[[modern_web]]
== The modern Web

// @todo Link to related xrefs.

Cowboy is a server for the modern Web. This chapter explains
what it means and details all the standards involved.

Cowboy supports all the standards listed in this document.

=== HTTP/2

HTTP/2 is the most efficient protocol for consuming Web
services. It enables clients to keep a connection open
for long periods of time; to send requests concurrently;
to reduce the size of requests through HTTP headers
compression; and more. The protocol is binary, greatly
reducing the resources needed to parse it.

HTTP/2 also enables the server to push messages to the
client. This can be used for various purposes, including
the sending of related resources before the client requests
them, in an effort to reduce latency. This can also be used
to enable bidirectional communication.

Cowboy provides transparent support for HTTP/2. Clients
that know it can use it; others fall back to HTTP/1.1
automatically.

HTTP/2 is compatible with the HTTP/1.1 semantics.

HTTP/2 is defined by RFC 7540 and RFC 7541.

=== HTTP/1.1

HTTP/1.1 is the previous version of the HTTP protocol.
The protocol itself is text-based and suffers from numerous
issues and limitations. In particular it is not possible
to execute requests concurrently (though pipelining is
sometimes possible), and it's also sometimes difficult
to detect that a client disconnected.

HTTP/1.1 does provide very good semantics for interacting
with Web services. It defines the standard methods, headers
and status codes used by HTTP/1.1 and HTTP/2 clients and
servers.

HTTP/1.1 also defines compatibility with an older version
of the protocol, HTTP/1.0, which was never really standardized
across implementations.

The core of HTTP/1.1 is defined by RFC 7230, RFC 7231,
RFC 7232, RFC 7233, RFC 7234 and RFC 7235. Numerous RFCs
and other specifications exist defining additional HTTP
methods, status codes, headers or semantics.

=== Websocket

Websocket is a protocol built on top of HTTP/1.1 that provides
a two-ways communication channel between the client and the
server. Communication is asynchronous and can occur concurrently.

It consists of a Javascript object allowing setting up a
Websocket connection to the server, and a binary based
protocol for sending data to the server or the client.

Websocket connections can transfer either UTF-8 encoded text
data or binary data. The protocol also includes support for
implementing a ping/pong mechanism, allowing the server and
the client to have more confidence that the connection is still
alive.

A Websocket connection can be used to transfer any kind of data,
small or big, text or binary. Because of this Websocket is
sometimes used for communication between systems.

Websocket messages have no semantics on their own. Websocket
is closer to TCP in that aspect, and requires you to design
and implement your own protocol on top of it; or adapt an
existing protocol to Websocket.

The Websocket protocol is defined by RFC 6455.

=== Long-lived requests

Cowboy provides an interface that can be used to support
long-polling or to stream large amounts of data reliably,
including using Server-Sent Events.

Long-polling is a mechanism in which the client performs
a request which may not be immediately answered by the
server. It allows clients to request resources that may
not currently exist, but are expected to be created soon,
and which will be returned as soon as they are.

Long-polling is essentially a hack, but it is widely used
to overcome limitations on older clients and servers.

Server-Sent Events is a small protocol defined as a media
type, `text/event-stream`, along with a new HTTP header,
`Last-Event-ID`. It is defined in the EventSource W3C
specification.

Cowboy provides an interface known as loop handlers that
facilitates the implementation of long-polling or stream
mechanisms. It works regardless of the underlying protocol.

=== REST

REST, or REpresentational State Transfer, is a style of
architecture for loosely connected distributed systems.
It can easily be implemented on top of HTTP.

REST is essentially a set of constraints to be followed.
Many of these constraints are purely architectural and
solved by simply using HTTP. Some constraints must be
explicitly followed by the developer.

Cowboy provides an interface known as REST handlers that
simplifies the implementation of a REST API on top of
the HTTP protocol.
