Hi:
I am engineer from China, I review kafka mirror latest impl, but I do
not figure out the reason why PartitionState update like that:
```
// true if we should emit an offset sync
boolean update(long upstreamOffset, long downstreamOffset) {
boolean shouldSyncOffsets = false;
long upstreamStep = upstreamOffset - lastSyncUpstreamOffset;
long downstreamTargetOffset = lastSyncDownstreamOffset + upstreamStep;
if (lastSyncDownstreamOffset == -1L
|| downstreamOffset - downstreamTargetOffset >= maxOffsetLag
|| upstreamOffset - previousUpstreamOffset != 1L
|| downstreamOffset < previousDownstreamOffset) {
lastSyncUpstreamOffset = upstreamOffset;
lastSyncDownstreamOffset = downstreamOffset;
shouldSyncOffsets = true;
}
previousUpstreamOffset = upstreamOffset;
previousDownstreamOffset = downstreamOffset;
return shouldSyncOffsets;
}
}
```
I can not know why the condition is like that.
1. lastSyncDownstreamOffset == -1L: never sync, so call sync method
2. downstreamOffset - downstreamTargetOffset >= maxOffsetLag: offset is not
accurate, so sync. but why use maxOffsetLag? why not >0: meaning not
accurate
3. upstreamOffset - previousUpstreamOffset != 1L: meaning why?
4. downstreamOffset < previousDownstreamOffset： meaning why?
