[jira] [Created] (NIFI-2185) Implement two-phase read for GET requests when replicated across the cluster

["Mark Payne (JIRA)" <ji...@apache.org>]

Mark Payne created NIFI-2185:
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             Summary: Implement two-phase read for GET requests when replicated across the cluster
                 Key: NIFI-2185
                 URL: https://issues.apache.org/jira/browse/NIFI-2185
             Project: Apache NiFi
          Issue Type: Task
          Components: Core Framework
    Affects Versions: 1.0.0
            Reporter: Mark Payne
            Assignee: Mark Payne
            Priority: Blocker
             Fix For: 1.0.0


With the new zero-master clustering paradigm, we can have a scenario where a GET request and a POST request occur 'at the same time' but the requests are replicated to the nodes in different orders. For instance, if I have a Process Group with ID 1234, it's possible that the group could be read via a GET and modified via a POST request simultaneously.

If we have a 3 node cluster, this can result in Nodes 1 and 2 servicing the GET request before the POST request, while Node 3 services the POST request before the GET request. As a result, Nodes 1 and 2 return one view of the Process Group while Node 3 returns a different view of the Process Group.

This wasn't an issue with the NCM because the NCM provided a locking mechanism that prevented any GET requests from being replicated while a POST/PUT/DELETE was also being replicated.

In the new clustering model, in order to implement the same type of logic, we need a two-phase read, where the first phase simply obtains a Read Lock, and the second phase handles the logic of creating the response for the client to consume and then unlocking the Read Lock.



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