kfaraz opened a new pull request, #20247:
URL: https://github.com/apache/druid/pull/20247
### Description
When cloning historicals for a blue-green deployment using
`CoordinatorDynamicConfig.cloneServers`, it can often be difficult to determine
if the clones are already synced and can be safely terminated. This is
primarily because the source server keeps getting assigned new segments and the
clone target is always playing catch up.
The metric that has been used so far to determine if a clone is sufficiently
"synced" is the `segmentLoadsRemaining` returned by the `/cloneStatus` API, but
that number often fails to fall below any reasonable threshold thereby slowing
down the deployment.
### Changes
- Add a configurable `CloneSyncCriteria` to `CoordinatorDynamicConfig`
- The criteria is a function of segments "pending sync" during a coordinator
run i.e. segments which are already loaded on the source server but still
loading at the target server at the start of the run.
- If the criteria is met, update the state of the clone to `SYNCED` and emit
a metric
- In subsequent runs, it is possible for the state to go back to
`IN_PROGRESS` if the clone starts lagging behind again
<hr>
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- [ ] added integration tests.
- [ ] been tested in a test Druid cluster.
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