REST

Ivano MalavoltaIvano Malavolta

ivano.malavolta@univaq.it

http://www.di.univaq.it/malavolta

Roadmap

• The REST Architectural Style

• Resources• Resources

• Representations

• Actions

• Security

REST

It stands for

REpresentational State Transfer

Proposed by Roy Fieldingsin his PhD dissertation in 2000in his PhD dissertation in 2000

REST rules the architecture ofthe World Wide Web (HTTP)

Major players

REST Architectural Style

REST is not a technology, nor a framework

REST is an ArchitecturalArchitecturalArchitecturalArchitectural Style Style Style Style

� a set of principles + constraints

Thos constraints help us in developing applications Thos constraints help us in developing applications that are “easy” to maintain and extend

REST Main Constraints

A RESTful system should be

• clientclientclientclient----serverserverserverserver• statelessstatelessstatelessstateless

– there should be no need for the service to keep users’ sessions

– each request should be independent of others– each request should be independent of others

• it has to support a caching systemcaching systemcaching systemcaching system• it has to be uniformly accessibleuniformly accessibleuniformly accessibleuniformly accessible

– each resource must have a unique address and a valid point of access

The (static) Web as a RESTful system

1. you type a URL into your browser to reach a specific HTML pagespecific HTML page

2. the browser gets and displays the elements of the HTML page

� the browser is getting a representation representation representation representation � the browser is getting a representation representation representation representation

of the current state of that resourceresourceresourceresource

REST Overview

In most cases, client-server client-server comunicationrelies on HTTP

http://bit.ly/JALve1

REST Main Actors

These are the abstractions that make a RESTful system:

• ResourcesResourcesResourcesResources

• RepresentationsRepresentationsRepresentationsRepresentations

• ActionsActionsActionsActions

Roadmap

• The REST Architectural Style

• Resources• Resources

• Representations

• Actions

• Security

Resources

A A A A resourceresourceresourceresource isisisis ““““everythingeverythingeverythingeverything” the service can ” the service can ” the service can ” the service can provideprovideprovideprovide

States and functions of a remote application are also States and functions of a remote application are also States and functions of a remote application are also States and functions of a remote application are also considered as resourcesconsidered as resourcesconsidered as resourcesconsidered as resources

Example of resources:• title of a movie from IMDb• a Flash movie from YouTube• a Flash movie from YouTube• images from Flickr• order info from eBay• etc.

Resources

In general, a RESTful resource is anything that is anything that is anything that is anything that is addressable over the Webaddressable over the Webaddressable over the Webaddressable over the Webaddressable over the Webaddressable over the Webaddressable over the Webaddressable over the Web

AddressableAddressableAddressableAddressable ==== anything that can be accessed and transferred between clients and servers

� a resource must have a unique address over the Web� a resource must have a unique address over the Web

Under HTTP these are URIsURIsURIsURIs

URIs

Uniform Resource IdentifierUniform Resource IdentifierUniform Resource IdentifierUniform Resource Identifier

in a RESTful web service is a hyperlink hyperlink hyperlink hyperlink to a resource

It is the only means for clients and servers to exchange representations of resources

ex..../orderinfo?id=123

URIs

The URI is not meant to change over timeThe URI is not meant to change over timeThe URI is not meant to change over timeThe URI is not meant to change over time� it is the only means to locate a specific resource � it is the only means to locate a specific resource

URIs are also used to negotiate representations of a given resource

In the url you give certain parametersparametersparametersparameters that define which information you want the server to return to you (just information you want the server to return to you (just like giving GET variables to a page)

The server will respond you with a resource representation containing the information you’ve asked

URIs

URIs are also used to link resources together

ex.

Roadmap

• The REST Architectural Style

• Resources• Resources

• Representations

• Actions

• Security

Representations

The representationrepresentationrepresentationrepresentation of resources is what is sent back and forth between clients and serversand forth between clients and servers

So, we never send or receive resources, only their we never send or receive resources, only their we never send or receive resources, only their we never send or receive resources, only their representationsrepresentationsrepresentationsrepresentations

URL

Uniform Resource LocatorUniform Resource LocatorUniform Resource LocatorUniform Resource Locator

A URL is a specialization of URI that defines the network location of a specific resource

Unlike a URI, the URL defines how the resource can be obtained

es.http://some.domain.com/orderinfo?id=123

Representations

The format of the representation is determined by the contentcontentcontentcontent----typetypetypetypecontentcontentcontentcontent----typetypetypetype

The interaction of the representation on the resource is determined by the action (GET, SET, etc.)

Content-types

Since we are using HTTP to communicate, we can transfer any kind of information that can be passed between any kind of information that can be passed between clients and servers

ex. text files, PDF documents, images, videos, etc.

In any case, the data is streamed over TCP/IP and the In any case, the data is streamed over TCP/IP and the browser knows how to interpret the binary streams because of the HTTP protocol response header Content-Type

Representation Formats

Different clients are able to consume different representations of the same resource

A representation can take various formsA representation can take various formsA representation can take various formsA representation can take various forms, such as:

• image• a text file• an XML stream• an XML stream• a JSON stream

but its resource has to be available through the same but its resource has to be available through the same but its resource has to be available through the same but its resource has to be available through the same URIURIURIURI

Representation Formats

For human-generated requests through a web browser, a representation is typically in the form of an HTML a representation is typically in the form of an HTML page

For automated requests from other web services, For automated requests from other web services, readability is not as important and a more efficient representation can be used such as XML or JSONXML or JSONXML or JSONXML or JSON

Roadmap

• The REST Architectural Style

• Resources• Resources

• Representations

• Actions

• Security

Actions

Actions are used to operate on resources

For example, they can be used for

– getting info about a movie

– adding a photo to Flickr

– deleting a file from a folder

The data transmitted to and from the resource is a representation of it

HTTP-based Actions

Under HTTP, actions are standard HTTP request:

GETGETGETGET

POSTPOSTPOSTPOST

PUTPUTPUTPUT

DELETEDELETEDELETEDELETEDELETEDELETEDELETEDELETE

They make up the uniform interface used for client/server data transfers

HTTP-based Actions

RESTful web services can also execute logic at the server level, but remembering that every result every result every result every result must be a resource representationmust be a resource representationmust be a resource representationmust be a resource representation

HTTP as Uniform Interface

In In In In RESTfulRESTfulRESTfulRESTful systemssystemssystemssystems wewewewe focus on focus on focus on focus on resourceresourceresourceresource namesnamesnamesnames, whereasin traditional web systems we focussed on the actions toin traditional web systems we focussed on the actions tobe performed on resources

� In RESTful systems we have four specific actions that we can take upon resources — Create, Retrieve, Update, Create, Retrieve, Update, Create, Retrieve, Update, Create, Retrieve, Update, and Delete (CRUD)and Delete (CRUD)and Delete (CRUD)and Delete (CRUD)and Delete (CRUD)and Delete (CRUD)and Delete (CRUD)and Delete (CRUD)

In traditional web applications, we could have countless actions with no naming or implementation standards

The Classroom Example

Artificial example of a web service handling students in some classroomin some classroom

Location of the service = http://restfuljava.com/

Resources are represented as XML streams

The Classroom Example: URIs

Student (identified by name):

http://restfuljava.com/students/{name}http://restfuljava.com/students/{name}

List of students: http://restfuljava.com/students

The Classroom Example: Representations

Student:

<student>

<name>Jane</name>

<age>10</age>

<link>/students/Jane</link>

</student></student>

The Classroom Example: Representations

Students List:

<students>

<student>

<name>Jane</name>

<age>10</age>

<link>/students/Jane</link>

</student>

<student><student>

<name>John</name>

<age>11</age>

<link>/students/John</link>

</student>

</students>

GET

The method GET is used to RETRIEVE resources

It cannot have side-effects

�it can be done repeatedly without changing the state of the resource

It can also return only parts of the resource

� it can act as both a read operation and a query operation

GET Example

POST

The method POST is used to CREATE resources

Usually, the resource identity/URL is not known at creation time

� The URL of the newly created resource is usuallycreated automatically by the server

POST Example

PUT

The method PUT is used to UPDATE resources

Recurrent PUT workflow:

1. we first GET the representation of the resource we need to update

2. in the client we update the resource with the new value(s) value(s)

3. we update the resource using a PUT request together with the representation as its payload

PUT Example

The initialGET is

omitted here

DELETE

The method DELETE is used to DELETE resources

Similarly to PUT, also in this case we need the URI ofthe resource being deleted

DELETE Example

A note on PUT and DELETE

PUT and DELETE apply to the entire resourcePUT and DELETE apply to the entire resourcePUT and DELETE apply to the entire resourcePUT and DELETE apply to the entire resource

� when doing a PUT or DELETE operation, the entire resource is replaced/deleted

The PUT and DELETE operations are atomicThe PUT and DELETE operations are atomicThe PUT and DELETE operations are atomicThe PUT and DELETE operations are atomic

� if two PUT/DELETE operations occur simultaneously, one of them will win and determine the final state of the resource

HTTP Status Codes

RESTful services use these codes to return information about the response of the requestsabout the response of the requests

1xx informational message

2xx success message

3xx redirects the client to another URL

4xx client-side error4xx client-side error

5xx server-side error

Roadmap

• The REST Architectural Style

• Resources• Resources

• Representations

• Actions

• Security

Security

Here we will focus on securing user access to ourservicesservices

There are three main methods:

1.1.1.1. Custom Custom Custom Custom tokentokentokentoken authenticationauthenticationauthenticationauthenticationControl access

2.2.2.2. HTTP HTTP HTTP HTTP BasicBasicBasicBasic authenticationauthenticationauthenticationauthentication

3.3.3.3. OAuthOAuthOAuthOAuth

Control accessto resources

Accessing serviceson behalf of users

Custom Token Authentication

2-steps process

1. The server generates a unique token for a registered API user

2. The registered user sends the generated token for authentication with every request to the service

The token can be used to enable a specific user, to checkif traffic limits have been exceeded, etc.

Pros and Cons

+ + + + The generation of an access token is independent The generation of an access token is independent The generation of an access token is independent The generation of an access token is independent of the web service of the web service of the web service of the web service of the web service of the web service of the web service of the web service

++++ It is a simple approachIt is a simple approachIt is a simple approachIt is a simple approach– while creating a user registration process, the server

generates a unique token per accountAccess

++++ data exchange can be logged and verifieddata exchange can be logged and verifieddata exchange can be logged and verifieddata exchange can be logged and verified++++ data exchange can be logged and verifieddata exchange can be logged and verifieddata exchange can be logged and verifieddata exchange can be logged and verified– since access is controlled for each request

- ThisThisThisThis methodmethodmethodmethod isisisis notnotnotnot securesecuresecuresecure– The passed token can be copied and reused without

authorization

How to send the token?

The authentication token is sent with every request in two ways: two ways:

1. it can be part of the URI

2. it can be added to the HTTP request header

HTTP Basic authentication

The client sends the (cleartext Base64 encoded) username and password pair in the HTTP header username and password pair in the HTTP header Authorization

Username and password must be sent for every HTTP request for the authorization to be validated

http://bit.ly/JFGCQW

Pros and Cons

+ clients must manage server authorization requests

- in general, it is not secure- because usernames and passwords are only encoded using

Base64 encoding, which can be easily deciphered

+ this potential security hole can be solved by using + this potential security hole can be solved by using HTTPS (SSL)

Client/server transaction

It can take 2 forms:

1. a client makes a request to the server without without without without authentication credentialsauthentication credentialsauthentication credentialsauthentication credentials

– the server sends a response with an HTTP error code of 401 (unauthorized access)

– we need to programmatically intercept the 401 response and then provide valid credentials to complete the original requestthen provide valid credentials to complete the original request

2. a client makes a request to the server with server with server with server with authentication credentials from the beginningauthentication credentials from the beginningauthentication credentials from the beginningauthentication credentials from the beginning

Example of Request

<input type="text" name=“u" id=“u" value="" />

<input type="password" name=“p" id=“p" value="" />

var username = $('#u').val();

var password = MD5($('#p').val());

$.ajax({

type: 'POST',

url: ‘https://www.domain.com/login.php',

data: {

username: username,username: username,

password: password

},

success: function(result) {

console.log(“logged in”);

}

});

Oauth 2.0

OAuth's authorization protocol is becoming the preferred authorization schemepreferred authorization scheme

It is simplesimplesimplesimple and easy to easy to easy to easy to

integrate to integrate to integrate to integrate to RESTfulRESTfulRESTfulRESTful servicesservicesservicesservices

Open source pen source pen source pen source protocol

What are we talking about...

http://slidesha.re/JdfBGy

OAuth

your Service yourapp

Service provider

OAuth 2.0

It is used for accessing web services on the behalf ofthe userthe user

OAuthOAuthOAuthOAuth is an authorization protocol that allows is an authorization protocol that allows is an authorization protocol that allows is an authorization protocol that allows thirdthirdthirdthird----party web service creators (you) to get party web service creators (you) to get party web service creators (you) to get party web service creators (you) to get access to users' data stored in a different web access to users' data stored in a different web access to users' data stored in a different web access to users' data stored in a different web serviceserviceserviceservice

This can happen only with users' consent and without a username and password exchange

OAuth 2.0

Before OAuth, users needed to pass login information to multiple third party servicesto multiple third party services

With OAuth, users don’t divulge their login information

� authorization is granted from the provider service, where both user’s data and credentials are stored

� the consumer service only receives an authorization token that is used to access data from the provider service

OAuth Basics

AuthenticationAuthenticationAuthenticationAuthentication• Need to log in to access parts of a website• Need to log in to access parts of a website

– ex: view user profile– post a photo– add a friend– view private messages

TokenTokenTokenToken----based Authenticationbased Authenticationbased Authenticationbased Authentication• Logged-in user has a unique token unique token unique token unique token used to access

data from your app

Intuition behind OAuth

OAuth 2.0 Authentication flowthe user

yourappapp

Auth Server(ex. Facebook)

http://tools.ietf.org/html/draft-ietf-oauth-v2-26

The server hosting protected resources

(ex. Facebook)

Example: Google+

References

http://bit.ly/JA1UPT

Cordova plugin for FB:

http://bit.ly/JdjoUh