[GitHub] zookeeper pull request #167: ZOOKEEPER-2684 commitProcessor does not crash w...

[fpj <gi...@git.apache.org>]

Github user fpj commented on a diff in the pull request:

    https://github.com/apache/zookeeper/pull/167#discussion_r121006265
  
    --- Diff: src/java/main/org/apache/zookeeper/server/quorum/CommitProcessor.java ---
    @@ -254,24 +254,23 @@ public void run() {
                             // If session queue != null, then it is also not empty.
                             Request topPending = sessionQueue.poll();
                             if (request.cxid != topPending.cxid) {
    -                            LOG.error(
    -                                    "Got cxid 0x"
    -                                            + Long.toHexString(request.cxid)
    -                                            + " expected 0x" + Long.toHexString(
    -                                                    topPending.cxid)
    -                                    + " for client session id "
    -                                    + Long.toHexString(request.sessionId));
    -                            throw new IOException("Error: unexpected cxid for"
    -                                    + "client session");
    +                            // we can get commit requests that is not at the queue head when 
    +                            // session moves (see ZOOKEEPER-2684). We will just pass the 
    +                            // commit to the next processor and put the pending back with
    +                            // a warning, we should not see this often under normal load
    +                            LOG.warn("Got request " + request + 
    +                                " but we are expecting request " + topPending);
    +                            sessionQueue.addFirst(topPending);
    +                        } else {                            
    --- End diff --
    
    Is it the case that for a given session, once we execute the else block once, executing the if block would be incorrect? If so, would it make sense to have a flag per session indicating that the else block has not been executed for the session? It might not even be a flag per session, but perhaps a set of session ids instead that we remove from once we execute the else block.   


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