Brokenice0415 commented on code in PR #939:
URL: https://github.com/apache/ratis/pull/939#discussion_r1361478687
##########
ratis-grpc/src/main/java/org/apache/ratis/grpc/server/GrpcLogAppender.java:
##########
@@ -143,7 +143,12 @@ private void resetClient(AppendEntriesRequest request,
boolean onError) {
if (request != null && request.isHeartbeat()) {
return;
}
- getFollower().decreaseNextIndex(nextIndex);
+ // decrease next index
+ final long oldNextIndex = getFollower().getNextIndex();
+ final long matchIndex = getFollower().getMatchIndex();
+ getFollower().updateNextIndex(
+ Math.max(matchIndex + 1
+ , oldNextIndex <= 0L? oldNextIndex:
Math.min(oldNextIndex - 1, nextIndex)));
Review Comment:
@szetszwo, I agree that we should not update next index blindly.
The patch I wrote is according to the operations to next index in
`FollowerInfoImpl`, which aims to add a low bound `match index + 1` to the
decreased next index.
And I think I should call `setNextIndex` instead of `updateNextIndex` to
ensure the decrease.
```java
@Override
public void decreaseNextIndex(long newNextIndex) {
nextIndex.updateUnconditionally(old -> old <= 0L? old: Math.min(old - 1,
newNextIndex),
message -> info("decreaseNextIndex", message));
}
@Override
public void setNextIndex(long newNextIndex) {
nextIndex.updateUnconditionally(old -> newNextIndex >= 0 ? newNextIndex :
old,
message -> info("setNextIndex", message));
}
@Override
public void updateNextIndex(long newNextIndex) {
nextIndex.updateToMax(newNextIndex,
message -> debug("updateNextIndex", message));
}
```
The bug I describe happens in case below:
1. Leader 0 append nop log [0] and get consensus in cluster;
2. Now follower 1's next index is 1 and match index is 0.
3. Leader 0 send heartbeat to follower 1.
4. Follower 1 crashes before replying the heartbeat.
5. Leader 0 decrease 1's next index to match index 0 for assigning the new
next index by `min(old - 1, newNextIndex)`
```
2023-10-17 10:59:26 WARN GrpcLogAppender:122 -
1@group-6F7570313233->0-AppendLogResponseHandler: Failed appendEntries:
org.apache.ratis.thirdparty.io.grpc.StatusRuntimeException: UNAVAILABLE:
Network closed for unknown reason
2023-10-17 10:59:26 INFO FollowerInfo:51 - 1@group-6F7570313233->0:
nextIndex: updateUnconditionally 1 -> 0
```
6. Now next index is not larger than match index.
> since it got an error, it should try resending the same (previous log
index + 1) even if (previous log index + 1 <= match index).
Although resending the same request will be always right, which can only
bring some redundant request, it will be better to optimize it.
> One reason could be due to inconsistency; see getNextIndexForInconsistency.
In this case, the patch works the same before if using `setNextIndex`
instead of `updateNextIndex`.
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