/Nov/23  Resolved: 18/Apr/22

Status: Closed
Project: Documentation
Component/s: manual, Server
Affects Version/s: None
Fix Version/s: 5.0.0, 5.3.0, 6.0.0, Server_Docs_20231030, Server_Docs_20231106, Server_Docs_20231105, Server_Docs_20231113

Type: Task Priority: Minor - P4
Reporter: Chris Kelly Assignee: Ian Fogelman
Resolution: Fixed Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
Related
related to DOCS-14135 Investigate changes in SERVER-53247: ... Closed
Participants:
Days since reply: 1 year, 42 weeks, 2 days ago
Epic Link: DOCSP-11702

 Description   

We ran into an issue where the documentation was a bit unclear about the downsides of using ReadConcern: Majority on PSA replicasets here.

 

Using ReadConcern: majority after doing writes with a WriteConcern value less than the majority can cause the user to receive stale data on all versions.

Additionally, using ReadConcern: Majority in PSA replicasets can cause immobilization of the entire deployment through increased cache pressure on failure of majority of the data-bearing nodes on older versions of MongoDB.  Stennie made some notes on a stackexchange post here

"The caveat on Read Concern Majority applies to a PSSAA deployment (or any where you can lose a majority of data-bearing voting nodes but still maintain a primary). With a PSSAA deployment degraded to PSxAA, the replica set can no longer acknowledge majority writes and there will be increased cache pressure which will degrade performance and eventually immobilise the deployment unless Read Concern Majority is disabled."



 Comments   
Comment by Githook User [ 18/Apr/22 ]

Author:

{'name': 'ianf-mongodb', 'email': '85948430+ianf-mongodb@users.noreply.github.com', 'username': 'ianf-mongodb'}

Message: Docs-15210 v5.0 (#973)

  • writeConcern -> write concern
  • CR 1 Feedback
  • update verbiage
  • remove duplicate your
  • remove ref
Comment by Githook User [ 18/Apr/22 ]

Author:

{'name': 'ianf-mongodb', 'email': '85948430+ianf-mongodb@users.noreply.github.com', 'username': 'ianf-mongodb'}

Message: Docs-15210 v5.0 (#973)

  • writeConcern -> write concern
  • CR 1 Feedback
  • update verbiage
  • remove duplicate your
  • remove ref
Comment by Githook User [ 18/Apr/22 ]

Author:

{'name': 'ianf-mongodb', 'email': '85948430+ianf-mongodb@users.noreply.github.com', 'username': 'ianf-mongodb'}

Message: Docs-15210 v5.3 (#972)

  • writeConcern -> write concern
  • CR 1 Feedback
  • update verbiage
  • remove duplicate your
  • remove ref
Comment by Githook User [ 18/Apr/22 ]

Author:

{'name': 'ianf-mongodb', 'email': '85948430+ianf-mongodb@users.noreply.github.com', 'username': 'ianf-mongodb'}

Message: Docs-15210 v5.3 (#972)

  • writeConcern -> write concern
  • CR 1 Feedback
  • update verbiage
  • remove duplicate your
  • remove ref
Comment by Githook User [ 18/Apr/22 ]

Author:

{'name': 'ianf-mongodb', 'email': '85948430+ianf-mongodb@users.noreply.github.com', 'username': 'ianf-mongodb'}

Message: DOCS-15210 add readConcern mention (#923)

  • writeConcern -> write concern
  • CR 1 Feedback
  • update verbiage
  • remove duplicate your
  • remove ref
Comment by Chris Kelly [ 05/Apr/22 ]

Hi Joseph,

I think I miscommunicated the exact issue. Specifically, it is not the enableMajorityReadConcern that is the issue, but rather actually using the read concern "majority" in a PSA replica set. In the case referenced in HELP-32506, it is not actually poor performance that is the issue - it is the fact the user attempts to query with readConcernlevel: "majority" on a write to the replicaset that uses w:1, and receives stale results.

In this case, we request a note to be made about using readConcern: majority, advising against using a global default readConcern of majority if writeConcern is less than the size of the majority (1 is implicit default), as there is a chance of not being able to read data back to the user that isn't fully replicated. Because of the implicit default writeConcern of 1, and enableMajorityReadConcern enabled by default in MongoDB 5.0 and above, this is a pitfall that can adversely affect those using PSA replica sets and may not be very obvious to troubleshoot, experienced in HELP-29194.

Comment by Joseph Dougherty [ 04/Apr/22 ]

I'm not sure there's much point in going into too much detail here since enableMajorityReadConcern has one setting (`true`) and cannot be set to false in v5.0 and later.

Would it help to point users who want add'l info to the Mitigate Performance Issues with PSA Replica Set page?

Generated at Thu Feb 08 08:12:17 UTC 2024 using Jira 9.7.1#970001-sha1:2222b88b221c4928ef0de3161136cc90c8356a66.