1. DISTRIBUTED COMPUTING PARADIGMS

2. Paradigm?
A MODEL

3. Distributed Computing Paradigms and their level of abstraction

4. Paradigms for Distributed Appl’ns
Message passing Paradigm
Client-Server Paradigm
Peer-to-Peer Paradigm
Message System Paradigm
Remote Procedure call Paradigm

5. Message Passing Paradigm
The basic approach to interprocess communications is message passing.
The data representing messages are exchanged between two processes, a sender and a receiver.
A Process sends a message representing a request. The message is delivered to a receiver, which processes the request and sends a message in response.

6. Message Passing Paradigm
The basic operations required to support the message-passing paradigm are send and receive.
The abstraction provided by this model, the interconnected processes perform input and output to each other, in a manner similar to file input and output.
As with file I/O, the operations serve to encapsulate the detail of network communication at the operating system level, so that a programmer may make use of the operations to send and receive message without having to deal with underlying detail.

7. Message Passing Paradigm

8. Client-Server Paradigm
The best known paradigm for network applications, the client-server model.
One process, the server, plays the role of service provider, waiting passively for the arrival of requests.
The Other process, the client issues specific requests to the server and awaits the server’s response.

9. Client-Server Paradigm
The client-server model provides an efficient abstraction for the delivery of network services.
Operations required include those for a server process to listen and to accept requests, and for a client process to issue requests and accepts responses.

10. Client-Server Paradigm

11. Peer-to-peer Paradigm
The client – server paradigm has the role for server fixed to be a listener, it cannot initiate any action.
The peer-to-peer paradigm, the participating processes play equal roles, with equivalent capabilities and responsibilities.
Each participant may issue a request to another participant and receive a response..

12. PEER-TO-PEER

13. Message System Paradigm(MOM)
Also called as Message-Oriented Middleware(MOM) paradigm.
In this paradigm a message system serves as an intermediary among separate, independent processes.
The message system acts as a switch for messages through which processes exchange messages asynchronously, in a decoupled manner.

14. Message System Paradigm
A sender deposits a message with the message system, which forwards it to a message queue associated with each receiver.
Once a message sent, the sender is free to move on to other tasks.

15. Message System Paradigm

16. Point-to-Point Message Model
In this paradigm, a message system forwards a message from the sender to the receiver’s message queue.
Unlike the basic message-passing model, the middleware provides a message repository and allows the sending and receiving to decoupled. Via the middleware, a sender deposits a message in the message queue of the receiving process.

17. Point-to-Point Message Model
A receiving process extracts the messages from its message queue and handles each message accordingly.
Compared to the basic message-passing model, the point-to-point message paradigm provides an additional abstraction for asynchronous operations.

18. Publish/subscribe Message Model
In this model, each message is associated with a specific topic or event.
Applications interested in the occurrence of a specific event may subscribe to messages for that event.
When the awaited event occurs, the process publishes a message announcing the event or topic.

19. Publish/subscribe Message Model
The publish/subscribe message model offers a powerful abstraction for multicasting or group communcation.
The publish operation allows a process to multicast to a group of processes, and the subscribe operation allows a process to listen for such a multicast.

20. RPC Model
The message passsing model works well for basic network protocols and for basic network applications.
As applications grew increasingly complex, it became necessary for further abstractions to be provided for network programming.
In particular, it was desirable to have a paradigm that allows distributed software to be programmed in a manner similar to convetional applications that run on a single processor.

21. RPC Model
The Remote Procedure Call(RPC) provides such an asbstraction.
In this model, communication between two processes is carried out using a concept similar to that for a local procedure call, which is familiar to application programmers.

22. RPC Model
A remote procedure call involves two independent processes, which may reside on separate machines.
A process A, wishing to make a request to another process B, issues a procedure call to B, passing with the call a list of argument values.
A remote procedure call triggers a predefined action in a procedure provided by process B.

23. RPC Model
At the completion of the procedure, process B returns a value to process A.
RPC allows programmers to build network applications using a programming construct similar to the local procedure call, providing a convenient abstraction for both interprocess communications and event synchronization.

24. RPC Model

25. Distributed Objects Paradigms
RMI
ORB
The Object space Paradigm
The Mobile Agent Paradigm
The Network services Paradigm
The Groupware Paradigm

26. RMI Remote Method Invocation is the Object Oriented Equivalent of RPC.
In this model, a process invokes the methods in an object, which may reside on the remote host.

27. ORB Object Request Broker Paradigm
A Process issues request to an appropriate object that provides the desired service.
The paradigm closely resembles the RMI model in its support for remote objects.

28. ORB
The difference is that the object request broker in this paradigm functions as middleware, allowing an application, as an object requestor, to potentially access multiple remote objects.
The request broker may also function as a mediator for heterogenous objects

29. Object Space Paradigm
The object space paradigm assumes the existence of logical entities known as object spaces.
The participants of an application converge in a common object space.
A provider places objects as entries into an object space, and requestor who subscribe to the space may access the entries.

30. Mobile Agent Paradigm
Mobile Agent is a transportable program or object.
An agent is launched from an originating host.
The agent then travels autonomously from host to host according to an itinerary that it carries.
The paradigm offers the abstraction for a transportable program or object.

31. Mobile Agent Paradigm
In lieu of message exchanges, data is carried by the program/object as the program is itself transported among the participants.

32. Choosing a Paradigm