34 lines
1.9 KiB
Markdown
34 lines
1.9 KiB
Markdown
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# Erlang, tcp sockets, and active true
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If you don't know erlang then [you're missing out](http://learnyousomeerlang.com/content).
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If you do know erlang, you've probably at some point done something with tcp sockets. Erlang's
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highly concurrent model of execution lends itself well to server programs where a high number
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of active connections is desired. Each thread can autonomously handle its single client,
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greatly simplifying the logic of the whole application while still retaining
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[great performance characteristics](http://www.metabrew.com/article/a-million-user-comet-application-with-mochiweb-part-1).
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# Background
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For an erlang thread which owns a single socket there are three different ways to receive data
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off of that socket. These all revolve around the `active` [setopts](http://www.erlang.org/doc/man/inet.html#setopts-2)
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flag. A socket can be set to one of:
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* `{active,false}` - All data must be obtained through [recv/2](http://www.erlang.org/doc/man/gen_tcp.html#recv-2)
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calls. This amounts to syncronous socket reading.
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* `{active,true}` - All data on the socket gets sent to the controlling thread as a normal erlang
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message. It is the thread's responsibility to keep up with the buffered data
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in the message queue. This amounts to asyncronous socket reading.
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* `{active,once}` - When set the socket is placed in `{active,true}` for a single packet. That
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is, once set the thread can expect a single message to be sent to when data
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comes in. To receive any more data off of the socket the socket must either
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be read from using [recv/2](http://www.erlang.org/doc/man/gen_tcp.html#recv-2)
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or be put in `{active,once}` or `{active,true}`.
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# Which to use?
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<Explanation of how other sources claim you should use active,once, and why>
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# Why not to use it
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