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From: mxm <git@git.apache.org>
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Subject: [GitHub] flink issue #2917: [FLINK-2821] use custom Akka build to listen on all inter...
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Message-Id: <20161202111245.7A3C8E04BB@git1-us-west.apache.org>
Date: Fri,  2 Dec 2016 11:12:45 +0000 (UTC)
archived-at: Fri, 02 Dec 2016 11:13:39 -0000

Github user mxm commented on the issue:

    https://github.com/apache/flink/pull/2917
  
    Thanks for checking out the code!
    
    >If different parts of the code or the JDK do a subtle change of behavior (i.e. resolve the InetAddress), then some nodes may have a hostname in the URL, others an address.
    
    Fair point. Let's remove InetAddress and use a String instead.
    
    >If two machines have a slightly different network configuration (especially concerning preferences to represent/encode IPv6 addresses) and the users set the IP address as the JobManager host, then they might create different Akka URLs and the machines cannot talk to each other again.
    
    A problem of the old method was that hostnames might resolve differently depending on the container context. So that method was not reliable either. Using IP addresses will work with the new method. They just have to be consistent on all node configurations. I don't think that is a problem because a configuration is usually created once and then copied over to all nodes.
    
    >I am a bit unsure how to proceed from here. Is there any way we can keep using IP addresses in the Akka URLs? Or does that just inherently not make sense with "dynamic hostnames" as they are used in container environments?
    
    For this PR to address the core problems of FLINK-2821, we have to avoid resolving the hostname because the IP address would represent the internal container address which may be unreachable from the outside; even if it were resolvable, Akka would drop the messages because of its exact URL match policy. Using the hostname which now acts as a purely logical address ultimately is a more reliable way across different network environments. We just have to make sure we document the new requirement that the JobManager address (JOB_MANAGER_IPC_ADDRESS) is consistent across all Flink cluster nodes.
     

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