| Author |
Message |
< Erlang ~ Its this scenario correct and fault proof ? |
| forcewill |
 Posted: Wed Feb 09, 2011 6:43 pm |
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Joined: 12 Feb 2010
Posts: 19
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Scenario:
"To have a gen server which will handle concurrent requests but limits the current amount of concurrent requests it is handling."
I have implemented like this
handle_cast(concurrent_request, #st{max_push = Max, cur_push = Max} ->
%%OK im at the treshold
{noreply, St};
handle_cast(concurrent_request, St = #st{cur_requests = Cur}) ->
Pid = proc_lib:spawn_link(fun() -> .... end),
erlang:monitor(process, Pid),
{noreply, St#st{cur_requests = Cur + 1}
And then in handle_info
handle_info(_Info, State = #state{cur_requests = Cur}) ->
{noreply, State#state{cur_requests = Cur - 1}}.
My question is if this implementation is bullet prof ? Im asking this because if it isn't my server may stop handling requests. |
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| rvirding |
| Posted: Fri Feb 11, 2011 12:04 am |
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Joined: 30 Aug 2006
Posts: 452
Location: Stockholm, Sweden
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I just put a code wrapper around your code make it easier to read.
Code:
handle_cast(concurrent_request, #st{max_push = Max, cur_push = Max} ->
%%OK im at the treshold
{noreply, St};
handle_cast(concurrent_request, St = #st{cur_requests = Cur}) ->
Pid = proc_lib:spawn_link(fun() -> .... end),
erlang:monitor(process, Pid),
{noreply, St#st{cur_requests = Cur + 1}}.
handle_info(_Info, State = #state{cur_requests = Cur}) ->
{noreply, State#state{cur_requests = Cur - 1}}.
I assume either cur_push or cur_requests is a typo otherwise you will be counting and testing different fields in the state.
The basic idea will work but there are details which must be worked out.
- You are linking, through proc_lib:spawn_link, to the worker processes, so if the gen_server is not trapping exits and a worker crashes then the gen_server and all the other workers will die. However, when a worker terminates normally then gen_server will not notice this. This is the standard error handling mechanism.
- You are both linking and monitoring the worker processes so if you are trapping exits you will receive two messages notifying you about this: one from the link, and one from the monitor.
- Why are you spawning the workers using proc_lib? There is nothing wrong with this but it does cost a little more, which might be unnecessary unless you need the extra functionality (for example an error log if they crash).
- It is very unsafe not to test in handle_info what message you are receiving, it might be an exit/monitor message from a worker. The safest would be to keep a list of current workers and test if it is the right exit/monitor message from a known worker.
- You need to decide how to handle errors in the both the gen_server and in the workers. If an error occurs do you crash everything and restart, or do you carry on bravely and hope for the best?
Those are the immediate points. |
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Robert Virding, Erlang Solutions Ltd. |
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| forcewill |
| Posted: Fri Feb 11, 2011 10:21 am |
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Joined: 12 Feb 2010
Posts: 19
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Thanks for the reply, its not a typo i use cur_requests to keep track of the number of "workers" and max_push as a configuration value to limit the max amount of workers
so that when cur_push equals max_push i dont allow any new worker to do its work.
I'm using proc_lib because sometime ago i read somewhere that it was the way to do it with OTP because it would get me a good log about whats happened if i don't use proc lib i just get a "Pid crashed at node blá blá" i don't know the context of that Pid, like what process spawned it right ? |
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| rvirding |
| Posted: Mon Feb 14, 2011 12:37 am |
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Joined: 30 Aug 2006
Posts: 452
Location: Stockholm, Sweden
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forcewill wrote: Thanks for the reply, its not a typo i use cur_requests to keep track of the number of "workers" and max_push as a configuration value to limit the max amount of workers
so that when cur_push equals max_push i dont allow any new worker to do its work.
I realised that max_push is a configuration value but I meant that while you increment cur_requests when you start a new "worker" you use cur_push to test if you have reached the limit of number of workers. In your example you don't show how cur_requests become cur_push. That is what I meant by counting and testing different values. |
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Robert Virding, Erlang Solutions Ltd. |
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| forcewill |
| Posted: Mon Feb 14, 2011 11:39 am |
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Joined: 12 Feb 2010
Posts: 19
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Oh yes sorry you are correct cur_push is a typo
i only have max_push and cur_requests . |
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| dongdongwu |
| Posted: Thu Sep 20, 2012 6:45 am |
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Joined: 19 Sep 2012
Posts: 236
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