Vishal K updated ZOOKEEPER-900:

    Attachment: ZOOKEEPER-900.patch1

There are two enhancements that I am working on for QuorumCnxManager.

1. QCM uses blocking IO for communicating with other peers. It does
not set a timeout for network read/write operations. SO_TIMEOUT does
not work with SocketChannel.

2. Incoming requests are processed one at a time. As a result, if QCM is
processing a connection from a peer and that peer fails, then requests
from other peers won't be processed. Even if we add timeout to
read/write calls, other peers will be blocked for that amount of
time. I had proposed a change in my earlier post for this part (see
above). I am working on a fix.

The attached patch addresses the first problem. Earlier, QCM used
SocketChannels. Now it uses DataInputStream/DataOutputStream, which
will blocki only until SO_TIMEOUT expires.

There are also some formatting changes done automatically by my editor
according to Java coding standards. So some of the changes are just

I have tested this change by creating a 3 node cluster and rebooting
leader/follower several times. The patch also includes a simple test.

Please let me know your comments.

> 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: Vishal K
>            Priority: Critical
>             Fix For: 3.4.0
>         Attachments: ZOOKEEPER-900.patch1
> 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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