[GitHub] [pulsar] addisonj commented on pull request #14841: Enable TCP/IP keepalive by default for broker, proxy and client

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addisonj commented on pull request #14841:
URL: https://github.com/apache/pulsar/pull/14841#issuecomment-1077726407


   Thanks for starting this discussion Lari.
   
   In the bookkeeper protocol, we *do* rely on TCP level keep-alive as opposed to application level keep-alive... and by all accounts, we should just switch to an application level keep-alive instead.
   
   AFAICT, event if we were to enable TCP keep-alive, it likely is not going to have much of an impact because of the OS level defaults being what they are, which, imho, basically make it useless. See this link https://tldp.org/HOWTO/TCP-Keepalive-HOWTO/usingkeepalive.html for details, but essentially the default OS level settings are such that:
   
   * it only starts after a connection has already been active for 2 hours
   * it only pings every 75 seconds
   * it takes 9 failures to trigger a failed socket
   
   In summary it can take upwards of 11 minutes to time out a socket but only for connections active after 2 hours.
   
   So this, combined with the application level keep alive, I think makes it not that useful by default...
   
   All that said I am not against making it optional. The cost of TCP keep-alive is near zero, but for "power users" it can provide a redundant system against zombie sockets, though I suppose it is possible for some new bugs to surface if we get socket hang ups in other code paths, where right now a dead socket is most likely to be closed by the heartbeat failing.
   
   


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