
= Components Of A Hypermedia System
:chapter: 02
:url: /hypermedia-components/

A _hypermedia system_ consists of a number of components, including:

* A hypermedia, such as HTML.
* A network protocol, such as HTTP.
* A server that presents a hypermedia API responding to network requests with hypermedia responses.
* A client that properly interprets those responses.

In this chapter we will look at these components and their implementation in the context of the web.

Once we have reviewed the major components of the web as a hypermedia system, we will look at some key ideas behind this system -- especially as developed by Roy
Fielding in his dissertation, "`Architectural Styles and the Design of Network-based Software Architectures.`"  We will see where the
terms REpresentational State Transfer (REST), RESTful and Hypermedia As The Engine Of Application State (HATEOAS) come from,
and we will analyze these terms in the context of the web.

This should give you a stronger understanding of the theoretical basis of the web as a hypermedia system, how it is
supposed to fit together, and why Hypermedia-Driven Applications are RESTful, whereas JSON APIs -- despite the way the
term REST is currently used in the industry -- are not.

== Components Of A Hypermedia System

=== The Hypermedia

The fundamental technology of a hypermedia system is a hypermedia that allows a
client and server to communicate with one another in a dynamic, non-linear fashion.  Again, what makes a hypermedia
a hypermedia is the presence of _hypermedia controls_: elements that allow users to select
non-linear actions within the hypermedia. Users can _interact_ with the media in a manner beyond
simply reading from start to end.

We have already mentioned the two primary hypermedia controls in HTML, anchors and forms, which allow a browser to
present links and operations to a user through a browser.

((("Uniform Resource Locator (URL)")))
In the case of HTML, these links and forms typically specify the target of their operations using [.dfn]_Uniform Resource
Locators (URLs)_:

Uniform Resource Locator:: A uniform resource locator is a textual string that refers to, or _points to_ a location
on a network where a _resource_ can be retrieved from, as well as the mechanism by which the resource can be retrieved.

A URL is a string consisting of various subcomponents:

.URL Components
----
[scheme]://[userinfo]@[host]:[port][path]?[query]#[fragment]
----

Many of these subcomponents are not required, and are often omitted.

A typical URL might look like this:

.A simple URL
----
https://hypermedia.systems/book/contents/
----

This particular URL is made up of the following components:

* A protocol or scheme (in this case, `https`)
* A domain (e.g., `hypermedia.systems`)
* A path (e.g., `/book/contents`)

This URL uniquely identifies a retrievable _resource_ on the internet, to which an _HTTP Request_ can be issued by
a hypermedia client that "`speaks`" HTTPS, such as a web browser.  If this URL is found as the reference of a
hypermedia control within an HTML document, it implies that there is a _hypermedia server_ on the other side of the
network that understands HTTPS as well, and that can respond to this request with a _representation_ of the given
resource (or redirect you to another location, etc.)

Note that URLs are often not written out entirely within HTML.  It is very common to see anchor tags that look like this,
for example:

.A Simple Link
[source, html]
----
<a href="/book/contents/">Table Of Contents</a>
----

Here we have a _relative_ hypermedia reference, where the protocol, host and port are _implied_ to be that of the "`current
document,`" that is, the same as whatever the protocol and server were to retrieve the current HTML page.  So, if this
link was found in an HTML document retrieved from `https://hypermedia.systems/`, then the implied URL for this anchor
would be `https://hypermedia.systems/book/contents/`.

=== Hypermedia Protocols

The hypermedia control (link) above tells a browser: "`When a user clicks on this text, issue a request to
`https://hypermedia.systems/book/contents/` using the Hypertext Transfer Protocol,`" or HTTP.

HTTP is the _protocol_ used to transfer HTML (hypermedia) between browsers (hypermedia clients) and servers (hypermedia
servers) and, as such, is the key network technology that binds the distributed hypermedia system of the web together.

HTTP version 1.1 is a relatively simple network protocol, so lets take a look at what the `GET` request triggered by the anchor
tag would look like.  This is the request that would be sent to the server found at `hypermedia.systems`, on port `80`
by default:

[source, http]
----
GET /book/contents/ HTTP/1.1
Accept: text/html,*/*
Host: hypermedia.systems
----

The first line specifies that this is an HTTP `GET` request.  It then specifies the path of the resource being
requested.  Finally, it contains the HTTP version for this request.

After that are a series of HTTP _request headers_: individual lines of name/value pairs separated by a colon. The request headers provide
_metadata_ that can be used by the server to determine exactly how to respond to the client request.  In this case,
with the `Accept` header, the browser is saying it would prefer HTML as a response format, but will accept any server response.

Next, it has a `Host` header that specifies which server the request has been sent to. This is useful when multiple
domains are hosted on the same host.

An HTTP response from a server to this request might look something like this:

[source, http]
----
HTTP/1.1 200 OK
Content-Type: text/html; charset=utf-8
Content-Length: 870
Server: Werkzeug/2.0.2 Python/3.8.10
Date: Sat, 23 Apr 2022 18:27:55 GMT

<html lang="en">
<body>
  <header>
    <h1>HYPERMEDIA SYSTEMS</h1>
  </header>
  ...
</body>
</html>
----

In the first line, the HTTP Response specifies the HTTP version being used, followed by a _response code_ of `200`,
indicating that the given resource was found and that the request succeeded.  This is followed by a string, `OK` that
corresponds to the response code.  (The actual string doesn't matter, it is the response code that tells the client
the result of a request, as we will discuss in more detail below.)

After the first line of the response, as with the HTTP Request, we see a series of _response headers_ that provide
metadata to the client to assist in displaying the _representation_ of the resource correctly.

Finally, we see some new HTML content.  This content is the HTML _representation_ of the requested resource, in this
case a table of contents of a book.  The browser will use this HTML to replace the entire content in its display window,
showing the user this new page, and updating the address bar to reflect the new URL.


==== HTTP methods

((("HTTP methods")))
((("HTTP methods", GET)))
((("HTTP methods", POST)))
((("HTTP methods", PUT)))
((("HTTP methods", PATCH)))
((("HTTP methods", DELETE)))

The anchor tag above issued an HTTP `GET`, where `GET` is the _method_ of the request.  The particular method
being used in an HTTP request is perhaps the most important piece of information about it, after the actual resource that
the request is directed at.

There are many methods available in HTTP; the ones of most practical importance to developers are the following:

`GET`::
  A GET request retrieves the representation of the specified resource. GET requests should not mutate data.

`POST`::
  A POST request submits data to the specified resource. This will often result in a mutation of state on the server.

`PUT`::
  A PUT request replaces the data of the specified resource. This results in a mutation of state on the server.

`PATCH`::
  A PATCH request replaces the data of the specified resource. This results in a mutation of state on the server.

`DELETE`::
  A DELETE request deletes the specified resource. This results in a mutation of state on the server.

(((CRUD)))
These methods _roughly_ line up with the "`Create/Read/Update/Delete`" or CRUD pattern found in many applications:

* `POST` corresponds with Creating a resource.
* `GET` corresponds with Reading a resource.
* `PUT` and `PATCH` correspond with Updating a resource.
* `DELETE` corresponds, well, with Deleting a resource.

.Put vs. Post
****
While HTTP Actions correspond roughly to CRUD, they are not the same. The technical specifications for these methods make
no such connection, and are often somewhat difficult to read.  Here, for example, is the documentation
on the distinction between a `POST` and a `PUT` from https://www.rfc-editor.org/rfc/rfc2616[RFC-2616].

[quote, RFC-2616, https://www.rfc-editor.org/rfc/rfc2616#section-9.6]
____
The target resource in a POST request is intended to handle the enclosed representation according to the
resource's own semantics, whereas the enclosed representation in a PUT request is defined as replacing the state of the
target resource.  Hence, the intent of PUT is idempotent and visible to intermediaries, even though the exact
effect is only known by the origin server.
____

In plain terms, a `POST` can be handled by a server pretty much however it likes, whereas a `PUT` should be handled
as a "`replacement`" of the resource, although the language, once again allows the server to do pretty much whatever it
would like within the constraint of being https://developer.mozilla.org/en-US/docs/Glossary/Idempotent[_idempotent_].
****

In a properly structured HTML-based hypermedia system you would use an appropriate HTTP method for the operation a
particular hypermedia control performs. For example, if a hypermedia control such as a button _deletes_ a resource,
ideally it should issue an HTTP `DELETE` request to do so.

A strange thing about HTML, though, is that the native hypermedia controls can only issue HTTP `GET` and `POST` requests. 

Anchor tags always issue a `GET` request.

Forms can issue either a `GET` or `POST` using the `method` attribute.

Despite the fact that HTML -- the world's most popular hypermedia -- has been designed alongside
HTTP (which is the Hypertext Transfer Protocol, after all!): if you wish to issue `PUT`, `PATCH` or `DELETE` requests
you currently _have to_ resort to JavaScript to do so.  Since a `POST` can do almost anything, it ends up being used for
any mutation on the server, and `PUT`, `PATCH` and `DELETE` are left aside in plain HTML-based
applications.

This is an obvious shortcoming of HTML as a hypermedia; it would be wonderful to see this fixed in the
HTML specification. For now, in Chapter 4, we'll discuss ways to get around this.

==== HTTP response codes

HTTP request methods allow a client to tell a server _what_ to do to a given resource.  HTTP responses contain
_response codes_, which tell a client what the result of the request was.  HTTP response codes are numeric
values that are embedded in the HTTP response, as we saw above.

The most familiar response code for web developers is probably `404`, which stands for "`Not Found.`"  This
is the response code that is returned by web servers when a resource that does not exist is requested from them.

((("HTTP response", codes)))
HTTP breaks response codes up into various categories:

`100`-`199`::
  Informational responses that provide information about how the server is processing the response.

`200`-`299`::
  Successful responses indicating that the request succeeded.

`300`-`399`::
  Redirection responses indicating that the request should be sent to some other URL.

`400`-`499`::
  Client error responses indicating that the client made some sort of bad request (e.g., asking for something that didn't
exist in the case of `404` errors).

`500`-`599`::
  Server error responses indicating that the server encountered an error internally as it attempted to respond to the request.

Within each of these categories there are multiple response codes for specific situations.

Here are some of the more common or interesting ones:

`200 OK`::
  The HTTP request succeeded.

`301 Moved Permanently`::
  The URL for the requested resource has moved to a new location permanently, and the new URL will be provided in
  the `Location` response header.

`302 Found`::
  The URL for the requested resource has moved to a new location temporarily, and the new URL will be provided in
  the `Location` response header.

`303 See Other`::
  The URL for the requested resource has moved to a new location, and the new URL will be provided in
  the `Location` response header.  Additionally, this new URL should be retrieved with a `GET` request.

`401 Unauthorized`::
  The client is not yet authenticated (yes, authenticated, despite the name) and must be authenticated
  to retrieve the given resource.

`403 Forbidden`::
  The client does not have access to this resource.

`404 Not Found`::
  The server cannot find the requested resource.

`500 Internal Server Error`::
  The server encountered an error when attempting to process the response.

There are some fairly subtle differences between HTTP response codes (and, to be honest, some ambiguities between them).
The difference between a `302` redirect and a `303` redirect, for example, is that the former will issue the request to the
new URL using the same HTTP method as the initial request, whereas the latter will always use a `GET`.  This is a small
but often crucial difference, as we will see later in the book.

A well crafted Hypermedia-Driven Application will take advantage of both HTTP methods and HTTP response codes to create
a sensible hypermedia API.  You do not want to build a Hypermedia-Driven Application that uses a `POST` method for all
requests and responds with `200 OK` for every response, for example.  (Some JSON Data APIs built on top of HTTP do exactly
this!)

When building a Hypermedia-Driven Application, you want, instead, to go "`with the grain`" of the web and use HTTP methods
and response codes as they were designed to be used.

==== Caching HTTP responses

((("HTTP response", caching)))
A constraint of REST (and, therefore, a feature of HTTP) is the notion of caching responses: a server can indicate to
a client (as well as intermediary HTTP servers) that a given response can be cached for future requests to the same
URL.

((("HTTP response header","Cache-Control")))
The cache behavior of an HTTP response from a server can be indicated with the `Cache-Control` response header.  This
header can have a number of different values indicating the cacheability of a given response.  If, for example, the header
contains the value `max-age=60`, this indicates that a client may cache this response for 60 seconds, and need not issue
another HTTP request for that resource until that time limit has expired.

((("HTTP response header", Vary)))
Another important caching-related response header is `Vary`.  This response header can be used to indicate exactly what
headers in an HTTP Request form the unique identifier for a cached result.  This becomes important to allow the browser
to correctly cache content in situations where a particular header affects the form of the server response.

((("HTTP response header", custom)))
(((HX-Request, about)))
A common pattern in htmx-powered applications, for example, is to use a custom header set by htmx, `HX-Request`, to
differentiate between "`normal`" web requests and requests submitted by htmx.  To properly cache the response to these
requests, the `HX-Request` request header must be indicated by the `Vary` response header.

A full discussion of caching HTTP responses is beyond the scope of this chapter; see
the https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching[MDN Article on HTTP Caching] if you would like to know more
on the topic.

=== Hypermedia Servers

Hypermedia servers are any server that can respond to an HTTP request with an HTTP response.  Because HTTP is so simple,
this means that nearly any programming language can be used to build a hypermedia server.  There are a vast number of
libraries available for building HTTP-based hypermedia servers in nearly every programming language imaginable.

This turns out to be one of the best aspects of adopting hypermedia as your primary technology for building a web application: it removes
the pressure to adopt JavaScript as a backend technology.  If you use a JavaScript-heavy Single Page Application-based
front end, and you use JSON Data APIs, you are going to feel significant pressure to deploy JavaScript on the back end as
well.

In this latter situation, you already have a ton of code written in JavaScript.  Why maintain two separate code bases in
two different languages? Why not create reusable domain logic on the client-side as well as the server-side?  Now that
JavaScript has excellent server-side technologies available like Node and Deno, why not just use a single language for
everything?

In contrast, building a Hypermedia-Driven Application gives you a lot more freedom in picking the back end technology you want
to use.  Your decision can be based on the domain of your application, what languages and server software you are familiar
with or are passionate about, or just what you feel like trying out.

You certainly aren't writing your server-side logic in HTML!  And every major programming language has at least one good
web framework and templating library that can be used to handle HTTP requests cleanly.

If you are doing something in big data, perhaps you'd like to use Python, which has tremendous support for that
domain.

If you are doing AI work, perhaps you'd like to use Lisp, leaning on a language with a long history in that area of research.

Maybe you are a functional programming enthusiast and want to use OCaml or Haskell.  Perhaps you just really like Julia or Nim.

These are all perfectly valid reasons for choosing a particular server-side technology!

By using hypermedia as your system architecture, you are freed up to adopt any of these choices. There simply isn't a
large JavaScript code base on the front end pressuring you to adopt JavaScript on the back end.

.Hypermedia On Whatever you'd Like (HOWL)
****
In the htmx community we call this (with tongue in cheek) the HOWL stack: Hypermedia On Whatever you'd Like.  The htmx community
is multi-language and multi-framework, there are rubyists as well as pythonistas, lispers as well as haskellers.  There
are even JavaScript enthusiasts!  All these languages and frameworks are able to adopt hypermedia, and are able to still
share techniques and offer support to one another because they share a common underlying architecture: they are all using
the web as a hypermedia system.

Hypermedia, in this sense, provides a "`universal language`" for the web that we can all use.
****

=== Hypermedia Clients

((("web browsers")))
We now come to the final major component in a hypermedia system: the hypermedia client.  Hypermedia _clients_ are software
that understand how to interpret a particular hypermedia, and the hypermedia controls within it, properly.  The canonical
example, of course, is the web browser, which understands HTML and can present it to a user to interact with. Web browsers
are incredibly sophisticated pieces of software.  (So sophisticated, in fact, that they are often re-purposed away from
being a hypermedia client, to being a sort of cross-platform virtual machine for launching Single Page Applications.)

Browsers aren't the only hypermedia clients out there, however.  In the last section of this book we will look at
Hyperview, a mobile-oriented hypermedia.  One of the outstanding features of Hyperview is that it doesn't simply provide
a hypermedia, HXML, but also provides a _working hypermedia client_ for that hypermedia.  This makes building a proper
Hypermedia-Driven Application with Hyperview extremely easy.

A crucial feature of a hypermedia system is what is known as _the uniform interface_.  We discuss this concept in depth
in the next section on REST.  What is often ignored in discussions about hypermedia is how important the hypermedia
client is in taking advantage of this uniform interface.  A hypermedia client must know how to properly interpret and
present hypermedia controls found in a hypermedia response from a hypermedia server for the whole hypermedia system
to hang together.  Without a sophisticated client that can do this, hypermedia controls and a hypermedia-based API are
much less useful.

This is one reason why JSON APIs have rarely adopted hypermedia controls successfully: JSON APIs are typically consumed
by code that is expecting a fixed format and that isn't designed to be a hypermedia client.  This is totally understandable:
building a good hypermedia client is hard!  For JSON API clients like this, the
power of hypermedia controls embedded within an API response is irrelevant and often simply annoying:

[quote, Freddie Karlbom,https://techblog.commercetools.com/graphql-and-rest-level-3-hateoas-70904ff1f9cf]
____
The short answer to this question is that HATEOAS isn’t a good fit for most modern use cases for APIs. That is why
after almost 20 years, HATEOAS still hasn’t gained wide adoption among developers. GraphQL on the other hand is spreading
like wildfire because it solves real-world problems.
____

HATEOAS will be described in more detail below, but the takeaway here is that a good hypermedia client is a necessary
component within a larger hypermedia system.

== REST

Now that we have reviewed the major components of a hypermedia system, it's time to look more deeply into the concept of
REST.  The term "`REST`" comes from Roy Fielding's PhD dissertation on the architecture
of the web.  Fielding wrote his dissertation at U.C. Irvine, after having helped build much of the infrastructure of the early
web, including the Apache web server.  Roy was attempting to formalize and describe the novel distributed computing system
that he had helped to build.

We are going to focus on what we feel is the most important section of Fielding's writing, from a web development
perspective: Section 5.1. This section contains the core concepts (Fielding calls them _constraints_) of Representational
State Transfer, or REST.

Before we get into the muck, however, it is important to understand that Fielding discusses REST as a _network architecture_,
that is, as an entirely different way to architect a distributed system.  And, further, as a novel network
architecture that should be _contrasted_ with earlier approaches to distributed systems.

It is also important to emphasize that, at the time Fielding wrote his dissertation, JSON APIs and AJAX did not exist.
He was describing the early web, with HTML being transferred over HTTP by early browsers, as a hypermedia system.

Today, in a strange turn of events, the term "`REST`" is mainly associated with JSON Data APIs, rather than with HTML
and hypermedia.  This is extremely funny once you realize that the vast majority of JSON Data APIs aren't
RESTful, in the original sense, and, in fact, _can't_ be RESTful, since they aren't using a natural hypermedia format.

To re-emphasize: REST, as coined by Fielding, describes _the pre-API web_, and letting go of the current, common
usage of the term REST to simply mean "`a JSON API`" is necessary to develop a proper understanding of the idea.

=== The "`Constraints`" of REST

((("Fielding, Roy")))
(((REST, constraints)))
In his dissertation, Fielding defines various "`constraints`" to describe how a RESTful system must behave.  This approach
can feel a little round-about and difficult to follow for many people, but it is an appropriate approach for an academic
document.  Given a bit of time thinking about the constraints he outlines and some concrete examples of those
constraints it will become easy to assess whether a given system actually satisfies the architectural requirements of
REST or not.

Here are the constraints of REST Fielding outlines:

* It is a client-server architecture (section 5.1.2).
* It must be stateless; (section 5.1.3) that is, every request contains all information necessary to respond to that request.
* It must allow for caching (section 5.1.4).
* It must have a _uniform interface_ (section 5.1.5).
* It is a layered system (section 5.1.6).
* Optionally, it can allow for Code-On-Demand (section 5.1.7), that is, scripting.

Let's go through each of these constraints in turn and discuss them in detail, looking at how (and to what extent) the web
satisfies each of them.

=== The Client-Server Constraint

See https://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm#sec_5_1_2[Section 5.1.2] for the
Client-Server constraint.

The REST model Fielding was describing involved both _clients_ (browsers, in the case of the web) and _servers_ (such
as the Apache Web Server he had been working on) communicating via a network connection.  This was the context of his
work: he was describing the network architecture of the World Wide Web, and contrasting it with earlier architectures,
notably thick-client networking models such as the Common Object Request Broker Architecture (CORBA).

It should be obvious that any web application, regardless of how it is designed, will satisfy this requirement.

=== The Statelessness Constraint

See https://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm#sec_5_1_3[Section 5.1.3] for the Stateless constraint.

As described by Fielding, a RESTful system is stateless: every request should encapsulate all information necessary to
respond to that request, with no side state or context stored on either the client or the server.

In practice, for many web applications today, we actually violate this constraint: it is common to establish a
_session cookie_ that acts as a unique identifier for a given user and that is sent along with every request.  While this
session cookie is, by itself, not stateful (it is sent with every request), it is typically
used as a key to look up information stored on the server, in what is usually termed "`the session.`"

This session information is typically stored in some sort of shared storage across multiple web servers, holding things
like the current user's email or id, their roles, partially created domain objects, caches, and so forth.

This violation of the Statelessness REST architectural constraint has proven to be useful for building web applications
and does not appear to have had a major impact on the overall flexibility the web.  But it is worth bearing in mind that
even Web 1.0 applications often violate the purity of REST in the interest of pragmatic trade-offs.

And it must be said that sessions _do_ cause additional operational complexity headaches when deploying hypermedia
servers; these may need shared access to session state information stored across an entire cluster.  So
Fielding was correct in pointing out that an ideal RESTful system, one that did not violate this constraint, would be simpler and therefore more robust.

=== The Caching Constraint

See https://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm#sec_5_1_4[Section 5.1.4] for the Caching constraint.

This constraint states that a RESTful system should support the notion of caching, with explicit information on the
cache-ability of responses for future requests of the same resource.  This allows both clients as well as intermediary
servers between a given client and final server to cache the results of a given request.

As we discussed earlier, HTTP has a sophisticated caching mechanism via response headers that is often overlooked or
underutilized when building hypermedia applications.  Given the existence of this functionality, however, it is
easy to see how this constraint is satisfied by the web.

=== The Uniform Interface Constraint

Now we come to the most interesting and, in our opinion, most innovative constraint in REST: that of the _uniform interface_.

This constraint is the source of much of the _flexibility_ and _simplicity_ of a hypermedia system, so we are going to
spend some time on it.

See https://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm#sec_5_1_5[Section 5.1.5] for the Uniform Interface
constraint.

In this section, Fielding says:

[quote, Roy Fielding, Architectural Styles and the Design of Network-based Software Architectures]
____
The central feature that distinguishes the REST architectural style from other network-based styles is its emphasis on
a uniform interface between components... In order to obtain a uniform interface, multiple architectural constraints
are needed to guide the behavior of components. REST is defined by four interface constraints: identification of
resources; manipulation of resources through representations; self-descriptive messages; and, hypermedia as the engine
of application state
____

So we have four sub-constraints that, taken together, form the Uniform Interface constraint.

==== Identification of resources

In a RESTful system, resources should have a unique identifier.  Today the concept of Universal Resource Locators (URLs) is
common, but at the time of Fielding's writing they were still relatively new and novel.

What might be more interesting today is the notion of a _resource_, thus being identified: in a RESTful system, _any_ sort of
data that can be referenced, that is, the target of a hypermedia reference, is considered a resource.  URLs, though common
enough today, end up solving the very complex problem of uniquely identifying any and every resource on the internet.

==== Manipulation of resources through representations

In a RESTful system, _representations_ of the resource are transferred between clients and servers.  These
representations can contain both data and metadata about the request (such as "`control data`" like an HTTP
method or response code).  A particular data format or _media type_ may be used to present a given resource to a client,
and that media type can be negotiated between the client and the server.

We saw this latter aspect of the uniform interface in the `Accept` header in the requests above.

==== Self-descriptive messages

((("self-descriptive messages")))
The Self-Descriptive Messages constraint, combined with the next one, HATEOAS, form what we consider to be the core of
the Uniform Interface, of REST and why hypermedia provides such a powerful system architecture.

The Self-Descriptive Messages constraint requires that, in a RESTful system, messages must be _self-describing_.

This means that _all information_ necessary to both display _and also operate_ on the data being represented must be
present in the response.  In a properly RESTful system, there can be no additional "`side`" information necessary for a
client to transform a response from a server into a useful user interface.  Everything must "`be in`" the message itself,
in the form of hypermedia controls.

This might sound a little abstract so let's look at a concrete example.

Consider two different potential responses from an HTTP server for the URL `\https://example.com/contacts/42`.

Both responses will return information about a contact, but each response will take very different forms.

The first implementation returns an HTML representation:

[source,html]
----
<html lang="en">
<body>
<h1>Joe Smith</h1>
<div>
    <div>Email: joe@example.bar</div>
    <div>Status: Active</div>
</div>
<p>
    <a href="/contacts/42/archive">Archive</a>
</p>
</body>
</html>
----

The second implementation returns a JSON representation:

[source,json]
----
{
  "name": "Joe Smith",
  "email": "joe@example.org",
  "status": "Active"
}
----

What can we say about the differences between these two responses?

One thing that may initially jump out at you is that the JSON representation is smaller than the HTML
representation.  Fielding notes exactly this trade-off when using a RESTful architecture:

[quote, Roy Fielding, Architectural Styles and the Design of Network-based Software Architectures]
____
The trade-off, though, is that a uniform interface degrades efficiency, since information is transferred in a
standardized form rather than one which is specific to an application's needs.
____

So REST _trades off_ representational efficiency for other goals.

To understand these other goals, first notice that the HTML representation has a hyperlink in it to navigate to a page
to archive the contact.  The JSON representation, in contrast, does not have this link.

What are the ramifications of this fact for a _client_ of the JSON API?

((("JSON API", "vs. HTML")))
What this means is that the JSON API client must know _in advance_ exactly what other URLs (and request methods) are
available for working with the contact information.  If the JSON client is able to update this contact in some way, it
must know how to do so from some source of information _external_ to the JSON message.  If the contact has a different
status, say "`Archived`", does this change the allowable actions?  If so, what are the new allowable actions?

The source of all this information might be API documentation, word of mouth or, if the developer controls both the server
and the client, internal knowledge.  But this information is implicit and _outside_ the response.

Contrast this with the hypermedia (HTML) response.  In this case, the hypermedia client (that is, the browser) needs
only to know how to render the given HTML.  It doesn't need to understand what actions are available for this contact:
they are simply encoded _within_ the HTML response itself as hypermedia controls.  It doesn't need to understand what
the status field means.  In fact, the client doesn't even know what a contact is!

The browser, our hypermedia client, simply renders the HTML and allows the user, who presumably understands the concept
of a Contact, to make a decision on what action to pursue from the actions made available in the representation.

This difference between the two responses demonstrates the crux of REST and hypermedia, what makes them so powerful
and flexible: clients (again, web browsers) don't need to understand _anything_ about the underlying resources being
represented.

Browsers only (only! As if it is easy!) need to understand how to interpret and display hypermedia, in this case HTML.  This
gives hypermedia-based systems unprecedented flexibility in dealing with changes to both the backing representations and
to the system itself.


==== Hypermedia As The Engine of Application State (HATEOAS)

(((HATEOAS)))
The final sub-constraint on the Uniform Interface is that, in a RESTful system, hypermedia should be "`the engine of
application state.`" This is sometimes abbreviated as "`HATEOAS`", although Fielding prefers to use the terminology
"`the hypermedia constraint`" when discussing it.

This constraint is closely related to the previous self-describing message constraint.  Let us consider again the two different
implementations of the endpoint `/contacts/42`, one returning HTML and one returning JSON.  Let's update the situation
such that the contact identified by this URL has now been archived.

What do our responses look like?

The first implementation returns the following HTML:

[source,html]
----
<html lang="en">
<body>
<h1>Joe Smith</h1>
<div>
    <div>Email: joe@example.bar</div>
    <div>Status: Archived</div>
</div>
<p>
    <a href="/contacts/42/unarchive">Unarchive</a>
</p>
</body>
</html>
----

The second implementation returns the following JSON representation:

[source,json]
----
{
  "name": "Joe Smith",
  "email": "joe@example.org",
  "status": "Archived"
}
----

The important point to notice here is that, by virtue of being a self-describing message, the HTML response now shows that
the "`Archive`" operation is no longer available, and a new "`Unarchive`" operation has become available.  The HTML representation
of the contact _encodes_ the state of the application; it encodes exactly what can and cannot be done with this particular
representation, in a way that the JSON representation does not.

A client interpreting the JSON response must, again, understand not only the general concept of a Contact,
but also specifically what the "`status`" field with the value "`Archived`" means.  It must know exactly what operations
are available on an "`Archived`" contact, to appropriately display them to an end user.  The state of the application is
not encoded in the response, but rather conveyed through a mix of raw data and side channel information such as
API documentation.

Furthermore, in the majority of front end SPA frameworks today, this contact information would live _in memory_ in a
JavaScript object representing a model of the contact, while the page data is held in the browser's https://developer.mozilla.org/en-US/docs/Web/API/Document_Object_Model[Document Object Model] (DOM).  The DOM would be updated based on changes to this model, that
is, the DOM would "`react`" to changes to this backing JavaScript model.

This approach is certainly _not_ using Hypermedia As The Engine Of Application State: rather, it is using a JavaScript
model as the engine of application state, and synchronizing that model with a server and with the browser.

With the HTML approach, the Hypermedia is, indeed, The Engine Of Application State: there is no additional model on the
client side, and all state is expressed directly in the hypermedia, in this case HTML.  As state changes on the server,
it is reflected in the representation (that is, HTML) sent back to the client.  The hypermedia client (a browser) doesn't know
anything about contacts, what the concept of "`Archiving`" is, or anything else about the particular domain model for this
response: it simply knows how to render HTML.

Because a hypermedia client doesn't need to know anything about the server model beyond how to render hypermedia to
a client, it is incredibly flexible with respect to the representations it receives and displays to users.

==== HATEOAS & API churn

This last point is critical to understanding the flexibility of hypermedia, so let's look
at a practical example of it in action. Consider a situation where a new feature has been added to the web application with these
two end points.  This feature allows you to send a message to a given Contact.

How would this change each of the two responses--HTML and JSON--from the server?

The HTML representation might now look like this:

[source,html]
----
<html lang="en">
<body>
<h1>Joe Smith</h1>
<div>
    <div>Email: joe@example.bar</div>
    <div>Status: Active</div>
</div>
<p>
    <a href="/contacts/42/archive">Archive</a>
    <a href="/contacts/42/message">Message</a>
</p>
</body>
</html>
----

The JSON representation, on the other hand, might look like this:

[source,json]
----
{
  "name": "Joe Smith",
  "email": "joe@example.org",
  "status": "Active"
}
----

Note that, once again, the JSON representation is unchanged.  There is no indication of this new functionality.  Instead,
a client must _know_ about this change, presumably via some shared documentation between the client and the server.

Contrast this with the HTML response.  Because of the uniform interface of the RESTful model and, in particular,
because we are using Hypermedia As The Engine of Application State, no such exchange of documentation is necessary!  Instead,
the client (a browser) simply renders the new HTML with this operation in it, making this operation available for the end user
without any additional coding changes.

A pretty neat trick!

Now, in this case, if the JSON client is not properly updated, the error state is relatively benign: a new bit of functionality
is simply not made available to users.  But consider a more severe change to the API: what if the archive functionality
was removed?  Or what if the URLs or the HTTP methods for these operations changed in some way?

In this case, the JSON client may be broken in a much more serious manner.

The HTML response, however, would simply be updated to exclude the removed options or to update the URLs used for them.  Clients
would see the new HTML, display it properly, and allow users to select whatever the new set of operations happens to be.  Once
again, the uniform interface of REST has proven to be extremely flexible: despite a potentially radically new layout
for our hypermedia API, clients continue to work.

An important fact emerges from this:  due to this flexibility, hypermedia APIs _do not have the versioning headaches
that JSON Data APIs do_.

Once a Hypermedia-Driven Application has been "`entered into`" (that is, loaded through some entry point URL), all functionality
and resources are surfaced through self-describing messages.  Therefore, there is no need to exchange documentation with
the client: the client simply renders the hypermedia (in this case HTML) and everything works out.  When a change occurs,
there is no need to create a new version of the API: clients simply retrieve updated hypermedia, which encodes the new
operations and resources in it, and display it to users to work with.

=== Layered System

The final "`required`" constraint on a RESTful system that we will consider is The Layered System constraint.  This constraint can be found in https://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm#sec_5_1_6[Section 5.1.6] of Fielding's dissertation.

To be frank, after the excitement of the uniform interface constraint, the "`layered system`" constraint is a bit of a
let down.  But it is still worth understanding and it is actually utilized effectively by The web.  The constraint
requires that a RESTful architecture be "`layered,`" allowing for multiple servers to act as intermediaries between
a client and the eventual "`source of truth`" server.

These intermediary servers can act as proxies, transform intermediate requests and responses and so forth.

A common modern example of this layering feature of REST is the use of Content Delivery Networks (CDNs) to deliver unchanging
static assets to clients more quickly, by storing the response from the origin server in intermediate servers more
closely located to the client making a request.

This allows content to be delivered more quickly to the end user and reduces load on the origin server.

Not as exciting for web application developers as the uniform interface, at least in our opinion, but useful
nonetheless.

=== An Optional Constraint: Code-On-Demand

We called The Layered System constraint the final "`required`" constraint because
Fielding mentions one additional constraint on a RESTful system. This Code On Demand constraint is somewhat awkwardly
described as "`optional`" (Section 5.1.7).

In this section, Fielding says:

[quote, Roy Fielding, Architectural Styles and the Design of Network-based Software Architectures]
____
REST allows client functionality to be extended by downloading and executing code in the form of applets or scripts. This
simplifies clients by reducing the number of features required to be pre-implemented. Allowing features to be downloaded
after deployment improves system extensibility. However, it also reduces visibility, and thus is only an optional constraint
within REST.
____

So, scripting was and is a native aspect of the original RESTful model of the web, and thus
should of course be allowed in a Hypermedia-Driven Application.

However, in a Hypermedia-Driven Application the presence of scripting should _not_ change the fundamental networking
model: hypermedia should continue to be the engine of application state, server communication should still consist of
hypermedia exchanges rather than, for example, JSON data exchanges, and so on. (JSON Data API's certainly have their
place; in Chapter 10 we'll discuss when and how to use them).

Today, unfortunately, the scripting layer of the web, JavaScript, is quite often used to _replace_, rather than augment
the hypermedia model.  We will elaborate in a later chapter what scripting that does not replace the underlying hypermedia
system of the web looks like.

== Conclusion

After this deep dive into the components and concepts behind hypermedia systems -- including Roy Fielding's insights into their operation -- we hope you have much better understanding of REST,
and in particular, of the uniform interface and HATEOAS. We hope you can see _why_ these characteristics make hypermedia
systems so flexible.

If you were not aware of the full significance of REST and HATEOAS before now, don't feel bad: it took some of us over a decade of
working in web development, and building a hypermedia-oriented library to boot, to understand the
special nature of HTML, hypermedia and the web!


:sectnums!:

[.html-note]
== HTML Notes: HTML5 Soup

[quote,Confucius]
The beginning of wisdom is to call things by their right names.

Elements like `<section>`, `<article>`, `<nav>`, `<header>`, `<footer>`, `<figure>` have become a sort of shorthand for HTML.

By using these elements, a page can make false promises, like `<article>` elements being self-contained, reusable entities, to clients like browsers, search engines and scrapers that can't know better. To avoid this:

* Make sure that the element you're using fits your use case. Check the HTML spec.
* Don't try to be specific when you can't or don't need to.
  Sometimes, `<div>` is fine.

(((HTML, spec)))
The most authoritative resource for learning about HTML is the HTML specification.
The current specification lives on link:https://html.spec.whatwg.org/multipage[].footnote:[
The single-page version is too slow to load and render on most computers. 
There's also a developers' edition at /dev, but the standard version has nicer styling.]
There's no need to rely on hearsay to keep up with developments in HTML.

Section 4 of the spec features a list of all available elements,
including what they represent, where they can occur, and what they are allowed to contain.
It even tells you when you're allowed to leave out closing tags!
