Hi All, I had posted this message as a comment for ZOOKEEPER-822. I thought it might be a good idea to give a wider attention so that it will be easier to collect feedback.
I found few problems in the FLE implementation while debugging for: https://issues.apache.org/jira/browse/ZOOKEEPER-822. Following the email below might require some background. If necessary, please browse the JIRA. I have a patch for 1. a) and 2). I will send them out soon. 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. 2. Buggy senderWorkerMap handling: The code that manages senderWorkerMap is very buggy. It is causing multiple election rounds. While debugging I found that sometimes after FLE a node will have its sendWorkerMap empty even if it has SenderWorker and RecvWorker threads for each peer. a) The receiveConnection() method calls the finish() method, which removes an entry from the map. Additionally, the thread itself calls finish() which could remove the newly added entry from the map. In short, receiveConnection is causing the exact condition that you mentioned above. b) Apart from the bug in finish(), receiveConnection is making an entry in senderWorkerMap at the wrong place. Here's the buggy code: SendWorker vsw = senderWorkerMap.get(sid); senderWorkerMap.put(sid, sw); if(vsw != null) vsw.finish(); It makes an entry for the new thread and then calls finish, which causes the new thread to be removed from the Map. The old thread will also get terminated since finish() will interrupt the thread. 3. Race condition in receiveConnection and initiateConnection: *In theory*, two peers can keep disconnecting each other's connection. Example: T0: Peer 0 initiates a connection (request 1) T1: Peer 1 receives connection from peer 0 T2: Peer 1 calls receiveConnection() T2: Peer 0 closes connection to Peer 1 because its ID is lower. T3: Peer 0 re-initiates connection to Peer 1 from manger.toSend() (request 2) T3: Peer 1 terminates older connection to peer 0 T4: Peer 1 calls connectOne() which starts new sendWorker threads for peer 0 T5: Peer 1 kills connection created in T3 because it receives another (request 2) connect request from 0 The problem here is that while Peer 0 is accepting a connection from Peer 1 it can also be initiating a connection to Peer 1. So if they hit the right frequencies they could sit in a connect/disconnect loop and cause multiple rounds of leader election. I think the cause here is again blocking connects()/accepts(). A peer starts to take action (to kill existing threads and start new threads) as soon as a connection is established at the* *TCP level. That is, it does not give us any control to synchronized connect and accepts. We could use non-blocking connects and accepts. This will allow us to a) tell a thread to not initiate a connection because the listener is about to accept a connection from the remote peer (use isAcceptable() and isConnectable()methods of SelectionKey) and b) prevent a thread from initiating multiple connect request to the same peer. It will simplify synchronization. Any thoughts? -Vishal
