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----
-title: "Erlang, tcp sockets, and active true"
-description: >-
- Using `{active:once}` isn't always the best way to handle connections.
----
-
-If you don't know erlang then [you're missing out][0]. If you do know erlang,
-you've probably at some point done something with tcp sockets. Erlang's highly
-concurrent model of execution lends itself well to server programs where a high
-number of active connections is desired. Each thread can autonomously handle its
-single client, greatly simplifying the logic of the whole application while
-still retaining [great performance characteristics][1].
-
-## Background
-
-For an erlang thread which owns a single socket there are three different ways
-to receive data off of that socket. These all revolve around the `active`
-[setopts][2] flag. A socket can be set to one of:
-
-* `{active,false}` - All data must be obtained through [recv/2][3] calls. This
- amounts to syncronous socket reading.
-
-* `{active,true}` - All data on the socket gets sent to the controlling thread
- as a normal erlang message. It is the thread's
- responsibility to keep up with the buffered data in the
- message queue. This amounts to asyncronous socket reading.
-
-* `{active,once}` - When set the socket is placed in `{active,true}` for a
- single packet. That is, once set the thread can expect a
- single message to be sent to when data comes in. To receive
- any more data off of the socket the socket must either be
- read from using [recv/2][3] or be put in `{active,once}` or
- `{active,true}`.
-
-## Which to use?
-
-Many (most?) tutorials advocate using `{active,once}` in your application
-\[0]\[1]\[2]. This has to do with usability and security. When in `{active,true}`
-it's possible for a client to flood the connection faster than the receiving
-process will process those messages, potentially eating up a lot of memory in
-the VM. However, if you want to be able to receive both tcp data messages as
-well as other messages from other erlang processes at the same time you can't
-use `{active,false}`. So `{active,once}` is generally preferred because it
-deals with both of these problems quite well.
-
-## Why not to use `{active,once}`
-
-Here's what your classic `{active,once}` enabled tcp socket implementation will
-probably look like:
-
-```erlang
--module(tcp_test).
--compile(export_all).
-
--define(TCP_OPTS, [
- binary,
- {packet, raw},
- {nodelay,true},
- {active, false},
- {reuseaddr, true},
- {keepalive,true},
- {backlog,500}
-]).
-
-%Start listening
-listen(Port) ->
- {ok, L} = gen_tcp:listen(Port, ?TCP_OPTS),
- ?MODULE:accept(L).
-
-%Accept a connection
-accept(L) ->
- {ok, Socket} = gen_tcp:accept(L),
- ?MODULE:read_loop(Socket),
- io:fwrite("Done reading, connection was closed\n"),
- ?MODULE:accept(L).
-
-%Read everything it sends us
-read_loop(Socket) ->
- inet:setopts(Socket, [{active, once}]),
- receive
- {tcp, _, _} ->
- do_stuff_here,
- ?MODULE:read_loop(Socket);
- {tcp_closed, _}-> donezo;
- {tcp_error, _, _} -> donezo
- end.
-```
-
-This code isn't actually usable for a production system; it doesn't even spawn a
-new process for the new socket. But that's not the point I'm making. If I run it
-with `tcp_test:listen(8000)`, and in other window do:
-
-```bash
-while [ 1 ]; do echo "aloha"; done | nc localhost 8000
-```
-
-We'll be flooding the the server with data pretty well. Using [eprof][4] we can
-get an idea of how our code performs, and where the hang-ups are:
-
-```erlang
-1> eprof:start().
-{ok,<0.34.0>}
-
-2> P = spawn(tcp_test,listen,[8000]).
-<0.36.0>
-
-3> eprof:start_profiling([P]).
-profiling
-
-4> running_the_while_loop.
-running_the_while_loop
-
-5> eprof:stop_profiling().
-profiling_stopped
-
-6> eprof:analyze(procs,[{sort,time}]).
-
-****** Process <0.36.0> -- 100.00 % of profiled time ***
-FUNCTION CALLS % TIME [uS / CALLS]
--------- ----- --- ---- [----------]
-prim_inet:type_value_2/2 6 0.00 0 [ 0.00]
-
-....snip....
-
-prim_inet:enc_opts/2 6 0.00 8 [ 1.33]
-prim_inet:setopts/2 12303599 1.85 1466319 [ 0.12]
-tcp_test:read_loop/1 12303598 2.22 1761775 [ 0.14]
-prim_inet:encode_opt_val/1 12303599 3.50 2769285 [ 0.23]
-prim_inet:ctl_cmd/3 12303600 4.29 3399333 [ 0.28]
-prim_inet:enc_opt_val/2 24607203 5.28 4184818 [ 0.17]
-inet:setopts/2 12303598 5.72 4533863 [ 0.37]
-erlang:port_control/3 12303600 77.13 61085040 [ 4.96]
-```
-
-eprof shows us where our process is spending the majority of its time. The `%`
-column indicates percentage of time the process spent during profiling inside
-any function. We can pretty clearly see that the vast majority of time was spent
-inside `erlang:port_control/3`, the BIF that `inet:setopts/2` uses to switch the
-socket to `{active,once}` mode. Amongst the calls which were called on every
-loop, it takes up by far the most amount of time. In addition all of those other
-calls are also related to `inet:setopts/2`.
-
-I'm gonna rewrite our little listen server to use `{active,true}`, and we'll do
-it all again:
-
-```erlang
--module(tcp_test).
--compile(export_all).
-
--define(TCP_OPTS, [
- binary,
- {packet, raw},
- {nodelay,true},
- {active, false},
- {reuseaddr, true},
- {keepalive,true},
- {backlog,500}
-]).
-
-%Start listening
-listen(Port) ->
- {ok, L} = gen_tcp:listen(Port, ?TCP_OPTS),
- ?MODULE:accept(L).
-
-%Accept a connection
-accept(L) ->
- {ok, Socket} = gen_tcp:accept(L),
- inet:setopts(Socket, [{active, true}]), %Well this is new
- ?MODULE:read_loop(Socket),
- io:fwrite("Done reading, connection was closed\n"),
- ?MODULE:accept(L).
-
-%Read everything it sends us
-read_loop(Socket) ->
- %inet:setopts(Socket, [{active, once}]),
- receive
- {tcp, _, _} ->
- do_stuff_here,
- ?MODULE:read_loop(Socket);
- {tcp_closed, _}-> donezo;
- {tcp_error, _, _} -> donezo
- end.
-```
-
-And the profiling results:
-
-```erlang
-1> eprof:start().
-{ok,<0.34.0>}
-
-2> P = spawn(tcp_test,listen,[8000]).
-<0.36.0>
-
-3> eprof:start_profiling([P]).
-profiling
-
-4> running_the_while_loop.
-running_the_while_loop
-
-5> eprof:stop_profiling().
-profiling_stopped
-
-6> eprof:analyze(procs,[{sort,time}]).
-
-****** Process <0.36.0> -- 100.00 % of profiled time ***
-FUNCTION CALLS % TIME [uS / CALLS]
--------- ----- --- ---- [----------]
-prim_inet:enc_value_1/3 7 0.00 1 [ 0.14]
-prim_inet:decode_opt_val/1 1 0.00 1 [ 1.00]
-inet:setopts/2 1 0.00 2 [ 2.00]
-prim_inet:setopts/2 2 0.00 2 [ 1.00]
-prim_inet:enum_name/2 1 0.00 2 [ 2.00]
-erlang:port_set_data/2 1 0.00 2 [ 2.00]
-inet_db:register_socket/2 1 0.00 3 [ 3.00]
-prim_inet:type_value_1/3 7 0.00 3 [ 0.43]
-
-.... snip ....
-
-prim_inet:type_opt_1/1 19 0.00 7 [ 0.37]
-prim_inet:enc_value/3 7 0.00 7 [ 1.00]
-prim_inet:enum_val/2 6 0.00 7 [ 1.17]
-prim_inet:dec_opt_val/1 7 0.00 7 [ 1.00]
-prim_inet:dec_value/2 6 0.00 10 [ 1.67]
-prim_inet:enc_opt/1 13 0.00 12 [ 0.92]
-prim_inet:type_opt/2 19 0.00 33 [ 1.74]
-erlang:port_control/3 3 0.00 59 [ 19.67]
-tcp_test:read_loop/1 20716370 100.00 12187488 [ 0.59]
-```
-
-This time our process spent almost no time at all (according to eprof, 0%)
-fiddling with the socket opts. Instead it spent all of its time in the
-read_loop doing the work we actually want to be doing.
-
-## So what does this mean?
-
-I'm by no means advocating never using `{active,once}`. The security concern is
-still a completely valid concern and one that `{active,once}` mitigates quite
-well. I'm simply pointing out that this mitigation has some fairly serious
-performance implications which have the potential to bite you if you're not
-careful, especially in cases where a socket is going to be receiving a large
-amount of traffic.
-
-## Meta
-
-These tests were done using R15B03, but I've done similar ones in R14 and found
-similar results. I have not tested R16.
-
-* \[0] http://learnyousomeerlang.com/buckets-of-sockets
-* \[1] http://www.erlang.org/doc/man/gen_tcp.html#examples
-* \[2] http://erlycoder.com/25/erlang-tcp-server-tcp-client-sockets-with-gen_tcp
-
-[0]: http://learnyousomeerlang.com/content
-[1]: http://www.metabrew.com/article/a-million-user-comet-application-with-mochiweb-part-1
-[2]: http://www.erlang.org/doc/man/inet.html#setopts-2
-[3]: http://www.erlang.org/doc/man/gen_tcp.html#recv-2
-[4]: http://www.erlang.org/doc/man/eprof.html