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| PAWilson |
 Posted: Sat Jan 20, 2007 2:43 am |
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Joined: 20 Jan 2007
Posts: 1
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Hi,
I am just trying to get to grips with this intriguing language / platform. I've worked through the "Gen event behaviour demystified" article in the trapexit cookbook and have a question.
The handler_guard module is supposed to "upon receiving a {gen_event_EXIT,Handler,Reason} message from the event manager reinstall[] the event handler.
If this is the case, why does the handler_guard return {stop, ...}, which according to the manual calls Module:terminate/2 and shuts the module down.
I was expecting to see the handle_info reregister the handler somehow. Perhaps by calling init?
Not really sure *how* the code reregisters the event handler in the event manager process. Any clarification would be welcome.
Thank you
PAWilson |
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| anders_n |
| Posted: Mon Jan 22, 2007 6:58 pm |
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User
Joined: 28 Feb 2005
Posts: 155
Location: Saltillo, Mexico
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Hi
The problem with gen_event is that Your event_handler is not
running as a separate process, instead it is just one of possibly
many callback modules registered in one event manager. So when
an event_handler fails it is just removed by the event manager.
The "trick" with the handler_guard is that it creates a separate
process that has only one purpose, to receive the
{gen_event_EXIT,Handler,Reason} message and terminate.
The reason here is that normally in Erlang You have a supervisor
that is responsible for supervising processes and restart them
if they crash. So the supervisor that started the handler_guard
will restart it for You.
/Anders |
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