Running Phoenix on a Unix socket

Phoenix Forum Questions / Help
1

Any experiences and performance results compared to running the Phoenix server on a TCP port? Any minimum system requirements to use Unix socket?

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2

I donā€™t think you can run Cowboy (which is the only server Phoenix officially works with AFAIK) on unix sockets.

EDIT: someone says you can since OTP 19.

As far as performance goes, itā€™s not specific to Erlang and should be somewhat faster.

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3

Whatā€™s the state of this?

There was a PR for OTP. Itā€™s marked as closed on Github, but the discussion seems to indicate that it got put into OTP-19.0-rc2.

I want to write a Docker plugin in Elixir, but it requires communicated with Docker over a Unix socket.

Thanks for the help.

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4

With :gen_tcp you can use the tuple {:local, path} to connect to a unix socket. It is mentioned in the :inet module docs:
http://erlang.org/doc/man/inet.html#type-local_address

If you want to make HTTP requests to a unix socket you can use HTTPoison/Hackney with the ā€œhttp+unix://ā€ scheme, which is a bit of a hidden feature:

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5

For posterity, this worksā€¦

{:ok, pid} = Plug.Adapters.Cowboy.http(MyPlug, [], ip: {:local, "my_plug.sock"}, port: 0)

The port: 0 is important; it wonā€™t work without it.

Also, you gotta rm my_plug.sock when youā€™re done otherwise it wonā€™t start up again: :eaddrinuse (address already in use)

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6

Any suggestions on getting the Phoenix.Endpoint to accept these options?
Iā€™m getting an error in ranch_listener_sup:

config :hello, HelloWeb.Endpoint,
  http: [port: 0, ip: {:local, "hello.sock"}],
  debug_errors: true,
  code_reloader: false,
  check_origin: false,
  watchers: []
Error Details 
** (Mix) Could not start application hello: Hello.Application.start(:normal, []) returned an error: shutdown: failed to start child: HelloWeb.Endpoint
    ** (EXIT) shutdown: failed to start child: Phoenix.Endpoint.Handler
        ** (EXIT) shutdown: failed to start child: {:ranch_listener_sup, HelloWeb.Endpoint.HTTP}
            ** (EXIT) an exception was raised:
                ** (Protocol.UndefinedError) protocol String.Chars not implemented for {:error, :einval}. This protocol is implemented for: Atom, BitString, Date, DateTime, Float, Integer, List, NaiveDateTime, Time, URI, Version, Version.Requirement
                    (elixir) /private/tmp/elixir-20180316-64850-zsrybb/elixir-1.6.4/lib/elixir/lib/string/chars.ex:3: String.Chars.impl_for!/1
                    (elixir) /private/tmp/elixir-20180316-64850-zsrybb/elixir-1.6.4/lib/elixir/lib/string/chars.ex:22: String.Chars.to_string/1
                    (phoenix) lib/phoenix/endpoint/cowboy_handler.ex:115: Phoenix.Endpoint.CowboyHandler.info/3
                    (phoenix) lib/phoenix/endpoint/cowboy_handler.ex:100: Phoenix.Endpoint.CowboyHandler.start_link/3
                    (stdlib) supervisor.erl:365: :supervisor.do_start_child/2
                    (stdlib) supervisor.erl:348: :supervisor.start_children/3
                    (stdlib) supervisor.erl:314: :supervisor.init_children/2
                    (stdlib) gen_server.erl:365: :gen_server.init_it/2
                    (stdlib) gen_server.erl:333: :gen_server.init_it/6
                    (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
7

What would be the reason to do this? Are plain Unix sockets faster than using a TCP port (when putting Phoenix+Cowboy behind a webserver like Nginx or Apache?)

8

@Qqwy There is a noticeable speed boost from using sockets over tcp. From what I recall it is 10-15%, in addition to avoiding the overhead of TLS.

Iā€™ve explored using a socket to connect to a legacy app running in a different languageā€”like terraformer, but on the same host and without the extra overhead.

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9

AFAIK a Unix socket does not suffer from the TCP slow start. Especially for short lived connections this is a noticeable bottleneck.

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10

So, is it possible to run Phoenix on a Unix socket?

11

In general it seems to be possible, as you can inject the socket after it has been established into cowboy, Iā€™m not sure though how this fits into phoenix startup procedure.

How it will work in Erlang using cowboy is explained briefly in this issue:

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12

Iā€™m primarily interested in unix sockets as a way to implement zero downtime deploys by updating a symlink to the current release and reloading nginx config. Never used Capistrano but I understand it worked similarly.

After trying to get this working Iā€™ve run into a couple more issues. The error above just requires a small change to the startup message phoenix outputs which assumes a port number.

The next issue I hit is the Plug.Static plug. Eventually it calls into the :file.sendfile kernel function, which fails with {:error, :badarg} when given a unix socket. There is a work-around in :ranch_transport.sendfile but Iā€™m not sure if the best place to fix this is in erlang, ranch, cowboy or plug?

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13

Or maybe you could serve static files from nginx since you are already using it anyway?

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14

Turns out the new file system in OTP 21 fixed the issue, so Phoenix should now run over a unix socket.

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15

Much of this depends on the version of kernel. It is true that since the beginning of time, communication over the loopback subnet went through the entire TCP stack, twice, ā€œoutgoingā€ and ā€œincomingā€. But in some recentā€™ish Linux kernel short-circuiting was added so that those comms now skip that overhead and work much more like domain sockets.

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16

Ooooo I hope this gets documented, should switch my server over. ^.^

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17

If you follow the trail from the Phoenix endpoint docs, you land at: https://hexdocs.pm/plug_cowboy/Plug.Cowboy.html

Which does mention how to set the ip and port appropriately for unix sockets.

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18

Great, looking forward to test it.

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19

Indeed, it is possible like this:
http: [ip: {:local, '/srv/phx/project/development.socket'}, port: 0],

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