[jira] [Created] (KAFKA-1155) Kafka server can miss zookeeper watches during long zkclient callbacks

["Neha Narkhede (JIRA)" <ji...@apache.org>]

Neha Narkhede created KAFKA-1155:
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             Summary: Kafka server can miss zookeeper watches during long zkclient callbacks
                 Key: KAFKA-1155
                 URL: https://issues.apache.org/jira/browse/KAFKA-1155
             Project: Kafka
          Issue Type: Bug
          Components: controller
    Affects Versions: 0.8, 0.8.1
            Reporter: Neha Narkhede
            Assignee: Neha Narkhede
            Priority: Critical


On getting a zookeeper watch, zkclient invokes the blocking user callback and only re-registers the watch after the callback returns. This leaves a possibly large window of time when Kafka has not registered for watches on the desired zookeeper paths and hence can miss important state changes (on the controller). In any case, it is worth noting that even though zookeeper has a read-and-set-watch API, there can always be a window of time between the watch being fired, the callback and the read-and-set-watch API call. Due to the zkclient wrapper, it is difficult to handle this properly in the Kafka code unless we directly use the zookeeper client. One way of getting around this issue is to use timestamps on the paths and when a watch fires, check if the timestamp in zk is different from the one in the callback handler.



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