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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 Thu, 11 Nov 2010 16:05:21 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-900?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12931056#action_12931056
] 

Vishal K commented on ZOOKEEPER-900:
------------------------------------

Hi Flavio,

I have a question regarding the logic that determines which connection
to retain if peer 1 and peer 2 decide to communicate with each other.

Suppose peer 1 connects to peer 2. It first sends its sid as a
challenge. Peer 2 reads the sid and determines whether to keep the
connection or initiate a connection back to peer 1. Both determine
that peer 2 should be the one initiating the connection to peer 1
since sid of peer2 > sid of peer1.  I am concerned that they both 
may not be able to maintain any connection since the handshake is
one-way.

In the current implementation, peer1 disconnects immediately after
writing the challenge to peer 2. It can happen that peer 2 may get a
ClosedChannelException before it reads the challenge from peer 1. As a
result, peer 2 will not initiate a connection to peer 1.

Is this a legitimate problem? If it is, how about we ask peer 2 to
send back a ACK after it reads the challenge. Peer 1 will do a timed
read() after writing a challenge to peer 2. This will hopefully give
peer 2 enough time to read the challenge and take appropriate
action. If peer 2 is really slow, peer 1 will timeout on the read
operation.

-Vishal

> 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 !
 peer.
> 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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