1. Distributed web based systemsBy
Bindu Rechamballi Veeranna

2. Contents Architecture Tradition web based systems
Multitiered Architecture
Web Services
Fundamentals
Composition and Coordination
References

3. Architecture As same as other distributed systems
Static documents to dynamically generated containing active elements.
Instead just documents supports services.

4. Architecture Traditional Web Based Systems
Relatively simple client-server architecture.
Core of the web site is formed by a process that has access to a local file system storing documents.
To refer to a document is by means of reference called Uniform Resource Locator(URL)
For transferring the document across the network URL specifies the application-lever protocol.

5. Architecture
Traditional Web Based Systems

6. Architecture Traditional Web Based Systems
Clients interacts with Web servers through a special application - Browser
Browser is responsible for displaying document
Communication between browser and web server is standardized
Both adhere to HTTP (HyperText Protocol)

7. Architecture Web Documents
All information comes in the form of a document
Documents includes all kinds of dynamic features such as audio, video and so on.
Helper application needed to read that special document
Documents divide into 2 parts:
Main Part - Acts as template for second part, its writing in a markup language HTML, XML
Second Part - different bits and pieces.

8. Architecture Web Documents
Extensible Markup Language(XML) is much more flexibility and includes definitions of the elements that mark up a document.
HTML & XML can also include all kinds of tags that refer to embedded documents.
Embedded Documents and way to interpret them
Embedded Document associated with MIME Type
Multipurpose Internet Mail Exchange(MIME) provide information on content of the message body.
MIME makes a distinction between Top level and subtypes.

9. Architecture
Web Documents

10. Architecture Multitiered Architecture
Combination of HTML with scripting provided a powerful means for expressing documents.
First enhancements to the architecture was support for simple CGI(Common Gateway Interface)
CGL defines a std was by which a web server can execute a program taking user data as input.
Server can also process the document before passing it to client.

11. Architecture
Multitiered Architecture

12. Web services
Web based systems that offering services to remote application without immediate interactions from end users.
Service for example: naming, weather-report etc thats are made available over internet.
Special is that it adheres to a collection of standards that will allow it to be discovered and access over the internet by client application that follow those standard as well.

13. Web services fundamentals
This service adheres to the Universal Description, Discovery and Integration standard (UDDI). The UDDI prescribes the layout of a database containing service descriptions that will allow Web service clients to browse for relevant services.
Services are described by means of the Web Services Definition Language (WSDL) which is a formal language very much the same as the Interface Definition Languages (IDL) used to support RPC-based communication. A WSDL description contains the precise definitions of the interfaces provided by a service, that is, procedure specification, data types, the (logical) location of services, etc. An important issue of a WSDL description is that can be automatically translated to client side and server-side stubs, again, analogous to the generation of stubs in ordinary RPC-based systems.
Finally, a core element of a Web service is the specification of how communication takes place. To this end, the Simple Object Access Protocol (SOAP) is used, which is essentially a framework in which much of the communication between two processes can be standardized. We will discuss SOAP in detail below, where it will also become clear that calling the framework simple is not really justified.

14. Web services fundamentals

15. Web services composition and coordination
In the model so far, a Web service is offered in the form of a single invocation. In practice, much more complex invocation structures need to take place before a service can be considered as completed. For example, take an electronic bookstore.
1)Ordering a book requires selecting a book, 2) paying, 3) and ensuring its delivery. From a service perspective, the actual service should be modeled as a transaction consisting of multiple steps that need to be carried out in a specific order. In other words, we are dealing with a complex service that is built from a number of basic services.
Complexity increases when considering Web services that are offered by combining Web services from different providers. A typical example is devising a Web-based shop. Most shops consist roughly of three parts: a first part by which the goods that a client requires are selected, a second one that handles the payment of those goods, and a third one that takes care of shipping and subsequent tracking of goods.

16. Web services composition and coordination
In these scenarios it is important that a customer sees a coherent service:
Namely a shop where he can select, pay, and rely on proper delivery. However, internally we need to deal with a situation in which possibly three different organizations need to act in a coordinated way. Providing proper support for such composite services forms an essential element of Web services.
There are at least two classes of problems that need to be solved.
First, how can the coordination between Web services, possibly from different organizations, take place?
Second, how can services be easily composed?

17. Web services composition and coordination
Coordination among Web services is tackled through coordination protocols. Such a protocol prescribes the various steps that need to take place for (composite) service to succeed. The issue, is to enforce the parties taking part in such protocol take the correct steps at the right moment.
There are various ways to achieve this;
The simplest is to have a single coordinator that controls the messages exchanged between the participating parties.
However, although various solutions exist, from the Web services perspective It is important to standardize the commonalities in coordination protocols. For one, it is important that when a party wants to participate in a specific protocol, that it knows with which other process(es) it should communicate.
In addition, it may very well be that a process is involved in multiple coordination protocols at the same time. Therefore, identifying the instance of a protocol is important as well.
Finally, a process should know which role it is to fulfill.

18. Web services composition and coordination

19. Web services composition and coordination

20. Thank you

21. References:
Andrew S. Tanenbaum, Maarten Van Steen, “Distributed Systems: Principles and Paradigms”, Prentice-Hall,NJ,USA.
A framework of a Web-based distributed control system, Electrical and Computer Engineering, 2003, vol.2
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