Alexey Serbin created KUDU-2247:
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Summary: Update doc on the 'kudu tablet change_config
move_replica' after 3-4-3 enabled by default
Key: KUDU-2247
URL: https://issues.apache.org/jira/browse/KUDU-2247
Project: Kudu
Issue Type: Task
Components: documentation
Affects Versions: 1.7
Reporter: Alexey Serbin
Assignee: Alexey Serbin
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).
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