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https://issues.apache.org/jira/browse/ZOOKEEPER-900?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12927518#action_12927518
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Vishal K commented on ZOOKEEPER-900:
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Hi Flavio,
I have a suggestion for changing the blocking IO code in QuorumCnxManager. It
keeps the current code structure and requires a small amount of changes. I am
not sure if these comments should go in ZOOKEEPER-901. ZOOKEEPER-901 is
probably addressing netty as well. Please feel free to close this JIRA if you
intend to make all the changes as a part of ZOOKEEPER-901.
Basically we jusy need to move parts of initiateConnection and
receiveConnection to SenderWorker and ReceiveWorker.
A. Current flow for receiving connection:
1. accept connection in Listener.run()
2. receiveConnection()
- Read remote server's ID
- Take action based on my ID and remote server's ID (disconnect and
reconnect if my ID is > remote server's ID).
- kill current set of SenderWorker and ReciveWorker threads
- Start a new pair
B Current flow for initiating connection:
1. In connectOne(), connect if not already connected. else return.
2. send my ID to the remote server
3. if my ID < remote server disconnect and return
4. if my ID > remote server
- kill current set of SenderWorker and ReceiveWorkter threads for the
remote server
- Start a new pair
Proposed changes:
Move the code that performs any blocking IO in SenderWorker and ReceiveWorker.
A. Proposed flow for receiving connection:
1. accept connection in Listener.run()
2. receiveConnection()
- kill current set of SenderWorker and ReciveWorker threads
- Start a new pair
Proposed changed to SenderWorker:
- Read remote server's ID
- Take action based on my ID and remote server's ID (disconnect and
reconnect if my ID is > remote server's ID).
- Proceed to normal operation
B Proposed flow for initiating connection:
1. in connectOne(), return if already connected
2. Start a new SenderWorker and ReceiveWorker pair
2. In SenderWorker
- connect to remote server
- write my ID
- if my ID < remote server disconnect and return (shutdown the pair).
- Proceed to normal operation
Questions:
- In QuorumCnxManager, is it necessary to kill the current pair and restart a
new one every time we receive a connect request?
- In receiveConnection we may choose to reject an accepted connection if a
thread in
SenderWorker is in the process of connecting. Otherwise a server with ID <
remote server may keep sending frequent connect request that will result in the
remote server closing connections for this peer. But I think we add a delay
before sending notifications, which might be good enough to prevent this
problem.
Let me know what you think about this. I can also help with the implementation.
-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: 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 !
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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