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


Cowboy aims to provide a complete HTTP stack in a small code base. It is optimized for low latency and low memory usage, in parts because it uses binary strings.

Cowboy provides routing capabilities, selectively dispatching requests to handlers written in Erlang.

Because it uses Ranch for managing connections, Cowboy can easily be embedded in any other application.

No parameterized module. No process dictionary. Clean Erlang code.

Quick start

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

Getting Started

  • Read the guide
  • Look at the examples in the examples/ directory
  • Build API documentation with make docs; open doc/index.html


This and all following sections will be removed as soon as their equivalent appear in the Cowboy guide.

Cowboy does nothing by default.

Cowboy uses Ranch for handling connections, and provides convenience functions to start and stop Ranch listeners. The Ranch application must always be started before Cowboy. The crypto application must also be started.

The cowboy:start_http/4 function will handle HTTP connections using the TCP transport. Similarly, cowboy:start_https/4 will handle HTTP connections using the SSL transport.

You can start as many listeners as you need to. To allow this, you are required to give a name to your listeners. It is the first argument to the start functions. The name can be of any type.

You can stop listeners using cowboy:stop_listener/1, giving it the name of the listener to be stopped.

The following example demonstrates the startup of a very simple HTTP listener. It redirects all requests to the my_handler module.

Dispatch = [
    %% {URIHost, list({URIPath, Handler, Opts})}
    {'_', [{'_', my_handler, []}]}
%% Name, NbAcceptors, TransOpts, ProtoOpts
cowboy:start_http(my_http_listener, 100, [{port, 8080}],
    [{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 need to write the handlers appropriate 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_req:reply(200, [], <<"Hello World!">>, Req),
    {ok, Req2, State}.

terminate(Req, State) ->

You can also write handlers that do not reply directly. Instead, such handlers will wait for an Erlang message from another process and only reply when receiving such message, or timeout if it didn't arrive in time.

This is especially useful for long-polling functionality, as Cowboy will handle process hibernation and timeouts properly, preventing mistakes if you were to write the code yourself. A handler of that kind can be defined like this:

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

-define(TIMEOUT, 60000).

init({tcp, http}, Req, Opts) ->
	{loop, Req, undefined_state, ?TIMEOUT, hibernate}.

info({reply, Body}, Req, State) ->
	{ok, Req2} = cowboy_req:reply(200, [], Body, Req),
	{ok, Req2, State};
info(Message, Req, State) ->
	{loop, Req, State, hibernate}.

terminate(Req, State) ->

It is of course possible to combine both type of handlers together as long as you return the proper tuple from init/3.

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 [<<"ninenines">>, <<"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 [<<"ninenines">>, '_'] 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 ['...', <<"ninenines">>, <<"eu">>] can match both "" and "" and path rule [<<"projects">>, '...'] can match both "/projects" and "/projects/cowboy/issues/42". The host leading segments and the path trailing segments can later be retrieved through cowboy_req:host_info/1 and cowboy_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, [<<"ninenines">>, ext] would bind the values <<"eu">> and <<"fr">> to the ext atom, that you can later retrieve in your handler by calling cowboy_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_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_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([websocket_init/3, websocket_handle/3,
    websocket_info/3, websocket_terminate/3]).

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

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

websocket_handle({text, Msg}, Req, State) ->
    {reply, {text, << "That's what she said! ", Msg/binary >>}, Req, State};
websocket_handle(_Data, Req, State) ->
    {ok, Req, State}.

websocket_info({timeout, _Ref, Msg}, Req, State) ->
    erlang:start_timer(1000, self(), <<"How' you doin'?">>),
    {reply, {text, Msg}, Req, State};
websocket_info(_Info, Req, State) ->
    {ok, 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.

As the Websocket protocol is still a draft the API is subject to change regularly when support to the most recent drafts gets added. Features may be added, changed or removed before the protocol gets finalized. Cowboy tries to implement all drafts transparently and give a single interface to handle them all, however.