Flavio Junqueira commented on ZOOKEEPER-900:

Hi Vishal, This is a good question. I'm actually assuming that the behavior of 
TCP is such that if I send a message and then close the channel properly 
(calling close()), due to the reliability and order guarantees of the 
connection, the message will get through before the connection closes. 
Essentially, I'm relying upon the TCP ACK to do exactly what you're proposing. 
However, it might be a good idea to make sure that the assumption is correct or 
if you know the answer already, just let me know. Overall I do agree that 
having an ACK is important.  

> 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
> 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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