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Use Cases

Concurrency support (http://issues.apache.org/jira/browse/HIVE-1293^{Image Removed}) is a must in databases and their use cases are well understood. At a minimum, we want to support concurrent readers and writers whenever possible. It would be useful to add a mechanism to discover the current locks which have been acquired. There is no immediate requirement to add an API to explicitly acquire any locks, so all locks would be acquired implicitly.

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Two new configurable parameters will be added to decide the number of retries for the lock and the wait time between each retry. If the number of retries are really high, it can lead to a live lock. Look at ZooKeeper recipes (http://hadoop.apache.org/zookeeper/docs/r3.1.2/recipes.html#sc_recipes_Locks^{Image Removed}) to see how read/write locks can be implemented using the zookeeper apis. Note that instead of waiting, the lock request will be denied. The existing locks will be released, and all of them will be retried after the retry interval.

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