From 4023e7f4e429179fd9c2cce4487c33646c6bd327 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lo=C3=AFc=20Hoguin?= <essen@ninenines.eu>
Date: Thu, 14 Jan 2016 13:35:25 +0100
Subject: Convert the documentation to Asciidoc

A few small revisions were made, and Erlang.mk has been updated.
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+[[modern_web]]
+== The modern Web
+
+Let's take a look at various technologies from the beginnings
+of the Web up to this day, and get a preview of what's
+coming next.
+
+Cowboy is compatible with all the technology cited in this
+chapter except of course HTTP/2.0 which has no implementation
+in the wild at the time of writing.
+
+=== The prehistoric Web
+
+HTTP was initially created to serve HTML pages and only
+had the GET method for retrieving them. This initial
+version is documented and is sometimes called HTTP/0.9.
+HTTP/1.0 defined the GET, HEAD and POST methods, and
+was able to send data with POST requests.
+
+HTTP/1.0 works in a very simple way. A TCP connection
+is first established to the server. Then a request is
+sent. Then the server sends a response back and closes
+the connection.
+
+Suffice to say, HTTP/1.0 is not very efficient. Opening
+a TCP connection takes some time, and pages containing
+many assets load much slower than they could because of
+this.
+
+Most improvements done in recent years focused on reducing
+this load time and reducing the latency of the requests.
+
+=== HTTP/1.1
+
+HTTP/1.1 quickly followed and added a keep-alive mechanism
+to allow using the same connection for many requests, as
+well as streaming capabilities, allowing an endpoint to send
+a body in well defined chunks.
+
+HTTP/1.1 defines the OPTIONS, GET, HEAD, POST, PUT, DELETE,
+TRACE and CONNECT methods. The PATCH method was added in more
+recent years. It also improves the caching capabilities with
+the introduction of many headers.
+
+HTTP/1.1 still works like HTTP/1.0 does, except the connection
+can be kept alive for subsequent requests. This however allows
+clients to perform what is called as pipelining: sending many
+requests in a row, and then processing the responses which will
+be received in the same order as the requests.
+
+=== REST
+
+The design of HTTP/1.1 was influenced by the REST architectural
+style. REST, or REpresentational State Transfer, is a style of
+architecture for loosely connected distributed systems.
+
+REST defines constraints that systems must obey to in order to
+be RESTful. A system which doesn't follow all the constraints
+cannot be considered RESTful.
+
+REST is a client-server architecture with a clean separation
+of concerns between the client and the server. They communicate
+by referencing resources. Resources can be identified, but
+also manipulated. A resource representation has a media type
+and information about whether it can be cached and how. Hypermedia
+determines how resources are related and how they can be used.
+REST is also stateless. All requests contain the complete
+information necessary to perform the action.
+
+HTTP/1.1 defines all the methods, headers and semantics required
+to implement RESTful systems.
+
+REST is most often used when designing web application APIs
+which are generally meant to be used by executable code directly.
+
+=== XmlHttpRequest
+
+Also know as AJAX, this technology allows Javascript code running
+on a web page to perform asynchronous requests to the server.
+This is what started the move from static websites to dynamic
+web applications.
+
+XmlHttpRequest still performs HTTP requests under the hood,
+and then waits for a response, but the Javascript code can
+continue to run until the response arrives. It will then receive
+the response through a callback previously defined.
+
+This is of course still requests initiated by the client,
+the server still had no way of pushing data to the client
+on its own, so new technology appeared to allow that.
+
+=== Long-polling
+
+Polling was a technique used to overcome the fact that the server
+cannot push data directly to the client. Therefore the client had
+to repeatedly create a connection, make a request, get a response,
+then try again a few seconds later. This is overly expensive and
+adds an additional delay before the client receives the data.
+
+Polling was necessary to implement message queues and other
+similar mechanisms, where a user must be informed of something
+when it happens, rather than when he refreshes the page next.
+A typical example would be a chat application.
+
+Long-polling was created to reduce the server load by creating
+less connections, but also to improve latency by getting the
+response back to the client as soon as it becomes available
+on the server.
+
+Long-polling works in a similar manner to polling, except the
+request will not get a response immediately. Instead the server
+leaves it open until it has a response to send. After getting
+the response, the client creates a new request and gets back
+to waiting.
+
+You probably guessed by now that long-polling is a hack, and
+like most hacks it can suffer from unforeseen issues, in this
+case it doesn't always play well with proxies.
+
+=== HTML5
+
+HTML5 is, of course, the HTML version after HTML4. But HTML5
+emerged to solve a specific problem: dynamic web applications.
+
+HTML was initially created to write web pages which compose
+a website. But soon people and companies wanted to use HTML
+to write more and more complex websites, eventually known as
+web applications. They are for example your news reader, your
+email client in the browser, or your video streaming website.
+
+Because HTML wasn't enough, they started using proprietary
+solutions, often implemented using plug-ins. This wasn't
+perfect of course, but worked well enough for most people.
+
+However, the needs for a standard solution eventually became
+apparent. The browser needed to be able to play media natively.
+It needed to be able to draw anything. It needed an efficient
+way of streaming events to the server, but also receiving
+events from the server.
+
+The solution went on to become HTML5. At the time of writing
+it is being standardized.
+
+=== EventSource
+
+EventSource, sometimes also called Server-Sent Events, is a
+technology allowing servers to push data to HTML5 applications.
+
+EventSource is one-way communication channel from the server
+to the client. The client has no means to talk to the server
+other than by using HTTP requests.
+
+It consists of a Javascript object allowing setting up an
+EventSource connection to the server, and a very small protocol
+for sending events to the client on top of the HTTP/1.1
+connection.
+
+EventSource is a lightweight solution that only works for
+UTF-8 encoded text data. Binary data and text data encoded
+differently are not allowed by the protocol. A heavier but
+more generic approach can be found in Websocket.
+
+=== 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.
+
+=== SPDY
+
+SPDY is an attempt to reduce page loading time by opening a
+single connection per server, keeping it open for subsequent
+requests, and also by compressing the HTTP headers to reduce
+the size of requests.
+
+SPDY is compatible with HTTP/1.1 semantics, and is actually
+just a different way of performing HTTP requests and responses,
+by using binary frames instead of a text-based protocol.
+SPDY also allows the server to send extra responses following
+a request. This is meant to allow sending the resources
+associated with the request before the client requests them,
+saving latency when loading websites.
+
+SPDY is an experiment that has proven successful and is used
+as the basis for the HTTP/2.0 standard.
+
+Browsers make use of TLS Next Protocol Negotiation to upgrade
+to a SPDY connection seamlessly if the protocol supports it.
+
+The protocol itself has a few shortcomings which are being
+fixed in HTTP/2.0.
+
+=== HTTP/2.0
+
+HTTP/2.0 is the long-awaited update to the HTTP/1.1 protocol.
+It is based on SPDY although a lot has been improved at the
+time of writing.
+
+HTTP/2.0 is an asynchronous two-ways communication channel
+between two endpoints.
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