Source: George Colouris, Jean Dollimore, Tim Kinderberg & Gordon Blair (2012). Distributed Systems: Concepts & Design (5 th Ed.). Essex: Addison-Wesley This Presentation was adapted from: ‎

 A web service provides a service interface enabling clients to interact with servers in a more general way than web browsers do.  Clients access the operations in the interface of a web service by means of requests and replies formatted in XML and usually transmitted over HTTP.

 Web services provide an infrastructure for maintaining a richer and more structured form of interoperability between clients and servers.  They provide a basis whereby a client program in one organization may interact with a server in another organization without human supervision.  External data representation and marshalling of messages exchanged between clients and web services is done in XML.

 A web service interface generally consists of a collection of operations that can be used by a client over the Internet.  The operations in a web service may be provided by a variety of different resources, for example, programs, objects, databases.  A web service may either be managed by a web server along with web pages; or it may be a totally separate service.

 Many well-known commercial web services including Amazon, Yahoo, Google and eBay offer web service interfaces that allow client to manipulate their web resources.  As an example, the web service offered by Amazon.com provides operations to allow clients to get information about products, to add an item to a shopping cart or to check the status of a transaction.

 The Amazon web services may be accessed either by SOAP or by REST (REpresentaional State Transfer).  The provision of a service interface allows its operations to be combined with those of other services to provide new functionality.

 A major task of many middleware platforms is to protect the programmer from the details of data representation and marshalling and sometimes with making remote invocations look like local ones.  These things are provided as a part of an infrastructure or middleware platform for web services.  At the simplest level, clients and servers may read and write their messages directly in SOAP, using XML.

 SOAP is defined to enable both client-server and asynchronous interaction over the Internet.  It defines a scheme for using XML to represent the contents of request and reply messages as well as a scheme for the communication of documents.  Originally, SOAP was based on HTTP, but the current version is designed to use a variety of transport protocols including SMTP, TCP or UDP.

 A SOAP message is carried in an “envelope”.  Inside the envelope there is an optional header and a body

 Components and operations Source: IBM 1.A Web service needs to be created, and its interfaces and invocation methods must be defined. 2.A Web service needs to be published to one or more intranet or Internet repositories for potential users to locate. 3.A Web service needs to be located to be invoked by potential users. 4.A Web service needs to be invoked to be of any benefit. 5.A Web service may need to be unpublished when it is no longer available or needed.

Service Registry/Broker/Directory