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  1. Cassandra
  2. CASSANDRA-14358

Partitioned outbound internode TCP connections can occur when nodes restart



    • Bug
    • Status: Resolved
    • Normal
    • Resolution: Fixed
    • 4.0-alpha1, 4.0
    • Normal


      edit summary: This primarily impacts networks with stateful firewalls such as AWS. I'm working on a proper patch for trunk but unfortunately it relies on the Netty refactor in 4.0 so it will be hard to backport to previous versions. A workaround for earlier versions is to set the net.ipv4.tcp_retries2 sysctl to ~5. This can be done with the following:

      $ cat /etc/sysctl.d/20-cassandra-tuning.conf
      $ # Reload all sysctls
      $ sysctl --system

      Original Bug Report:

      I've been trying to debug nodes not being able to see each other during longer (~5 minute+) Cassandra restarts in 3.0.x and 2.1.x which can contribute to UnavailableExceptions during rolling restarts of 3.0.x and 2.1.x clusters for us. I think I finally have a lead. It appears that prior to trunk (with the awesome Netty refactor) we do not set socket connect timeouts on SSL connections (in 2.1.x, 3.0.x, or 3.11.x) nor do we set SO_TIMEOUT as far as I can tell on outbound connections either. I believe that this means that we could potentially block forever on connect or recv syscalls, and we could block forever on the SSL Handshake as well. I think that the OS will protect us somewhat (and that may be what's causing the eventual timeout) but I think that given the right network conditions our OutboundTCPConnection threads can just be stuck never making any progress until the OS intervenes.

      I have attached some logs of such a network partition during a rolling restart where an old node in the cluster has a completely foobarred OutboundTcpConnection for ~10 minutes before finally getting a java.net.SocketException: Connection timed out (Write failed) and immediately successfully reconnecting. I conclude that the old node is the problem because the new node (the one that restarted) is sending ECHOs to the old node, and the old node is sending ECHOs and REQUEST_RESPONSES to the new node's ECHOs, but the new node is never getting the ECHO's. This appears, to me, to indicate that the old node's OutboundTcpConnection thread is just stuck and can't make any forward progress. By the time we could notice this and slap TRACE logging on, the only thing we see is ~10 minutes later a SocketException inside writeConnected's flush and an immediate recovery. It is interesting to me that the exception happens in writeConnected and it's a connection timeout (and since we see Write failure I believe that this can't be a connection reset), because my understanding is that we should have a fully handshaked SSL connection at that point in the code.

      Current theory:

      1. "New" node restarts,  "Old" node calls newSocket
      2. Old node starts creating a new SSL socket 
      3. SSLSocket calls createSocket, which conveniently calls connect with a default timeout of "forever". We could hang here forever until the OS kills us.
      4. If we continue, we get to writeConnected which eventually calls flush on the output stream and also can hang forever. I think the probability is especially high when a node is restarting and is overwhelmed with SSL handshakes and such.

      I don't fully understand the attached traceback as it appears we are getting a Connection Timeout from a send failure (my understanding is you can only get a connection timeout prior to a send), but I think it's reasonable that we have a timeout configuration issue. I'd like to try to make Cassandra robust to networking issues like this via maybe:

      1. Change the SSLSocket getSocket methods to provide connection timeouts of 2s (equivalent to trunk's timeout)
      2. Appropriately set recv timeouts via SO_TIMEOUT, maybe something like 2 minutes (in old versions via setSoTimeout, in trunk via SO_TIMEOUT
      3. Since we can't set send timeouts afaik (thanks java) maybe we can have some kind of watchdog that ensures OutboundTcpConnection is making progress in its queue and if it doesn't make any progress for ~30s-1m, forces a disconnect.

      If anyone has insight or suggestions, I'd be grateful. I am going to rule out if this is keepalive duration by setting tcp_keepalive_probes to like 1 and maybe tcp_retries2 to like 8 get more information about the state of the tcp connections the next time this happens. It's a very rare bug and when it does happen I only have 10 minutes to jump on the nodes and fix it before it fixes itself so I'll do my best.


        1. 10 Minute Partition.pdf
          127 kB
          Joey Lynch

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              jolynch Joey Lynch
              jolynch Joey Lynch
              Joey Lynch
              Ariel Weisberg
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