[erlang-bugs] dist_ac deadlock

Decker, Nils <>
Tue Nov 27 13:37:55 CET 2007

Hello everyone,

is sent this message last week, but it does not appear in the mailinglist archive. Please excuse possible duplicates.

I have found a deadlock in the dist_ac process during a failover of a distributed application on 4 nodes.

I am using Release R11B2 (debian stable).
R11B5 (11.b.5dfsg-8 debian testing) shows the same behaviour.

The situation:
  - 2 distributed applications defined to run on 4 nodes
    with the same priority.
    {distributed, [
      {cmm_adm, [{'', ... ''}]},
      {cmm_db,  [{'', ... ''}]}
  - takeover to node4 using application:takeover(cmm_db, permanent), 
    same for cmm_adm
  - kill this node using ^C a


- The application is started on node1
  - application:info() shows the cmm_db application as running
    on node1
  - application:info() on the other nodes show the applications
    running on the killed node
  - dist_ac:info() times out

I generated a crashdump on node 1 - 3. 

dist_ac (0.17.0) on node1:
  - Program counter: gen_server:loop
  - Msg Queue Length: 0
  - state of the application ( from stack ): local

dist_ac (0.17.0) on node2:
  - Program counter: dist_ac:collect_answers/4
  - Msg Queue Length: 5
  - state of the application ( from stack ):

dist_ac (0.17.0) on node3:
  - Program counter: dist_ac:collect_answers/4
  - Msg Queue Length: 6
  - state of the application ( from stack ):

The comment before the function dist_ac:collect_answers/4 states that dist_ac must always be prepared to handle dist_ac_weight messages. Yet collect_answers does not handle this message.

This seems to be a problem, if more than two nodes have the same priority for an application.

I have changed my application to have different priorities on the nodes.
(list of nodes instead of tuple with nodes) This works but has the drawback that a takeover is performed when the failed node comes back online. (forcing the re-initialisation of the communication to a number of external systems)

I could provide crashdumps or try to reproduce the error with a minimal application if needed.

  Nils Decker

- MCI alles aus einer Hand -

Nils Decker

Studio Hamburg Media Consult International (MCI) GmbH Jenfelder Allee 80
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