• 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

• 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

• Provide ways to
• Discover a service (UDDI)
• Describe the service (WSDL)
• Interact with the service (SOAP - XML)
• Interoperability is very important

• 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

• 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

• 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

• 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

Representational State Transfer (REST)

• More of an architectural style - not a standard per se
• REST service request, XML or JSON service response

• 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

• The REST architecture defines the means to transport data using HTTP
• POST: insert
• PUT: update
• GET: list
• DELETE: delete an item in a collection

• 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

• REST takes advantage of HTTP request methods
• GET, POST, PUT, DELETE
  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