[erlang-questions] What causes nodes to become disconnected/reconnected?

József Bérces jozsef.berces@REDACTED
Mon May 28 06:46:19 CEST 2012

Is there a way to detect from Erlang that windows just woke up from sleeping?

From: David Mercer [mailto:dmercer@REDACTED]
Sent: Friday, May 25, 2012 20:39
To: József Bérces; erlang-questions@REDACTED
Subject: RE: [erlang-questions] What causes nodes to become disconnected/reconnected?

These might be related, though.  You are right, my case does not occur due to the host going to sleep, but both my hosts are Windows, and perhaps something is happening - whether it be with the network, firewall, virus protection, or whatever - that is causing the nodes to get disconnected, and they do not properly reconnect when the temporary issue is resolved.

It might be the same thing in the Windows port of Erlang that prevents them from reconnecting after sleep that is preventing my nodes from reconnecting after something else.



From: erlang-questions-bounces@REDACTED [mailto:erlang-questions-bounces@REDACTED] On Behalf Of József Bérces
Sent: Friday, May 25, 2012 4:30 AM
To: erlang-questions@REDACTED
Subject: Re: [erlang-questions] What causes nodes to become disconnected/reconnected?

It is somewhat offtopic as I do not believe David experenced this, but I have similar disconnection when Windows goes to sleep. After the wake-up, the nodes do not see each other. Does anyone have any idea how to fix this?


From: erlang-questions-bounces@REDACTED<mailto:erlang-questions-bounces@REDACTED> [mailto:erlang-questions-bounces@REDACTED] On Behalf Of Martynas Pumputis
Sent: Friday, May 25, 2012 16:24
To: David Mercer
Cc: erlang-questions@REDACTED<mailto:erlang-questions@REDACTED>
Subject: Re: [erlang-questions] What causes nodes to become disconnected/reconnected?
Erlang doesn't detect net splits by itself. You could start looking at net_kernel:set_net_ticktime/2 (try to increase this value if your node is suffering from high load/traffic) and kernel/src/dist_utils.erl to get a grasp how erlang handles node connections.

On Thu, May 24, 2012 at 6:23 PM, David Mercer <dmercer@REDACTED<mailto:dmercer@REDACTED>> wrote:
(Yes, I am still working on my issue with a distributed application that keeps losing its connection to the other node.)

It is not a problem only when I have 2 nodes on the same host.  I was running the nodes on different hosts last night, and this morning the failover node had lost its connection to the main and so had started its own instance of the application.  Calling nodes() on the failover returned [].

Then I started a new node on the same host as the main (to see if it would restore the connections), and, yes, it did.  After starting the third node, nodes() now on the failover node returns a list of two nodes, the two on the main host.  However, the application on the failover node did not shut down, and so it is still running on both the main and failover nodes.

To summarize:

1.       Distributed application running on a node on host A ("main@REDACTED"), failover on node on host B ("failover@REDACTED").

2.       At some point, failover@REDACTED becomes disconnected from main@REDACTED, and the application starts on failover@REDACTED  Now there are two instances of the application running.

3.       From a network point of view, there is still (or again) a valid network connection between hosts A and B.  I can't say for sure if some network/firewall/other issue caused a temporary disconnect, but I can say that by the time I got in this morning, when the application was running on both nodes, there was a firm network connection between the two hosts.

4.       Calling nodes() on failover@REDACTED returns [].

5.       A new (failover) node was started on host A ("failover@REDACTED").  It does not start the application (which is correct, since it is already running, albeit on both other nodes instead of just one).

6.       Calling nodes() on failover@REDACTED now returns [failover@REDACTED,main@REDACTED].

7.       The application, however, is still running on failover@REDACTED, despite the fact that nodes/0 reports a connection to main@REDACTED

I don't need someone to diagnose this for me.  If someone could just educate me a little on how the connections work, how net splits are detected and nodes disconnected, etc., I might be able to take it from there.  Anyone know enough and have the time to type out a little blurb?  I can read source code, but having a little background knowledge would help put it into context for me.




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