Hi devs, It is a long time issue that OpCode.check can cause client disconnection and server unusable or even database corruption. It has been reported twice in ZOOKEEPER-2623[1] and ZOOKEEPER-4680[2]. Both are reported by third-party client authors.
I opened pr-1988[3](merged now) to make OpCode.check a pure read operation and return nothing to match OpResult.CheckResult in MultiResponse. But I am rethinking this when investigating ZOOKEEPER-4750[4] and wonder whether UNIMPLEMENTED is more appropriate for reasons: 1. OpCode.check was designed to function inside OpCode.multi but not individually. 2. We have OpCode.exists, it covers the OpCode.check behavior pr-1988[3] introduced. 3. It could confuse us when inspecting logs and metrics if we record operation statistics somewhere. This probably introduces a long term maintenance burden. So I am here to seek a consensus on this. I prefer to UNIMPLEMENTED even pr-1988[3] has been merged. @anmolnar might want to backport this to 3.8 and 3.9 series. I am supportive of backport. So I think we probably should align before backporting. Any thoughts ? [1]: https://issues.apache.org/jira/browse/ZOOKEEPER-2623 [2]: https://issues.apache.org/jira/browse/ZOOKEEPER-4680 [3]: https://github.com/apache/zookeeper/pull/1988 [4]: https://issues.apache.org/jira/browse/ZOOKEEPER-4750
