Introduction to Web Services

By J. H. Wang

Nov. 28, 2011

Outline

• Overview

• RESTful Web services

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

• Web service– An interface described in a machine-processable form

at (WSDL, or Web Services Description Language)– Other systems interact with the Web service using SO

AP 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

Web Services Architecture

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

SOAP Message• POST /InStock HTTP/1.1

Host: www.example.org Content-Type: application/soap+xml; charset=utf-8 Content-Length: 299 SOAPAction: "http://www.w3.org/2003/05/soap-envelope" <?xml version="1.0"?> <soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope"> <soap:Header> </soap:Header> <soap:Body> <m:GetStockPrice xmlns:m="http://www.example.org/stock"> <m:StockName>IBM</m:StockName> </m:GetStockPrice> </soap:Body> </soap:Envelope>

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

Web Services in a Service-Oriented Architecture

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, DEL

ETE– Interacting with stateful resources, rather than messg

aes or operations

RPC Web Services

• Basic unit: WSDL operation• Widely deployed and supported, but not loosely

coupled• Other approaches: CORBA, DCE/RPC, Java

RMI

SOA Web Services

• Basic unit: message

• Supported by most major vendors, loose coupling

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

Representation of concepts in WSDL 1.1 and 2.0 documents

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

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

Constraints

• Client-server• Stateless

– 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 serv

er, or an intermediary

• Code on demand (optional)– Servers are able to temporarily extend the functionality of a clien

t

• Uniform interface

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

Key Goals

• Scalability of component interactions

• Generality of interfaces

• Independent deployment of components

• Intermediary components to reduce latency, enforce security, and encapsulate legacy systems

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

• No official standard for RESTful services– But Web standard protocols are often used

RESTful Web services: Basics

• Use HTTP methods explicitly

• Be stateless

• Expose directory structure-like URIs

• Transfer XML, JSON, or both

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/1.1Host: myserverContent-Type: application/xml<?xml version="1.0"?><user>

<name>Robert</name></user>

• Another example– Before

• 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>

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

Expose directory structure-like URIs

• Ex.– http://www.myservice.org/discussion/topics/{topic}– http://www.myservice.org/discussion/2008/12/10/{topic}

• Guidelines– Hide the server-side scripting technology file extensions (.jsp, .p

hp, .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 othe

r)– 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.

Transfer XML, JSON, or both

• Ex.– <?xml version="1.0"?>

<discussion date="{date}" topic="{topic}"><comment>{comment}</comment><replies>

<reply from="joe@mail.com" href="/discussion/topics/{topic}/joe"/>

<reply from="bob@mail.com"href="/discussion/topics/{topic}/bob"/>

</replies></discussion>

• Common MIME types– JSON: application/json– XML: application/xml– XHTML: application/xhtml+xml

References

• http://en.wikipedia.org/wiki/Web_service

• RESTful Web services: the basics, by Alex Rodriguez, IBM developerWorks, available at: http://www.ibm.com/developerworks/webservices/library/ws-restful/.

Thanks for Your Attention!