[erlang-questions] Terminate process if it is slow at receiving messages

[Michael Truog]

Polling the message_queue_len of an Erlang pid is sort of like trying to catch a falling knife, and it is wasteful, processing time-wise.  If you could set a limit on the message queue size of an Erlang pid, that might help, but it doesn't really help you handle error conditions.  The best solution is to queue the messages you care about, within the Erlang pid's data, if they can't be handled immediately, so it becomes a queueing problem.

To handle slow clients, you just give each client a timeout value.  The OTP/Erlang source code doesn't decrement timeout values as time elapses, so it is possible that a timeout doesn't terminate a message call path in a way consistent with the timeout value.  The top-level function call throws an exception, but other pids may continue to execute despite the timeout expiration.  Often, people don't care about the timeout value not handling cumulative delay.  If you do care about handling cumulative delay, you should look at my open source project here http://cloudi.org , since CloudI services handle the timeout decrements and the queueing (if you would want the most accurate timeout adjustments possible, there are CloudI service options for that here: https://github.com/okeuday/CloudI/blob/master/src/lib/cloudi_core/src/cloudi_configuration.hrl#L78).

On 09/20/2013 08:08 AM, akonsu wrote:
> Hello,
>
> I have posted this question to stackoverflow (http://stackoverflow.com/questions/18895614/terminate-process-if-it-is-slow-at-receiving-messages) and received one helpful response, but I am still not clear about the right way to do it.
>
>  I also have seen this thread: http://erlang.org/pipermail/erlang-questions/2011-June/059335.html, which is somewhat similar but it still did not hep me.
>
> I would appreciate more help. Here is the question:
>
>
>
> I have a data source process that sends messages to worker processes. To keep memory consumption under control, I need to terminate the workers that are slow retrieving their messages from their mailboxes.
>
> I am new to Erlang, I would appreciate any pointers. If this is difficult to accomplish with Erlang messages, maybe I can use sockets? If so, are there examples?
>
> EDIT:
>
> I have a registered process that reads from the web and generates a lot of data. It sends these data to all the "subscribed" processes using Erlang messages. For each particular piece of data, it sends the same message to all subscribers.
>
> I also have a web server that streams the data that the registered process reads. So, when a http client connects, the web server creates a process and this process subscribes to the registered process and starts receiving its messages.
>
> The registered process uses monitors to monitor subscribers. The subscribers are controlled by the web server, and when a connection is closed, the process that were serving this connection, dies.
>
> There is no acknowledgement, that is, subscribers do not respond when a message is sent to them. Although I can program them this way, but I think it is too much traffic.
>
> Basically I want to close the connection if a http client is too slow.
>
>
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