Small, fast, modular HTTP server.
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Cowboy is a small, fast and modular HTTP server written in Erlang.


Cowboy aims to provide the following advantages:

  • Small code base.
  • Damn fast.
  • Modular: transport and protocol handlers are replaceable.
  • Binary HTTP for greater speed and lower memory usage.
  • Easy to embed inside another application.
  • Selectively dispatch requests to handlers, allowing you to send some requests to your embedded code and others to a FastCGI application in PHP or Ruby.
  • No parameterized module. No process dictionary. Clean Erlang code.

The server is currently in early development. Comments and suggestions are more than welcome. To contribute, either open bug reports, or fork the project and send us pull requests with new or improved functionality. You should discuss your plans with us before doing any serious work, though, to avoid duplicating efforts.

Quick start

  • Add Cowboy as a rebar or agner dependency to your application.
  • Start Cowboy and add one or more listeners.
  • Write handlers for your application.

Getting Started

At heart, Cowboy is nothing more than an TCP acceptor pool. All it does is accept connections received on a given port and using a given transport, like TCP or SSL, and forward them to a request handler for the given protocol. Acceptors and request handlers are of course supervised automatically.

It just so happens that Cowboy also includes an HTTP protocol handler. But Cowboy does nothing by default. You need to explicitly ask Cowboy to listen on a port with your chosen transport and protocol handlers. To do so, you must start a listener.

A listener is a special kind of supervisor that manages both the acceptor pool and the request processes. It is named and can thus be started and stopped at will.

An acceptor pool is a pool of processes whose only role is to accept new connections. It's good practice to have many of these processes as they are very cheap and allow much quicker response when you get many connections. Of course, as with everything else, you should benchmark before you decide what's best for you.

Cowboy includes a TCP transport handler for HTTP and an SSL transport handler for HTTPS. The transport handlers can of course be reused for other protocols like FTP or IRC.

The HTTP protocol requires one last thing to continue: dispatching rules. Don't worry about it right now though and continue reading, it'll all be explained.

You can start and stop listeners by calling cowboy:start_listener/6 and cowboy:stop_listener/1 respectively.

The following example demonstrates the startup of a very simple listener.

Dispatch = [
    %% {Host, list({Path, Handler, Opts})}
    {'_', [{'_', my_handler, []}]}
%% Name, NbAcceptors, Transport, TransOpts, Protocol, ProtoOpts
cowboy:start_listener(http, 100,
    cowboy_tcp_transport, [{port, 8080}],
    cowboy_http_protocol, [{dispatch, Dispatch}]

This is not enough though, you must also write the my_handler module to process the incoming HTTP requests. Of course Cowboy comes with predefined handlers for specific tasks but most of the time you'll want to write your own handlers for your application.

Following is an example of a "Hello World!" HTTP handler.

-export([init/3, handle/2, terminate/2]).

init({tcp, http}, Req, Opts) ->
    {ok, Req, undefined_state}.

handle(Req, State) ->
    {ok, Req2} = cowboy_http_req:reply(200, [], <<"Hello World!">>, Req),
    {ok, Req2, State}.

terminate(Req, State) ->

Continue reading to learn how to dispatch rules and handle requests.

Dispatch rules

Cowboy allows you to dispatch HTTP requests directly to a specific handler based on the hostname and path information from the request. It also lets you define static options for the handler directly in the rules.

To match the hostname and path, Cowboy requires a list of tokens. For example, to match the "" domain name, you must specify [<<"dev-extend">>, <<"eu">>]. Or, to match the "/path/to/my/resource" you must use [<<"path">>, <<"to">>, <<"my">>, <<"resource">>]. All the tokens must be given as binary.

You can use the special token '_' (the atom underscore) to indicate that you accept anything in that position. For example if you have both "" and "" domains, you can use the match spec [<<"dev-extend">>, '_'] to match any top level extension.

Finally, you can also match multiple leading segments of the domain name and multiple trailing segments of the request path using the atom '...' (the atom ellipsis) respectively as the first host token or the last path token. For example, host rule ['...', <<"dev-extend">>, <<"eu">>] can match both "" and "" and path rule [<<"projects">>, '...'] can math both "/projects" and "/projects/cowboy/issues/42". The host leading segments and the path trailing segments can later be retrieved through cowboy_http_req:host_info/1 and cowboy_http_req:path_info/1.

Any other atom used as a token will bind the value to this atom when matching. To follow on our hostnames example, [<<"dev-extend">>, ext] would bind the values <<"eu">> and <<"fr">> to the ext atom, that you can later retrieve in your handler by calling cowboy_http_req:binding/{2,3}.

You can also accept any match spec by using the atom '_' directly instead of a list of tokens. Our hello world example above uses this to forward all requests to a single handler.

There is currently no way to match multiple tokens at once.

Requests handling

Requests are passed around in the Request variable. Although they are defined as a record, it is recommended to access them only through the cowboy_http_req module API.

You can retrieve the HTTP method, HTTP version, peer address and port, host tokens, raw host, used port, path tokens, raw path, query string values, bound values from the dispatch step, header values from the request. You can also read the request body, if any, optionally parsing it as a query string. Finally, the request allows you to send a response to the client.

See the cowboy_http_req module for more information.


The Websocket protocol is built upon the HTTP protocol. It first sends an HTTP request for an handshake, performs it and then switches to Websocket. Therefore you need to write a standard HTTP handler to confirm the handshake should be completed and then the Websocket-specific callbacks.

A simple handler doing nothing but sending a repetitive message using Websocket would look like this:

-export([init/3, handle/2, terminate/2]).
-export([websocket_init/3, websocket_handle/3, websocket_terminate/3]).

init({tcp, http}, Req, Opts) ->
    {upgrade, protocol, cowboy_http_websocket}.

handle(Req, State) ->
    error(foo). %% Will never be called.

terminate(Req, State) ->
    error(foo). %% Same for that one.

websocket_init(TransportName, Req, _Opts) ->
    erlang:start_timer(1000, self(), <<"Hello!">>),
    {ok, Req, undefined_state}.

websocket_handle({timeout, _Ref, Msg}, Req, State) ->
    erlang:start_timer(1000, self(), <<"How' you doin'?">>),
    {reply, Msg, Req, State};
websocket_handle({websocket, Msg}, Req, State) ->
    {reply, << "That's what she said! ", Msg/binary >>, Req, State}.

websocket_terminate(_Reason, _Req, _State) ->

Of course you can have an HTTP handler doing both HTTP and Websocket handling, but for the sake of this example we're ignoring the HTTP part entirely.

Using Cowboy with other protocols

One of the strengths of Cowboy is of course that you can use it with any protocol you want. The only downside is that if it's not HTTP, you'll probably have to write the protocol handler yourself.

The only exported function a protocol handler needs is the start_link/3 function, with arguments Socket, Transport and Opts. Socket is of course the client socket; Transport is the module name of the chosen transport handler and Opts is protocol options defined when starting the listener. Anything you do past this point is up to you!

You should definitely look at the cowboy_http_protocol module for a great example of fast request handling if you need to. Otherwise it's probably safe to use {active, once} mode and handle everything as it comes.

Note that while you technically can run a protocol handler directly as a gen_server or a gen_fsm, it's probably not a good idea, as the only call you'll ever receive from Cowboy is the start_link/3 call. On the other hand, feel free to write a very basic protocol handler which then forwards requests to a gen_server or gen_fsm. By doing so however you must take care to supervise their processes as Cowboy only knows about the protocol handler itself.