Se3316a 2013 09b REST-full-webservices

1Copyright [2005]. Roger L. Costello, Timothy D. Kehoe. All Rights Reserved.

REST (Representational State Transfer)

Roger L. Costello Timothy D. Kehoe


Overview

The Web's simple set of operations, i.e., the Web's API

The Web's fundamental components REST Questions and Answers The REST design pattern (summary) References


The Web's Simple Set of Operations

(i.e., the Web's API*)

* Application Programming Interface


Web Basics: Retrieving Information using HTTP GET

AmazonWeb Server

GET / HTTP/1.1Host: http://www.amazon.com

http://www.amazon.com

- The user types in at his browser: http://www.amazon.com - The browser software creates an HTTP header (no payload)

- The HTTP header identifies:- The desired action: GET ("get me resource")- The target machine (www.amazon.com)


Web Basics: Updating Information using HTTP POST

AmazonWeb Server

POST / HTTP/1.1Host: http://www.amazon.com

Book: DaVince CodeCredit Card: VisaNumber: 123-45-6789Expiry: 12-04-06

- The user fills in the Web page's form- The browser software creates an HTTP header with a payload

comprised of the form data- The HTTP header identifies:

- The desired action: POST ("here's some update info")- The target machine (amazon.com)

- The payload contains:- The data being POSTed (the form data)

Book: Da Vince CodeCredit Card: VisaNumber: 123-45-6789Expiry: 12-04-06


Terminology: Header and Payload

AmazonWeb Server

POST / HTTP/1.1Host: http://www.amazon.com

Book: DaVince CodeCredit Card: VisaNumber: 123-45-6789Expiry: 12-04-06

Book: Da Vince CodeCredit Card: VisaNumber: 123-45-6789Expiry: 12-04-06

This information (the form data) is called the payload.

This information(the HTTP data)

is called the HTTP header


Web Basics: Simple Set of Operations, via the HTTP API HTTP provides a simple set of operations.

Amazingly, all Web exchanges are done using this simple HTTP API: GET = "give me some info" (Retrieve) POST = "here's some update info" (Update) PUT = "here's some new info" (Create) DELETE = "delete some info" (Delete)

The HTTP API is CRUD (Create, Retrieve, Update, and Delete)


Fundamental Web Components


Web Component: Firewalls(Firewall: "Should I allow POSTing to geocities?")

POST / HTTP/1.1Host: http://www.geocities.com/crafts/woodworking/fred

Item: Woodworking setCredit Card: VisaNumber: 123-45-6789Expiry: 12-04-06

Item: Woodworking setCredit Card: VisaNumber: 123-45-6789Expiry: 12-04-06

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Firewall Rules & PoliciesProhibit all POSTs to the geocities Web site.

The firewall decides whether an HTTP message should pass through

All decisions are based purely upon the HTTP header. The firewall never looks in the payload. This is fundamental!

This message is rejected!


Basic Web Components: Firewalls, Routers, Caches

The Web is comprised of some basic components: Firewalls: these components decide what HTTP messages get out,

and what get in. These components enforce Web security.

Routers: these components decide where to send HTTP messages. These components manage Web scaling.

Caches: these components decide if a saved copy can be used. These components increase Web speed.

All these components base their decisions and actions purely upon information in the HTTP header. "Thou shalt never peek inside the HTTP payload!"


Web Components Operate using only Information found in the

HTTP Header!

HTTP HeaderHTTP Payload

Web Message

Component(firewall, router, cache)


Letter Analogy

Why do Web components base their decisions solely on information in the HTTP header?

My company has a receiving warehouse. All letters and packages go there first, and from there they are distributed.

No one in the receiving warehouse may look inside any letter or package. All decisions about what to do with letters and packages must be made purely by looking at the addressing on the outside. Any attempt to peek inside of letters and packages is a violation of Federal Law (U.S.).


REST


What is REST?

" REST " was coined by Roy Fielding in his Ph.D. dissertation [1] to describe a design pattern for implementing networked systems.

[1] http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm


Why is it called "Representational State Transfer? "

ResourceClienthttp://www.boeing.com/aircraft/747

Boeing747.html

The Client references a Web resource using a URL. A representation of the resource is returned (in this case as an HTML document).The representation (e.g., Boeing747.html) places the client in a new state. When the client selects a hyperlink in Boeing747.html, it accesses another resource. The new representation places the client application into yet another state. Thus, the client application transfers state with each resource representation.

Fuel requirementsMaintenance schedule...


Representational State Transfer"Representational State Transfer is intended to evoke an image of how a well-designed Web application behaves: a network of web pages (a virtual state-machine), where the user progresses through an application by selecting links (state transitions), resulting in the next page (representing the next state of the application) being transferred to the user and rendered for their use."

- Roy Fielding


Motivation for REST

The motivation for developing REST was to create a design pattern for how the Web should work, such that it could serve as the guiding framework for the Web standards and designing Web services.


REST - Not a Standard REST is not a standard

You will not see the W3C putting out a REST specification. You will not see IBM or Microsoft or Sun selling a REST

developer's toolkit. REST is just a design pattern

You can't bottle up a pattern. You can only understand it and design your Web services to it.

REST does prescribe the use of standards: HTTP URL XML/HTML/GIF/JPEG/etc. (Resource Representations) text/xml, text/html, image/gif, image/jpeg, etc. (Resource Types,

MIME Types)


REST defined

Resources are identified by uniform resource identifiers (URIs)

Resources are manipulated through their representations

Messages are self-descriptive and stateless Multiple representations are accepted or

sent Hypertext is the engine of application state

Representation

Resources are just concepts URIs tell a client that there's a concept

somewhere Clients can then request a specific

representation of the concept from the representations the server makes available

There may be HTML representation (web page), XML representation, JPEG etc

State

State means application/session state Maintained as part of the content

transferred from client to server back to client

Thus any server can potentially continue transaction from the point where it was left off

State is never left in limbo

Transfer of state

Connectors (client, server, cache, resolver, tunnel) are unrelated to sessions

State is maintained by being transferred from clients to servers and back to clients

Nouns and Verbs

Nouns are URIs Each noun (URI) represents a concept Trillions of nouns to represent all the concepts

in all the servers Verbs are actions that are applied to nouns

ReST prescribes four verbs for every noun Get, Put, Post, Delete Or CRUD (Create, Retrieve, Update, Delete)

Four verbs for every noun

GET to retrieve information POST to add new information, showing its

relation to old information PUT to update information DELETE to discard information

Put or Post

Both can be used to Create and Update What you pick may depend on

Does the URI exist already? (Put if it doesnt) Do you create the name (Put) or does the server

(Post) Does repeated invocations keep building up the

resource (Post) or has no effect (Put why?)


Learn by Example

The REST design pattern is best explained with an example.

I will present an example of a company deploying three Web services using the REST design pattern.


Parts Depot Web Services

Parts Depot, Inc has deployed some web services to enable its customers to: get a list of parts get detailed information about a particular part submit a Purchase Order (PO)


The REST way of Designing the Web Services

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HTTP POST URL 3PO(HTML/XML)

HTTP GET request URL 1

HTTP responseURL to submitted PO

PartsList

PartData

PO

HTTP responseResponse(HTML/XML doc)

HTTP responseResponse(HTML/XML doc)

HTTP GET request URL 2


Web Service for Clients to Retrieve a List of Parts

Service: Get a list of parts The web service makes available a URL

to a parts list resource. A client uses this URL to get the parts list: http://www.parts-depot.com/parts Note that how the web service generates the

parts list is completely transparent to the client. This is loose coupling.


REST Fundamentals Create a resource for every service. Identify each resource using a URL.


Data Returned - Parts List

Note that the parts list has links to get detailed info about each part. This is a key feature of the REST design pattern. The client transfers from one state to the next by examining and choosing from among the alternative URLs in the response document.


REST Fundamentals The data that a Web service returns

should link to other data. Thus, design your data as a network of information.

Contrast with OO design, which says to encapsulate information.


Web Service for Clients to Retrieve a Particular Part

Service: Get detailed information about a particular part The web service makes available a URL to

each part resource. For example, here's how a client requests a specific part: http://www.parts-depot.com/parts/00345


Data Returned - Part 00345

00345Widget-AThis part is used within the frap assembly

0.1010

Again observe how this data is linked to still more data - the specification for this part may be found by traversing the hyperlink. Each response document allows the client to drill down to get more detailed information.


QuestionWhat if Parts Depot has a million parts, will there be a million static pages? For example:

http://www.parts-depot/parts/000000http://www.parts-depot/parts/000001...

http://www.parts-depot/parts/999999


AnswerWe need to distinguish between a logical and a physical entity. The above URLs are logical. They express what resource is desired.

They do not identify a physical object. The advantage of using alogical identifier (URL) is that changes to the underlying implementation of the resource will be transparent to clients (that's loose coupling!).

Quite likely, Parts Depot will store all parts data in a database. Code at the Parts Depot web site will receive each logical URL request, parse it to determine which part is being requested, query the database, and generate the part response document to return to the client.


Answer (cont.)Contrast the above logical URLs with these physical URLs:

http://www.parts-depot/parts/000000.htmlhttp://www.parts-depot/parts/000001.html...

http://www.parts-depot/parts/999999.html

These URLs are clearly pointing to physical (HTML) pages. If there are a million parts it will not be very attractive to have a million static pages. Furthermore, changes to how these parts data is represented will effect all clients that were using the old representation.


QuestionWhat if I have a complex query?

For example: Show me all parts whose unit cost is under $0.50 and for which the quantity is less than 10

How would you do that with a simple URL?


AnswerFor complex queries, Parts Depot will provide a service (resource) for a client to retrieve a form that the client then fills in.

When the client hits "Submit" the browser will gather up the clients responses and generate a URL based on the responses.

Thus, oftentimes the client doesn't generate the URL (think about using Amazon - you start by entering the URL to amazon.com; from then on you simply fill in forms, and the URLs are automatically created for you).


Summary of theREST Design Pattern


The REST Design Pattern Create a resource for every service. Uniquely identify each resource with a

logical URL. Design your information to link to other

information. That is, the information that a resource returns to a client should link to other information in a network of related information.


The REST Design Pattern (cont.) All interactions between a client and a web

service are done with simple operations. Most web interactions are done using HTTP and just four operations: retrieve information (HTTP GET) create information (HTTP PUT) update information (HTTP POST) delete information (HTTP DELETE)


The REST Design Pattern (cont.) Remember that Web components (firewalls,

routers, caches) make their decisions based upon information in the HTTP Header. Consequently, the destination URL must be placed in the HTTP header for Web components to operate effectively. Conversely, it is anti-REST if the HTTP header

just identifies an intermediate destination and the payload identifies the final destination.


References Paul Prescod has written several excellent

articles on REST: Second Generation Web Services

http://www.xml.com/pub/a/2002/02/06/rest.html REST and the Real World

http://www.xml.com/pub/a/2002/02/20/rest.html SOAP, REST and Interoperability

http://www.prescod.net/rest/standardization.html Evaluating XML for Protocol Control Data

http://www.prescod.net/xml/envelopes/

Further Reading

Lessons learnt from designing the Red Hat

Enterprise Virtualization API

restful-api-design.readthedocs.org/en/latest

Presentation by Brian Mulloy at Apigee where he

starts with a typical poorly-designed API and

iterate it into a well-behaved RESTful API

blog.apigee.com/detail/restful_api_design

How to Avoid Common REST Mistakes

[Long URL. JGI]

6 Examples [rest.elkstein.org]

The Google Glass API: youtu.be/JpWmGX55a40

Twitter API: dev.twitter.com/docs/api

Flickr API: www.flickr.com/services/api/

Amazon S3: docs.aws.amazon.com/AmazonS3/2006-03-

01/API/APIRest.html

Atom: RFC 4287 and 5023

Tesla Model S: docs.timdorr.apiary.io/

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