[DOCS-12471] Docs for SERVER-35651: Arbiters do not track FCV changes Created: 12/Feb/19  Updated: 13/Nov/23  Resolved: 08/Aug/19

Status: Closed
Project: Documentation
Component/s: manual, Server
Affects Version/s: None
Fix Version/s: 4.1.8, Server_Docs_20231030, Server_Docs_20231106, Server_Docs_20231105, Server_Docs_20231113

Type: Task Priority: Major - P3
Reporter: Kay Kim (Inactive) Assignee: Ravind Kumar (Inactive)
Resolution: Done Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
Documented
documents SERVER-35651 Arbiters do not track FCV changes Closed
Related
is related to DOCS-13029 Note that arbiters will always return... Closed
Participants:
Days since reply: 4 years, 26 weeks, 6 days ago
Epic Link: DOCS: 4.2 Server/Tools

 Description   

Description

Description:

4.2 binaries running as arbiters will not downgrade their data files. Assuming WT bumps the file version format to a value that 4.0 does not understanding, downgrading binaries will require deleting the local data files.

This should not cause additional steps for the user. When the downgraded binary comes up and learns it should become an arbiter, it will seamlessly transition itself into the role without requiring the previous data files.

Engineering Ticket Description:

Because arbiters do not track FCV (since they do not follow the oplog and thus do not currently know when it is upgraded), the WiredTiger data format for arbiters is stuck on 2.9 (the version associated with MongoDB 3.2). We must keep dragging along the parsing code and associated test code for that version for each new release of MongoDB, indefinitely.

Some ways to fix this would be:

  • upgrade arbiters to the 4.2 data format immediately, the first time a 4.2 binary is run, and change the downgrade instructions to delete all arbiter datafiles on downgrade.
  • change the way FCV changes are propagated so arbiters hear about them.
  • deprecate and remove support for non-replicating arbiters.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)



 Comments   
Comment by Githook User [ 08/Aug/19 ]

Author:

{'name': 'rk-mongo', 'email': 'ravind.kumar@mongodb.com'}

Message: DOCS-12471: Arbiters Downgrade Requires Clearing Data Directory
Branch: master
https://github.com/mongodb/docs/commit/de9e3a9943fff3c6fc2236fe21345f2eaf50748e

Comment by Ravind Kumar (Inactive) [ 08/Aug/19 ]

Note: At this time it doesn't seem like the WT data version changes are in (4.2.0-rc5). However, the change in logic stands such that if the data version does change, there will be issues on downgrade.

As of such, moving forward with updating the upgrade/downgrade/compat sections w.r.t. arbiters to explicitly guide users to nuking the local DB.

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