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

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

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, 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 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

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 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>

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 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/webservices/library/ws-restful/.

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