Introduction to Cowboy: A web server for Erlang

April 17, 2024

Among the options available for web servers, Cowboy stands out as a robust and versatile web server and HTTP framework tailored specifically for the Erlang programming language.

Cowboy offers developers a powerful toolkit for building high-performance web applications with Erlang, harnessing the language’s concurrency model and fault-tolerant design principles. In this blog post, we’ll embark on a journey to explore Cowboy, delving into its features, capabilities, and the myriad of benefits it brings to developers and businesses alike.

Our aim is to provide a comprehensive introduction to Cowboy, shedding light on its role as a cornerstone of Erlang web development. Whether you’re a seasoned Erlang veteran or a newcomer eager to explore the world of web servers, this blog post will serve as your guide to unlocking the potential of Cowboy and leveraging its strengths for your next web development project.


What is Cowboy?

At its core, Cowboy is more than just a web server; it’s a comprehensive HTTP framework tailored specifically for Erlang, designed to empower developers in building high-performance and scalable web applications. Let’s break down what Cowboy brings to the table.

Definition and role: Cowboy serves as a robust web server and HTTP framework, providing Erlang developers with the tools they need to handle incoming HTTP requests, manage WebSocket connections, and build dynamic web applications with ease. Its primary role is to act as the backbone of Erlang-based web services, offering a reliable foundation for handling web traffic efficiently.

Key features and characteristics: Cowboy boasts a plethora of features that set it apart from traditional web servers. From its lightning-fast performance to its asynchronous processing model, Cowboy excels in delivering seamless user experiences while handling a high volume of concurrent connections. Additionally, its support for modern web protocols like HTTP/1.1 and WebSocket enables developers to build real-time, interactive applications with ease.

History and evolution: Since its inception, Cowboy has evolved into one of the go-to choices for Erlang developers seeking to build robust web applications. Over the years, it has undergone continuous refinement and improvement, incorporating feedback from the community and adapting to the ever-changing landscape of web development. Today, Cowboy stands as a testament to Erlang’s versatility and resilience in the realm of web servers.


Key features of Cowboy

Cowboy is renowned for its rich feature set, which empowers developers to build high-performance and scalable web applications with ease. Let’s delve into some of Cowboy’s key features that make it a standout choice for Erlang web development.


High performance

Cowboy is engineered for speed, boasting exceptional performance and low resource consumption. Leveraging Erlang’s concurrency model and lightweight processes, Cowboy excels in handling a large number of concurrent connections efficiently, ensuring lightning-fast response times even under heavy loads.


Asynchronous processing

One of Cowboy’s standout features is its asynchronous, event-driven architecture. By utilizing non-blocking I/O operations and Erlang’s message-passing concurrency model, Cowboy can handle multiple requests concurrently without blocking the main server process. This asynchronous processing model contributes to Cowboy’s exceptional scalability and responsiveness.


HTTP/1.1 and WebSocket support

Cowboy offers comprehensive support for the HTTP/1.1 protocol, allowing developers to build RESTful APIs, serve static files, and handle various HTTP methods seamlessly. Additionally, Cowboy provides built-in support for WebSocket communication, enabling developers to implement real-time, bidirectional communication between clients and servers effortlessly.


Routing and middleware

Cowboy simplifies request routing and processing through its flexible routing mechanism and support for middleware components. Developers can define routes based on URL patterns and HTTP methods, and leverage middleware to add pre-processing and post-processing logic to request handling pipelines. This enables developers to organize their code effectively and implement cross-cutting concerns such as authentication, logging, and error handling with ease.


Security features

Cowboy includes robust security features to safeguard web applications against common threats and vulnerabilities. It offers mechanisms for enforcing HTTPS/TLS encryption, request validation, input sanitization, and protection against cross-site scripting (XSS) and cross-site request forgery (CSRF) attacks. With Cowboy, developers can ensure that their applications adhere to security best practices and maintain data integrity and confidentiality.



Cowboy’s modular and extensible design allows developers to customize and extend its functionality to suit their specific requirements. Whether it’s integrating with third-party libraries, adding custom middleware, or implementing advanced routing logic, Cowboy provides developers with the flexibility and freedom to tailor their web applications to meet their unique needs.


Getting started with Cowboy

Getting started with Cowboy is straightforward, and in this section, we’ll walk you through the process of setting up a development environment and creating a simple Cowboy application.


Setting up a development environment

Install Erlang/OTP: Cowboy requires Erlang/OTP to run. You can download and install Erlang from the official Erlang website.

Install Cowboy: Cowboy can be installed using rebar3, Erlang’s de facto build tool. If you haven’t installed rebar3 yet, you can follow the instructions on the rebar3 GitHub repository. Once rebar3 is installed, you can create a new Cowboy project by running the following command:

rebar3 new cowboy my_cowboy_app

Configure Your Application: After creating your Cowboy project, navigate to the project directory and configure your application according to your requirements. You can define routes, handlers, and middleware components in the application’s configuration file (e.g., sys.config or config/sys.config).


Creating a simple Cowboy application

Now that you have set up your development environment, let’s create a basic Cowboy application to get you started.

Define routes: Define the routes for your application in the Cowboy configuration file. Routes specify the URL patterns and associated handler modules for processing incoming requests. For example:

{cowboy, [
   {middlewares, []},
   {transports, [http]},
   {listeners, [{http, [{port, 8080}]}]},
   {handler_opts, [{env, [{dispatch, cowboy_router:compile([{_, _, {[], cowboy_static, static_handler, [{dir, {priv_dir, my_cowboy_app,<<"static">>}}]}]})}]}]}


Create handler modules: Implement handler modules to handle incoming requests. Handler modules are Erlang modules that define callback functions for processing different types of HTTP requests. For example:

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

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

handle_req(Req, _State) ->
   {ok, Req, <<"Hello, Cowboy!">>}.


Run your application: With your Cowboy application configured and handler modules implemented, you can now start your application by running the following command:

rebar3 shell

This command will compile your application and start a shell with your application running. You can then visit http://localhost:8080 in your web browser to see your Cowboy application in action.

Congratulations! You’ve successfully created a basic Cowboy application and taken your first steps into the world of Erlang web development. From here, you can continue to explore Cowboy’s features and capabilities, building more complex web applications and harnessing the power of Erlang to create robust and scalable solutions.


Advanced topics and best practices

As you delve deeper into Erlang web development with Cowboy, you’ll encounter advanced features and best practices that enhance the security, reliability, and performance of your web applications. Let’s explore some of these topics.


Request validation and input sanitization

Cowboy provides mechanisms for validating incoming requests and sanitizing user input to prevent security vulnerabilities such as injection attacks (e.g., SQL injection, XSS). Implementing request validation and input sanitization ensures that your application handles user input safely and securely.


HTTPS/TLS support

Security is paramount in web applications, especially when handling sensitive data. Cowboy offers built-in support for HTTPS/TLS encryption, allowing you to secure communication between clients and servers. By configuring HTTPS/TLS support in Cowboy, you can ensure the confidentiality and integrity of data transmitted over the network.


Error handling and logging

Effective error handling and logging are essential for diagnosing and troubleshooting issues in your web applications. Cowboy provides mechanisms for handling errors gracefully and logging relevant information, such as request details, errors, and application events. By implementing robust error handling and logging strategies, you can identify and resolve issues efficiently, improving the overall reliability of your application.


Performance optimization

Optimizing the performance of your Cowboy applications is crucial for delivering fast and responsive user experiences. Techniques such as caching, asynchronous processing, and code optimization can significantly improve the performance of your web applications. Additionally, monitoring and profiling tools can help identify performance bottlenecks and areas for optimization, allowing you to fine-tune your application for optimal performance.


Concurrency and scalability

Erlang’s concurrency model enables Cowboy applications to handle a large number of concurrent connections efficiently. By leveraging Erlang’s lightweight processes and supervision mechanisms, you can build highly scalable and fault-tolerant web applications with Cowboy. Understanding concurrency patterns and designing your application for scalability ensures that your application can handle increased load and scale gracefully as your user base grows.


Security best practices

Implementing security best practices is paramount for protecting your web applications against threats and vulnerabilities. This includes enforcing access controls, validating input data, protecting against common web vulnerabilities (e.g., XSS, CSRF), and keeping software dependencies up-to-date. By following security best practices and staying informed about emerging threats, you can safeguard your Cowboy applications and protect your users’ data from unauthorized access and exploitation.


By exploring advanced topics and adopting best practices in Cowboy web development, you can build robust, secure, and high-performance web applications that meet the demands of modern software development. Continuously learning and staying up-to-date with advancements in Erlang and web development technologies will empower you to build scalable and resilient applications that drive value for your users and your business.


Real-world use cases

Cowboy has been embraced by numerous companies and projects across various industries for building robust, scalable, and high-performance web applications. Let’s explore two real-world use cases where Cowboy has been successfully utilized.



RabbitMQ, a message broker software, is written in Erlang and leverages Cowboy for its HTTP-based management API. Cowboy enables RabbitMQ administrators to interact with the message broker via HTTP endpoints for tasks such as managing queues, exchanges, and bindings.



MongooseIM, an open-source XMPP messaging server, is implemented in Erlang and uses Cowboy for handling HTTP connections. Cowboy serves as the HTTP gateway for MongooseIM, enabling integration with web-based applications and services.



In conclusion, we’ve embarked on a journey to explore Cowboy, a powerful web server and HTTP framework tailored specifically for Erlang. Let’s recap the key points we’ve discussed in this blog post and reflect on the impact of Cowboy on the future of web development with Erlang.

Throughout this post, we’ve delved into the fundamental aspects of Cowboy, from its role as a high-performance web server to its asynchronous processing model and support for modern web protocols like HTTP/1.1 and WebSocket. We’ve explored advanced features such as request validation, input sanitization, and HTTPS/TLS support, as well as best practices for building robust and secure web applications with Cowboy.

Furthermore, we’ve highlighted real-world use cases where Cowboy has been successfully deployed, ranging from messaging platforms to cloud services to educational platforms. These examples showcase Cowboy’s versatility and reliability in diverse application scenarios, reaffirming its status as a top choice for Erlang web development.

In closing, Cowboy stands as a testament to Erlang’s capabilities in the realm of web servers, and its continued evolution and adoption pave the way for a bright future in web development with Erlang.


Additional resources

Check out the Ada Beat Functional Programming blog for more topics, including functional programming principles, summaries of MeetUps, language specific articles, and much more. Whether you’re interested in functional programming theory or practical application, we have something for everyone.