Details
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Task
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Status: Open
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Major
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Resolution: Unresolved
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1.7.0
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None
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None
Description
The way how the replica replacement works in 3-4-3 v1 design scheme has a few corner cases for very specific run-time scenarios. That's due to the absence of the SUPERCEDES attribute, which is in the full 3-4-3 design proposal (option E), but not in 3-4-3 v1 design.
- Initial configuration is [ A(V:+:), B(V:+:), C(V:+:) ]
- A voter replica A marked with the REPLACE attribute: [ A(V:+:REPLACE=true), B(V:+:), C(V:+:) ]
- A non-voter replica X is added to replace replica A: [ A(V:+:REPLACE=true), B(V:+:), C(V:+:), X(N:+:PROMOTE=true) ]
- Replica B fails, so the system adds another non-voter replica Y to replace the failed replica: [ A(V:+:REPLACE=true), B(V:-:), C(V:+:), X(N:+:PROMOTE=true), Y(N:+:PROMOTE=true) ]
- After some time, before replica tablet copying is complete for either of two replicas X or Y, replica B is back, so the system evicts replica X: [ A(V:+:REPLACE=true), B(V:+:), C(V:+:), Y(N:+:PROMOTE=true) ]
- Eventually, replica Y completes copying the data, catches up with the leader and is promoted by the leader replica: [ A(V:+:REPLACE=true), B(V:+:), C(V:+:), Y(V:+:) ]
- Next step is removing replica A, so the result configuration is [ B(V:+:), C(V:+:), Y(V:+:) ] instead of the expected [ B(V:+:), C(V:+:), X(V:+:) ]
In this context, it's necessary to document that the 'target' replica is not the guaranteed destination of the replica move process, but just a pivot. Also, it make sense to update the CLI tool to accept only the source replica as an argument, where the target replica is selected by the system itself (if it's not so already).
Attachments
Issue Links
- is related to
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KUDU-2246 Make "kudu tablet change_config move_replica" more reliable
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- Open
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