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
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
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/soapenvelope"
<?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,
DELETE
– Interacting with stateful resources, rather than
messgaes 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
server, or an intermediary
• Code on demand (optional)
– Servers are able to temporarily extend the functionality of a client
• 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/1.1
Host: myserver
Content-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, .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.
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/webs
ervices/library/ws-restful/.
Thanks for Your Attention!