This presentation has been developed in the context of the Mobile
Applications Development course at the Computer Science Department of the University of L’Aquila (Italy).
REST It stands for REpresentational State Transfer Proposed by Roy Fieldings in his PhD dissertation in 2000 in his PhD dissertation in 2000 REST rules the architecture of the World Wide Web (HTTP)
REST Architectural Style REST is not a technology, nor a framework REST is an Architectural Architectural Architectural Architectural 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 • client client client client- - - -server server server server • stateless stateless stateless stateless – 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 system caching system caching system caching system • it has to be uniformly accessible uniformly accessible uniformly accessible uniformly 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 page specific 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 resource resource resource resource
REST Main Actors These are the abstractions that make a RESTful system: • Resources Resources Resources Resources • Representations Representations Representations Representations • Actions Actions Actions Actions
Resources A A A A resource resource resource resource is is is is “ “ “ “everything everything everything everything” the service can ” the service can ” the service can ” the service can provide provide provide provide 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 resources considered as resources considered as resources considered 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 Web addressable over the Web addressable over the Web addressable over the Web addressable over the Web addressable over the Web addressable over the Web addressable over the Web Addressable Addressable Addressable Addressable = = = = 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 URIs URIs URIs URIs
URIs Uniform Resource Identifier Uniform Resource Identifier Uniform Resource Identifier Uniform 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 time The URI is not meant to change over time The URI is not meant to change over time The 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 parameters parameters parameters parameters 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
Representations The representation representation representation representation of resources is what is sent back and forth between clients and servers and 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 representations representations representations representations
URL Uniform Resource Locator Uniform Resource Locator Uniform Resource Locator Uniform 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 content content content content- - - -type type type type content content content content- - - -type type type type 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 forms A representation can take various forms A representation can take various forms A 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 URI URI URI URI
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 JSON XML or JSON XML or JSON XML or JSON
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: GET GET GET GET POST POST POST POST PUT PUT PUT PUT DELETE DELETE DELETE DELETE DELETE DELETE DELETE DELETE 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 representation must be a resource representation must be a resource representation must be a resource representation
HTTP as Uniform Interface In In In In RESTful RESTful RESTful RESTful systems systems systems systems we we we we focus on focus on focus on focus on resource resource resource resource names names names names, whereas in traditional web systems we focussed on the actions to in traditional web systems we focussed on the actions to be 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 classroom in 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
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
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 usually created automatically by the server
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
A note on PUT and DELETE PUT and DELETE apply to the entire resource PUT and DELETE apply to the entire resource PUT and DELETE apply to the entire resource PUT 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 atomic The PUT and DELETE operations are atomic The PUT and DELETE operations are atomic The 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 requests about the response of the requests 1xx informational message 2xx success message 3xx redirects the client to another URL 4xx client-side error 4xx client-side error 5xx server-side error
Security Here we will focus on securing user access to our services services There are three main methods: 1. 1. 1. 1. Custom Custom Custom Custom token token token token authentication authentication authentication authentication Control access 2. 2. 2. 2. HTTP HTTP HTTP HTTP Basic Basic Basic Basic authentication authentication authentication authentication 3. 3. 3. 3. OAuth OAuth OAuth OAuth Control access to resources Accessing services on 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 check if 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 approach It is a simple approach It is a simple approach It is a simple approach – while creating a user registration process, the server generates a unique token per accountAccess + + + + data exchange can be logged and verified data exchange can be logged and verified data exchange can be logged and verified data exchange can be logged and verified + + + + data exchange can be logged and verified data exchange can be logged and verified data exchange can be logged and verified data exchange can be logged and verified – since access is controlled for each request - This This This This method method method method is is is is not not not not secure secure secure secure – 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 credentials authentication credentials authentication credentials authentication 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 request then 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 beginning authentication credentials from the beginning authentication credentials from the beginning authentication credentials from the beginning
Oauth 2.0 OAuth's authorization protocol is becoming the preferred authorization scheme preferred authorization scheme It is simple simple simple simple and easy to easy to easy to easy to integrate to integrate to integrate to integrate to RESTful RESTful RESTful RESTful services services services services Open source pen source pen source pen source protocol
OAuth 2.0 It is used for accessing web services on the behalf of the user the user OAuth OAuth OAuth OAuth is an authorization protocol that allows is an authorization protocol that allows is an authorization protocol that allows is an authorization protocol that allows third third third third- - - -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 service service service service 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 services to 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 Authentication Authentication Authentication Authentication • 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 Token Token Token Token- - - -based Authentication based Authentication based Authentication based Authentication • Logged-in user has a unique token unique token unique token unique token used to access data from your app
OAuth 2.0 Authentication flow the user your app app Auth Server (ex. Facebook) http://tools.ietf.org/html/draft-ietf-oauth-v2-26 The server hosting protected resources (ex. Facebook)
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