[erlang-questions] Re: Distributed application takeover

[Ulf Wiger]

Hi Jeroen,

This is a bit tricky, and can be very confusing for someone trying to build their own cluster controller. :)

The stopping of the application is done by the local application_controller when it is told that the application is running somewhere else. In other words, dist_ac kindly forwards the information to the local AC, and it is the local AC that in this particular case takes responsibility for 'knowing' that the local instance should be stopped.

See application_controller:handle_application_started/3

https://github.com/erlang/otp/blob/OTP_R14B02/lib/kernel/src/application_controller.erl#L913

(The dirty deed actually happens on line 946).

The reason I know this, is that in the AXD 301, we ran multiple instances of some applications, distributed across several mated pairs - each instance having its own standby node. This can be done by writing an own distributed AC, but it has to be smart enough to know _when_ to forward the {started, Node} status to the local AC; if an instance was running locally - and _was supposed to_ do so (i.e. not involved in a takeover), the distributed AC had to suppress this message.

BR,
Ulf W

On 30 Mar 2011, at 14:11, Jeroen Koops wrote:

> Hi,
> 
> I'm trying to find out how the distributed application controller works internally. I'm especially interested in the implementation of an application takeover.
> 
> In case an application runs on node A, and is taken over by node B, what should happen is that it is first started on node B, so that there are two instances of the application running simultaneously for a brief period of time, and then stopped on node A.
> 
> However, I cannot figure out where this stopping happens in dist_ac.erl. If I understand correctly, this should happen in response to a ac_application_run message from the application_controller. This message is received by the dist_ac on node B, and a dist_ac_app_started message is then broadcast to the dist_acs on all connected nodes. The dist_ac of node A receives this message, notices that the application is still running locally, and decides to shut down the application on its own node -- at least that is what the comments say (dist_ac.erl, line 529):
> 
> %% Another node tookover from me; stop my application
> %% and update the running list.
> 
> But all I can see is that the dist_ac's list of applications is updated to indicate that the application is no longer running locally -- I cannot find where the application_controller is instructed to actually shutdown the application.
> 
> Can anyone point me in the right direction?
> 
> Thanks,
> 
> Jeroen _______________________________________________
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Ulf Wiger, CTO, Erlang Solutions, Ltd.
http://erlang-solutions.com



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