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Comp2005 - Lecture 13 - Web Services
Alejandro Saucedo - Comp2005 Lecture 13 FlashCard Set
Computer Science
Undergraduate 2

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What are the principles of 'The Web of Data'?
  • All entities of interest should be identified by unique identifiers
    • These should be deferencable - applications can look up an id over HTTP and GET data
  • Data should be provided using a standard format (XML, HTML, JSON, RDF, etc)
  • Data should be interlinked with other data


What are the Principles of Extensible Markup Language (XML)?
  • Essentially extends HTML concepts to allow exchange of structured data between systems rather than simply displaying information to people
  • Elements - Hierarchical decompostition
  • Attributes - Labeling elements
  • Entities - contain document fragments
  • DTD (Document Type Definition) provides grammar and defines elements that may appear in a document
What are the principles of Web services?
  • Provide ways to
    • Discover a service (UDDI)
    • Describe the service (WSDL)
    • Interact with the service (SOAP - XML)
  • Interoperability is very important
What is SOAP?
  • The Simple Object Access Protocol describes a standard way for systems to communicate
    • Provides a format for sending one-way, stateless messages, using XML representation/serialization
    • Allows exchange of structured and typed information between peers in a distributed environment
  • Used over a variety of transports
    • HTTP
    • Supported in a number of programming languages
    • Directly over TCP


What is WSDL?
  • Web Services Description Languages describe functionality offered by a web service
    • How it can be called
    • Parameters expected
    • Data structures returned
  • Can be used with SOAP and XML Schema
    • Client reads WSDL file to discover operations
    • Special data types included as XML Schema
  • WDSL 2.0 includes support for RESTful web services¬†
    • Specifically PUT and DELETE
What is UDDI?
  • Universal description discovery and Integration
    • A platform-independent registry based on XML
    • Allows registration and discovery of web service applications
    • Designed to use SOAP messages
  • Querying a UDDI registry leads to discovery of WSDL documents describing the message formats
What is the relationship between UDDI, WSDL and SOAP?
  • Service provider describes services with WSDL
    • Published to a repository of services
    • Repository may use UDDI
  • Service consumer queries UDDI repository to locate service and discover how to interact with it
  • Consumer selects service and retreives the associated WSDL description for it
  • Consumer then sends query to provider (XML)
  • Provider responds (XML)
  • All the above messages are transmitted by SOAP
    • Provides the envelope for the web service messages
    • Generally uses HTTP
What is an alternative approach to SOAP/WSDL?

Representational State Transfer (REST)

  • More of an architectural style - not a standard per se
  • REST service request, XML or JSON service response
What is REST?
  • Representational state tranfed is designed for client-server operation within distributed web systems
    • Uses HTTP methods
    • Messages are of the form request - respose
    • A response may be XML, CSV or JSON
    • REST includes transfer of representations of resources
  • A number of design principles:
    • Stateless, cacheable, uniform interface
    • Layered system(may include "transparent" proxies)
  • RESTful application
What are some REST HTTP methods?
  • The REST architecture defines the means to transport data using HTTP
  • POST: insert
  • PUT: update
  • GET: list
  • DELETE: delete an item in a collection
What are the principles of JSON?
  • Javascript Object Notation
    • Can represent simple data structures and arrays
    • Can specify format for JSON data with JSON Schema
  • Can be used for serialising and transmitting structured data over a network connection
    • An alternative to XML
State some characteristics of SOAP vs REST
  • REST takes advantage of HTTP request methods
      Calls easier to create, can be less resource heavy
  • REST is an "architectural style" rather than a defined protocol like SOAP
    • Leverages JSON
    • But it can still use standards including XML
  • SOAP
    • Provides structure, and potentially rich service descriptions via WSDL
    • Can run over but not limited to HTTP
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