1. Introduction to Web Services
By J. H. Wang
Nov. 28, 2011

2. Outline
Overview
RESTful Web services

3. What is a Web Service
“a method of communication between two electronic devices over the Web”
From Wikipedia entry on “Web service”
“a software system designed to support interoperable machine-to-machine interaction over a network”
From W3C definition

4. Two major classes of Web services
An interface described in a machine-processable format (WSDL, or Web Services Description Language)
Other systems interact with the Web service using SOAP messages, typically conveyed using XML/HTTP and other Web-related standards
SOAP: Simple Object Access Protocol
Two major classes of Web services
REST-compliant
To manipulate XML representations of Web resources using a uniform set of “stateless” operations
Arbitrary

5. Web Services Architecture

6. Simple Object Access Protocol (SOAP)
A protocol for exchanging structured information in the implementation of Web Services
XML: for message format
HTTP, SMTP: for message transmission

7. SOAP Message
POST /InStock HTTP/1.1 Host: Content-Type: application/soap+xml; charset=utf-8 Content-Length: 299 SOAPAction: " <?xml version="1.0"?> <soap:Envelope xmlns:soap=" <soap:Header> </soap:Header> <soap:Body> <m:GetStockPrice xmlns:m=" <m:StockName>IBM</m:StockName> </m:GetStockPrice> </soap:Body> </soap:Envelope>

8. Web APIs Moving from SOAP based services to REST based communications
REST: Representational State Transfer
Do not require XML, SOAP, WSDL
Typically a defined set of HTTP request messages along with the structure of response messages expressed in XML or JSON format
JSON: JavaScript Object Notation
They allow the combination of multiple Web services into new applications known as mashups

9. Web Services in a Service-Oriented Architecture

10. Three Most Common Styles of Use
RPC (Remote Procedure Calls)
A distributed function call interface
SOA (Service-Oriented Architecture)
The basic unit of communication is a message, rather than an operation
REST (Representational State Transfer)
Standard operations in HTTP: GET, POST, PUT, DELETE
Interacting with stateful resources, rather than messgaes or operations

11. RPC Web Services Basic unit: WSDL operation
Widely deployed and supported, but not loosely coupled
Other approaches: CORBA, DCE/RPC, Java RMI

12. SOA Web Services Basic unit: message
Supported by most major vendors, loose coupling

13. Representational State Transfer (REST)
Interacting with stateful resources, rather than messages or operations
Using HTTP standard operations such as GET, POST, PUT, DELETE
WSDL 2.0 offers support for binding to all HTTP request methods
WSDL 1.1 only GET and POST

14. Representation of concepts in WSDL 1.1 and 2.0 documents

15. Criticisms Too complex, not open source
A custom interface requires a custom client for every service
Concerns about performance due to XML and SOAP/HTTP in enveloping and transport

16. RESTful Web Services
Introduced by Roy Fielding in his doctoral dissertation
He is one of the principal authors of the HTTP specification version 1.0 and 1.1
Client-server
Clients initiate requests
Servers process requests and return appropriate responses
Requests and responses are built around the transfer of representations of resources

17. Constraints Client-server Stateless Cacheable Layered system
No client context is stored on the server between requests
The server can be stateful
Cacheable
Clients can cache responses
Layered system
Clients cannot tell whether it’s connected directly to the end server, or an intermediary
Code on demand (optional)
Servers are able to temporarily extend the functionality of a client
Uniform interface

18. Guiding Principles of the Interface
Identification of resources
E.g. URIs
Manipulation of resources through these representations
Self-descriptive messages
Hypermedia as the engine of application state
E.g. hyperlinks, hypertext

19. Key Goals Scalability of component interactions
Generality of interfaces
Independent deployment of components
Intermediary components to reduce latency, enforce security, and encapsulate legacy systems

20. RESTful Web API Four aspects Base URI for the Web service
Internet media type of the data supported by the Web service
E.g. JSON, XML, or YAML
The set of operations supported by the Web service using HTTP methods
E.g. GET, PUT, POST, or DELETE
The API must be hypertext driven

21. No official standard for RESTful services
But Web standard protocols are often used

22. RESTful Web services: Basics
Use HTTP methods explicitly
Be stateless
Expose directory structure-like URIs
Transfer XML, JSON, or both

23. Using HTTP Methods Explicitly
One-to-one mapping
GET: to retrieve a resource on the server
POST: to create a resource
PUT: to change the state of a resource or to update it
DELETE: to remove a resource
For example,
Before
GET /adduser?name=Robert HTTP/1.1
After
POST /users HTTP/1.1
Host: myserver
Content-Type: application/xml
<?xml version="1.0"?>
<user>
<name>Robert</name>
</user>

24. Another example Before After
GET /updateuser?name=Robert&newname=Bob HTTP/1.1
After
PUT /users/Robert HTTP/1.1
Host: myserver
Content-Type: application/xml
<?xml version="1.0"?>
<user>
<name>Bob</name>
</user>

25. Be Stateless
For scalability, clients are required to send complete, independent requests
include all data needed to be fulfilled so that the components in the intermediary servers may forward, route, and load-balance without any state being held locally in between requests

27. Expose directory structure-like URIs
Guidelines
Hide the server-side scripting technology file extensions (.jsp, .php, .asp), if any, so you can port to something else without changing the URIs
Keep everything lowercase
Substitute spaces with hyphens or underscores (one or the other)
Avoid query strings as much as you can
Instead of using the 404 Not Found code if the request URI is for a partial path, always provide a default page or resource as a response.

28. Transfer XML, JSON, or both
Ex.
<?xml version="1.0"?>
<discussion date="{date}" topic="{topic}">
<comment>{comment}</comment>
<replies>
<reply href="/discussion/topics/{topic}/joe"/>
<reply
href="/discussion/topics/{topic}/bob"/>
</replies>
</discussion>
Common MIME types
JSON: application/json
XML: application/xml
XHTML: application/xhtml+xml

29. References http://en.wikipedia.org/wiki/Web_service
RESTful Web services: the basics, by Alex Rodriguez, IBM developerWorks, available at:

30. Thanks for Your Attention!