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## Chapter 4. Introduction

Cowboy is a small, fast and modern HTTP server for Erlang/OTP.

Cowboy aims to provide a complete modern Web stack. This includes HTTP/1.1, HTTP/2, Websocket, Server-Sent Events and Webmachine-based REST.

Cowboy comes with functions for introspection and tracing, enabling developers to know precisely what is happening at any time. Its modular design also easily enable developers to add instrumentation.

Cowboy is a high quality project. It has a small code base, is very efficient (both in latency and memory use) and can easily be embedded in another application.

Cowboy is clean Erlang code. It includes hundreds of tests and its code is fully compliant with the Dialyzer. It is also well documented and features a Function Reference, a User Guide and numerous Tutorials.

## 4.1. Prerequisites

Beginner Erlang knowledge is recommended for reading this guide.

Knowledge of the HTTP protocol is recommended but not required, as it will be detailed throughout the guide.

## 4.2. Supported platforms

Cowboy is tested and supported on Linux, FreeBSD, Windows and OSX.

Cowboy has been reported to work on other platforms, but we make no guarantee that the experience will be safe and smooth. You are advised to perform the necessary testing and security audits prior to deploying on other platforms.

Cowboy is developed for Erlang/OTP 24.0 and newer.

## 4.3. License

Cowboy uses the ISC License.

    Copyright (c) 2011-2026, Loïc Hoguin <essen@ninenines.eu>

    Permission to use, copy, modify, and/or distribute this software for any
    purpose with or without fee is hereby granted, provided that the above
    copyright notice and this permission notice appear in all copies.

    THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
    WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
    MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
    ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
    ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

## 4.4. Versioning

Cowboy uses Semantic Versioning 2.0.0.

## 4.5. Conventions

In the HTTP protocol, the method name is case sensitive. All standard method names are uppercase.

Header names are case insensitive. When using HTTP/1.1, Cowboy converts all the request header names to lowercase. HTTP/2 requires clients to send them as lowercase. Any other header name is expected to be provided lowercased, including when querying information about the request or when sending responses.

The same applies to any other case insensitive value.

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