somandal opened a new pull request, #15468:
URL: https://github.com/apache/pinot/pull/15468
When running table rebalance, we need to specify the `minAvailableReplicas`
to keep up as part of the rebalance. Internally we track the number of replicas
based on the target assignment only which can be problematic for certain
scenarios.
For strict replica group based assignment, I found that the rebalance can
get into an infinite loop for the following conditions:
- Starting replication = 1
- Target replication = 3
- minAvailableReplicas = -1 (or 2) [basically higher than the starting
replication]
In the above scenario, when doing the assignment for the segments, we hit
the following condition:
```
if (availableInstances.size() >= minAvailableReplicas) {
...
} else {
// New assignment cannot be added, use the current instance
state map
nextAssignment.put(segmentName, currentInstanceStateMap);
return currentAvailableInstances;
}
```
Since `availableInstances.size()` is always 1 (since we start with just 1
replica), and `minAvailableReplicas` is 2 based on the configuration passed in
to rebalance, we never make progress and keep setting the current assignment
for such segments.
What I noticed is that for a few of the partitions it does move forward
initially, because of this block at the start:
```
if (currentAvailableInstances == null) {
// First segment assigned to these instances, use the new
assignment and update the available instances
nextAssignment.put(segmentName, assignment._instanceStateMap);
updateNumSegmentsToOffloadMap(numSegmentsToOffloadMap,
currentInstanceStateMap.keySet(), k);
return availableInstances;
} else {
```
This PR fixes the above by mandating that `minAvailableReplicas` is never
higher than the starting replication factor, and if detected that it is higher,
it is set to the starting replication factor. It also logs a warning when this
scenario is detected.
Testing: Added a test case to reproduce this scenario and verified that with
the fix the test passes
cc @Jackie-Jiang @xiangfu0 @klsince
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