Representational State Transfer (REST)

What is REST?

• Network Architectural style

• Overview:– Resources are defined and addressed– Transmits domain-specific data over HTTP

REST is not a standard

• No W3C specification (EDIT: But there is an

XML database that has such an API: eXist)• REST is built on the concepts:

– HTTP (transferring mechanism)– URL (resource address)– XML/HTML/GIF/JPEG (Resource representations)– text/xml, text/html, image/gif, image/jpeg (MIME

types)

Web services: example

• Build a web service to enable its customers to– Get list of items– Get details about a particular item– Purchase Order (PO)

Web

Ser

ver

HTTP POST PO.xmlPO

(HTML/XML)

HTTP GET request /items

HTTP responseURL to submitted PO

List ofItems

ItemData

PO

HTTP responseResponse(HTML/XML doc)

HTTP responseResponse(HTML/XML doc)

HTTP GET request /00345Time

A user session

Get list of items

• Access the URL: http://www.company.com/items• How the list is generated is invisible to the user (loose coupling)• The following XML document returned:

• Each item is retrieved via other links (key feature)

Get detail of an item• Accessing the URL: http://www.company.com/items/00345• Any data change will not affect the user as in accessing

http://www.company.com/items/00345.html (restricts the resource; whereas a database could be underneath)

• Again, more in-depth links appear, enabling to more detailed information

Characteristics• Traversing links pulls representations (pull-based client-server

interaction)• Each request must be self-sufficient, not taking advantage of any

store resource (stateless)• Capability of storing responses to frequent requests (cacheable or

non-cacheable)• Resources are access via (HTTP) GET, POST, PUT, DELETE

(uniform interface)• Every resource is named using an URL (named resources)• Resources are interconnected using URLs (interconnected resource

representations)• Intermediate components, e.g. proxy servers, gateways, firewalls,

etc. can be inserted between clients and resources for performance, security (layered components)

Steps in creating a RESTful system

1. Identify resources, e.g. list of items, detail of an item, purchase order

2. Create a URL for every resource[use nouns, not verbs]

http://www.company.com/items/00345Instead of

http://www.company.com/items/getItem?id=003453. Categorize resources:

• Accessible via HTTP GET (read-only)• Accessible also via POST, PUT, DELETE (modifiable)

4. Connect resources through hyperlinks 5. Specify the format of the response using a schema, e.g. DTD,

W3C schema, RelaxNG (also take care of POST or PUT services)

6. Describe how your services should be invoked (in a WSDL or HTML document)

REST Constrains (2)

• Uniform interface constrain– Implementation decoupled from interfaces- Adapting degrades efficiency

- Layered system constraint- Architecture can be build hierarchically- Added Overhead

- Code-on-demand (inactive code is executed on the client when the user requests it)

Advantage to SOAP technologyStructure Accessibility Extensibility Performance Security

REST Inter-connections

(hyperlinks)

GET, PUT, POST,

DELETE

(uniform interface)

Yes, through links

GET-base URIs are cacheable

Can apply {GET, PUT, POST, DELETE} permissions to a data object

SOAP RPC

OO paradigm of encapsulating

data

Specific and numerous methods

Cannot refer to other objects outside the system

Not cached by any existing technology

Have to implement suitable strategy