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From "Vishal K (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ZOOKEEPER-900) FLE implementation should be improved to use non-blocking sockets
Date Fri, 05 Nov 2010 14:02:45 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-900?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12928590#action_12928590

Vishal K commented on ZOOKEEPER-900:

Hi Flavio,

Thanks for your feedback. I will do the code changes.

For point 2 above, I was referring to the code that deletes the SenderWorker and ReceiveWorker
pair after receiving a connect request. I was concerned that a peer might send frequent connect
request before to the remote peer before the remote peer can initiate connection back. But
I think the                 Notification n = recvqueue.poll(notTimeout,  TimeUnit.MILLISECONDS);
in lookForLeader will prevent this scenario. Also, this won't be a concern if we decide to
remove the part that kills the pair for each connect.

I am also thinking of adding a sanity check that will accept connections only from peers that
are not listed in the zoo.cfg file or OBSERVER_ID.
I have not used observes so far. Can you please explain why a node will use OBSERVER_ID instead
of its sid? In particular, I am referring to the following code in QuorumCnxManager:
            // Read server id
            sid = Long.valueOf(msgBuffer.getLong());
            if(sid == QuorumPeer.OBSERVER_ID){
                 * Choose identifier at random. We need a value to identify
                 * the connection.
                sid = observerCounter--;
                LOG.info("Setting arbitrary identifier to observer: " + sid);

> FLE implementation should be improved to use non-blocking sockets
> -----------------------------------------------------------------
>                 Key: ZOOKEEPER-900
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-900
>             Project: Zookeeper
>          Issue Type: Bug
>            Reporter: Vishal K
>            Assignee: Flavio Junqueira
>            Priority: Critical
> From earlier email exchanges:
> 1. Blocking connects and accepts:
> a) The first problem is in manager.toSend(). This invokes connectOne(), which does a
blocking connect. While testing, I changed the code so that connectOne() starts a new thread
called AsyncConnct(). AsyncConnect.run() does a socketChannel.connect(). After starting AsyncConnect,
connectOne starts a timer. connectOne continues with normal operations if the connection is
established before the timer expires, otherwise, when the timer expires it interrupts AsyncConnect()
thread and returns. In this way, I can have an upper bound on the amount of time we need to
wait for connect to succeed. Of course, this was a quick fix for my testing. Ideally, we should
use Selector to do non-blocking connects/accepts. I am planning to do that later once we at
least have a quick fix for the problem and consensus from others for the real fix (this problem
is big blocker for us). Note that it is OK to do blocking IO in SenderWorker and RecvWorker
threads since they block IO to the respective !
> b) The blocking IO problem is not just restricted to connectOne(), but also in receiveConnection().
The Listener thread calls receiveConnection() for each incoming connection request. receiveConnection
does blocking IO to get peer's info (s.read(msgBuffer)). Worse, it invokes connectOne() back
to the peer that had sent the connection request. All of this is happening from the Listener.
In short, if a peer fails after initiating a connection, the Listener thread won't be able
to accept connections from other peers, because it would be stuck in read() or connetOne().
Also the code has an inherent cycle. initiateConnection() and receiveConnection() will have
to be very carefully synchronized otherwise, we could run into deadlocks. This code is going
to be difficult to maintain/modify.
> Also see: https://issues.apache.org/jira/browse/ZOOKEEPER-822

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