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From afine <...@git.apache.org>
Subject [GitHub] zookeeper pull request #411: ZOOKEEPER-2684 Fix a crashing bug in the mixed ...
Date Tue, 31 Oct 2017 20:20:12 GMT
Github user afine commented on a diff in the pull request:

    https://github.com/apache/zookeeper/pull/411#discussion_r148115810
  
    --- Diff: src/java/main/org/apache/zookeeper/server/quorum/CommitProcessor.java ---
    @@ -246,33 +246,51 @@ public void run() {
                         }
     
                         /*
    -                     * Check if request is pending, if so, update it with the
    -                     * committed info
    +                     * Check if request is pending, if so, update it with the committed
info
                          */
                         LinkedList<Request> sessionQueue = pendingRequests
                                 .get(request.sessionId);
                         if (sessionQueue != null) {
                             // 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");
    +                            // TL;DR - we should not encounter this scenario often under
normal load.
    +                            // We pass the commit to the next processor and put the pending
back with a warning.
    +
    +                            // Generally, we can get commit requests that are not at
the queue head after
    +                            // a session moved (see ZOOKEEPER-2684). Let's denote the
previous server of the session
    +                            // with A, and the server that the session moved to with
B (keep in mind that it is
    +                            // possible that the session already moved from B to a new
server C, and maybe C=A).
    +                            // 1. If request.cxid < topPending.cxid : this means that
the session requested this update
    +                            // from A, then moved to B (i.e., which is us), and now B
receives the commit
    +                            // for the update after the session already performed several
operations in B
    +                            // (and therefore its cxid is higher than that old request).
    +                            // 2. If request.cxid > topPending.cxid : this means that
the session requested an updated
    +                            // from B with cxid that is bigger than the one we know therefore
in this case we
    +                            // are A, and we lost the connection to the session. Given
that we are waiting for a commit
    +                            // for that update, it means that we already sent the request
to the leader and it will
    +                            // be committed at some point (in this case the order of
cxid won't follow zxid, since zxid
    +                            // is an increasing order). It is not safe for us to delete
the session's queue at this
    +                            // point, since it is possible that the session has newer
requests in it after it moved
    +                            // back to us. We just leave the queue as it is, and once
the commit arrives (for the old
    +                            // request), the finalRequestProcessor will see a closed
cnxn handle, and just won't send a
    +                            // response.
    +                            // Also note that we don't have a local session, therefore
we treat the request
    +                            // like any other commit for a remote request, i.e., we perform
the update without sending
    +                            // a response.
    +
    +                            LOG.warn("Got request " + request +
    +                                    " but we are expecting request " + topPending);
    +                            sessionQueue.addFirst(topPending);
    +                        } else {
    +                            // We want to send to the next processor our version of the
request,
    +                            // since it contains the session information that is needed
    +                            // for post update processing (e.g., using request.cnxn we
send a response to the client).
    +                            topPending.setHdr(request.getHdr());
    --- End diff --
    
    The explanation is really great. I was also hoping you could shed some light on this part
of the code. Why do we "want to send to the next processor our version of the request" and
why can we proceed in the other case if this is required here?


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