[erlang-questions] Question about Distributed Applications

[Ulf Wiger (TN/EAB)]

Chris Goffinet skrev:
> Hi
> 
> I was reading over:
> 
> http://www.erlang.org/doc/design_principles/distributed_applications.html
> 
> And saw that if you had 3 nodes (A,B,C), and A failed, B would take  
> over. If A came back, B would eventually shutdown and A would take  
> over. Is it possible at all to disable the takeover? So if A comes  
> back, as long as B is still running, nothing changes?

It is possible, but not exactly easy. (:

You can use application:permit(App, false) on A to tell the application
controller that you don't want it to start App there. Of course, you'd
have to do this early on, and you'd likely end up writing your own
simple cluster controller of sorts. One way to do this would be to
store a configuration in a mnesia table, and then start a process
early that checks the configuration and calls permit() accordingly.
It sounds simple enough, but the devil's in the details, as usual.

Another way to go about it is to in fact write your own distributed
application controller. To know how to do this, you pretty much
need to read the comments in application_controller.erl
One key function is application_controller:control_application(App).
This can only be called once per app (there is no un_control()),
so you need to make sure that the dist_ac is not used at the same
time.

A /very/ good idea is to spawn a process per app that becomes the
controlling process, and then send that process simple commands
like run/dont_run. The message passing interface to the OTP
application controller is described in the comments.

This second alternative is of course /very/ advanced. I'd personally
recommend using gen_leader for a cluster controller (that was partly
the original purpose for gen_leader). In fact, I and a few others are
slowly working on such a cluster controller component. Don't hold your
breath, though.

BR,
Ulf W