What Is Cowboy Web Server?


Angela Bailey

The Cowboy web server is a lightweight and high-performance HTTP server written in Erlang. It is designed to handle a large number of concurrent connections efficiently, making it an excellent choice for building scalable web applications. In this article, we will explore the features and benefits of using Cowboy as your web server.

Features of Cowboy Web Server

Cowboy comes with several features that make it a powerful tool for building robust web applications:

  • High performance: Cowboy is known for its excellent performance and can handle thousands of simultaneous connections with ease. This makes it suitable for applications that require high throughput and low latency.
  • Websocket support: Cowboy provides built-in support for Websockets, allowing you to easily incorporate real-time communication into your applications.
  • HTTP/2 support: Cowboy fully supports the HTTP/2 protocol, which offers improved performance and better resource utilization compared to its predecessor, HTTP/1.1.
  • Routing flexibility: Cowboy offers a flexible routing mechanism that allows you to define complex URL patterns and easily map them to specific handlers or resources.
  • Middleware support: Middleware functions can be used to intercept requests and modify them before they reach the handler. This enables you to add common functionality such as authentication or request logging to your application.

Getting Started with Cowboy

To start using Cowboy in your project, you first need to include it as a dependency in your Erlang application. You can do this by adding the following line to your project’s `rebar.config` file:

[{deps, [{cowboy, "2.9.0"}]}].

You can replace `2.0` with the latest version of Cowboy available.

Once you have added the dependency, you need to define a Cowboy handler module that will handle incoming requests. This module should implement the `cowboy_http_handler` behavior and define the necessary callback functions.

Here is an example of a simple Cowboy handler module:


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

init(_Transport, Req, _Opts) ->
    {ok, Req, undefined_state}.

handle(Req, State) ->
    {ok, Req2} = cowboy_req:reply(200,
        [{<<"content-type">>, <<"text/plain">>}],
        <<"Hello, Cowboy!">>),
    {ok, Req2, State}.

terminate(_Reason, _Req, _State) ->

In this example, the `init/3` function initializes the handler state and returns it along with the initial request. The `handle/2` function handles incoming requests and generates a response. Finally, the `terminate/3` function is called when the connection is closed.

To start the Cowboy server and listen for incoming connections on a specific port, you can use the following code:

    [{port, 8080}],
    #{env => #{dispatch => Dispatch}},

Dispatch = cowboy_router:compile([
    {'_', [
        {"/", my_handler, []}

In this code snippet, we start the server on port `8080` and define a routing table that maps the root URL (`/`) to our `my_handler` module.


Cowboy is a powerful and flexible web server that excels in handling concurrent connections and delivering high-performance web applications. Its support for Websockets, HTTP/2, and middleware functions make it an excellent choice for modern web development. By following the steps outlined in this article, you can quickly get started with Cowboy and leverage its features to build scalable and efficient web applications.

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