[jira] [Created] (ZOOKEEPER-3315) Exceptions in callbacks should be handlable by the application

["Steven McDonald (JIRA)" <ji...@apache.org>]

Steven McDonald created ZOOKEEPER-3315:
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             Summary: Exceptions in callbacks should be handlable by the application
                 Key: ZOOKEEPER-3315
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3315
             Project: ZooKeeper
          Issue Type: Improvement
            Reporter: Steven McDonald
         Attachments: ExceptionTest.java

Hi,

In [KAFKA-7898|https://issues.apache.org/jira/browse/KAFKA-7898], a {{NullPointerException}} in a {{MultiCallback}} caused a Kafka cluster to become unhealthy in such a way that manual intervention was needed to recover. The cause of this particular {{NullPointerException}} is fixed in Kafka 2.2.x (with a proposed documentation update in [ZOOKEEPER-3314|https://issues.apache.org/jira/projects/ZOOKEEPER/issues/ZOOKEEPER-3314]), but I am interested in improving the resiliency of Kafka (and by extension the Zookeeper client library) against such bugs.

Quoting the stack trace from KAFKA-7898:

{code}
[2019-02-05 14:28:12,525] ERROR Caught unexpected throwable (org.apache.zookeeper.ClientCnxn)
java.lang.NullPointerException
at kafka.zookeeper.ZooKeeperClient$$anon$8.processResult(ZooKeeperClient.scala:217)
at org.apache.zookeeper.ClientCnxn$EventThread.processEvent(ClientCnxn.java:633)
at org.apache.zookeeper.ClientCnxn$EventThread.run(ClientCnxn.java:508)
{code}

The "caught unexpected throwable" message comes from [within the Zookeeper client library|https://github.com/apache/zookeeper/blob/release-3.4.13/src/java/main/org/apache/zookeeper/ClientCnxn.java#L641]. I think that try/catch is pointless, because removing it causes the message to instead be logged [here|https://github.com/apache/zookeeper/blob/release-3.4.13/src/java/main/org/apache/zookeeper/server/ZooKeeperThread.java#L60], with no discernable change in behaviour otherwise. Explicitly exiting the {{EventThread}} when this happens does not help (I don't think it gets restarted).

I suspect the best approach here might be to allow the application to register a callback to notify it of unhandlable exceptions within the Zookeeper library, since Zookeeper has no way of knowing what approach makes sense for the application. Of course, this is already technically possible in this case by having the application catch all exceptions in every callback, but that doesn't seem very practical.

This is especially problematic with distributed applications, since they are generally designed to tolerate the loss of a node, so it is preferable to have the application be allowed to terminate itself rather than risk inconsistent state.

I am attaching a simple Zookeeper client which does nothing except throw a {{NullPointerException}} as soon as it receives a callback, to illustrate the problem. Running this results in:

{code}
232 [main-EventThread] ERROR org.apache.zookeeper.ClientCnxn  - Error while calling watcher 
java.lang.NullPointerException
        at ExceptionTest.process(ExceptionTest.java:31)
        at org.apache.zookeeper.ClientCnxn$EventThread.processEvent(ClientCnxn.java:539)
        at org.apache.zookeeper.ClientCnxn$EventThread.run(ClientCnxn.java:514)
{code}

This comes from [here|https://github.com/apache/zookeeper/blob/7256d01a26412cd35a46edab6de9ac8c5adf5bb3/zookeeper-server/src/main/java/org/apache/zookeeper/ClientCnxn.java#L541], which simply logs the occurrence but provides no way for my application to handle the failure.



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