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131
erlang-tcp-socket-pull-pattern.md
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# Erlang, tcp sockets, and active true
If you don't know erlang then [you're missing out](http://learnyousomeerlang.com/content).
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](http://www.metabrew.com/article/a-million-user-comet-application-with-mochiweb-part-1).
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](http://www.erlang.org/doc/man/inet.html#setopts-2)
flag. A socket can be set to one of:
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](http://www.erlang.org/doc/man/gen_tcp.html#recv-2)
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](http://www.erlang.org/doc/man/gen_tcp.html#recv-2)
or be put in `{active,once}` or `{active,true}`.
* `{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.
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:
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}]).
-define(TCP_OPTS, [
binary,
{packet, raw},
{nodelay,true},
{active, false},
{reuseaddr, true},
{keepalive,true},
{backlog,500}
]).
%Start listening
listen(Port) ->
@ -66,15 +82,16 @@ read_loop(Socket) ->
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:
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](http://www.erlang.org/doc/man/eprof.html) we can get an idea
of how our code performs, and where the hang-ups are:
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().
@ -111,18 +128,30 @@ 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`.
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:
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}]).
-define(TCP_OPTS, [
binary,
{packet, raw},
{nodelay,true},
{active, false},
{reuseaddr, true},
{keepalive,true},
{backlog,500}
]).
%Start listening
listen(Port) ->
@ -194,20 +223,30 @@ 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.
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.
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.
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/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

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goplus.md
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# Go and project root
Compared to other languages go has some strange behavior regarding its project root settings. If you
import a library called `somelib`, go will look for a `src/somelib` folder in all of the folders in
the `$GOPATH` environment variable. This works nicely for globally installed packages, but it makes
encapsulating a project with a specific version, or modified version, rather tedious. Whenever you go
to work on this project you'll have to add its path to your `$GOPATH`, or add the path permanently,
which could break other projects which may use a different version of `somelib`.
Compared to other languages go has some strange behavior regarding its project
root settings. If you import a library called `somelib`, go will look for a
`src/somelib` folder in all of the folders in the `$GOPATH` environment
variable. This works nicely for globally installed packages, but it makes
encapsulating a project with a specific version, or modified version, rather
tedious. Whenever you go to work on this project you'll have to add its path to
your `$GOPATH`, or add the path permanently, which could break other projects
which may use a different version of `somelib`.
My solution is in the form of a simple script I'm calling go+. go+ will search in currrent directory
and all of its parents for a file called `GOPROJROOT`. If it finds that file in a directory, it
prepends that directory's absolute path to your `$GOPATH` and stops the search. Regardless of whether
or not `GOPROJROOT` was found go+ will passthrough all arguments to the actual go call. The
My solution is in the form of a simple script I'm calling go+. go+ will search
in currrent directory and all of its parents for a file called `GOPROJROOT`. If
it finds that file in a directory, it prepends that directory's absolute path to
your `$GOPATH` and stops the search. Regardless of whether or not `GOPROJROOT`
was found go+ will passthrough all arguments to the actual go call. The
modification to `$GOPATH` will only last the duration of the call.
As an example, consider the following:
@ -23,8 +26,8 @@ As an example, consider the following:
/hello.go
```
If `hello.go` depends on `somelib`, as long as you run go+ from `/tmp/hello` or one of its children
your project will still compile
If `hello.go` depends on `somelib`, as long as you run go+ from `/tmp/hello` or
one of its children your project will still compile
Here is the source code for go+: