Comments on: Using Supervisors to Keep ErlyBank Afloat

By: David Weldon

David Weldon — Sat, 04 Oct 2008 22:27:06 +0000

As I’m writing this I’m watching Kevin Smith’s “Erlang in Practice” episode 8. In it he claims that you need to explicitly trap exits in order for a supervisor to notice that something has gone wrong. In our example we would add:

init([]) ->
process_flag(trap_exit, true),

to eb_server.erl. Did Kevin get it wrong on this one?

By: Mitchell

Mitchell — Wed, 17 Sep 2008 15:46:01 +0000

Jeremy,

Right, this is a good question. The technique I use is to use the gen_event:add_sup_handler/3 method. Its not as simple as throwing something into a supervision tree but at least using that method your handler will receive messages if the event handler crashes or if the handler somehow went bad.

Using these messages you can reattach the event handler as soon as the event server re-registers itself. Although theoretically this would require a timer to check if the event server is up, in my project its always been back up almost instantaneously. But to be safe, you should implement a timer that tries adding itself to the event server every two seconds or so.

Also be sure if you do use this that you add some logic to make sure that it doesn’t retry to connect forever and that it caps out at some point

This is what I do but I haven’t seen any “official” word on how to do it so if any of the other readers has a better way of doing this, I’d greatly appreciate it!

By: Jeremy

Jeremy — Wed, 17 Sep 2008 01:48:03 +0000

These tutorials have been immensely helpful. I appreciate them very much. I do have one question concerning the supervisor and the event manager. Using the supervisor, how should event handlers be registered? Particularly, how should the eb_withdrawal_handler be registered? Obviously, it can be done in the repl, but how would this be done in a running system?

By: Mitchell

Mitchell — Sun, 14 Sep 2008 02:40:14 +0000

I fixed the error you caught! Awesome, thanks And I’m glad the series has been helpful!

By: Jonathon Mah

Jonathon Mah — Sat, 13 Sep 2008 23:00:20 +0000

Thanks again, Mitchell! I’m really taking a lot away from your series.

Errata: The event manager child spec uses “[eb_event_manager]” in the first occurrence, but “dynamic” in init/1.