DESIGNING APPLICATIONS OVER MOBILE ENVIRONMENT

Prepared by:

Mugisha Robert, Maulik Patel

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OUTLINE Introduction Issues and challengesPublish/subscribe paradigmMessage PassingPoint-to-point/Message QueuingShared spacesRemote invocationReferences

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INTRODUCTION Important characteristics that distinguish distributed applications from

conventional applications which run on a single machine: Inter-process communication. Event synchronization.

Paradigm means “a pattern, example, or model.”

In the study of any subject of great complexity, it is useful to identify the basic patterns or models, and classify the detail according to these models.

This presentation aims to present a classification of the paradigms for distributed applications over mobile environment.

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INTRODUCTION CONT…

Abstractions Arguably the most fundamental concept in computer science, abstraction is

the idea of detail hiding.

We often use abstraction when it is not necessary to know the exact details of how something works or is represented, because we can still make use of it in its simplified form. Getting involved with the detail often tends to obscure what we are trying to understand, rather than illuminate it.

In software engineering, abstraction is realized with the provision of tools or facilities which allow software to be built without the developer having to be cognizant of some of the underlying complexities.

ISSUES AND CHALLENGES Widely varying modes of use

The system components are subject to wide variations in workload (e.g. some web pages have millions of hits a day and some may have no hits). Some applications have special requirements for high communication bandwidth and low latency (e.g multimedia apps).

Wide range of system environments A distributed system must accommodate heterogeneous hardware, operating

systems, and networks.

Internal problems Non-synchronized clocks, concurrency problems, many modes of hardware

and software failures involving the individual components of the system.

External threats Attacks on data integrity, ensuring confidentiality, denial of service.

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PUBLISH/SUBSCRIBE PARADIGM The publish/subscribe message model offers a powerful

abstraction for multicasting or group communication. The publish operation allows a process to multicast to a group of

processes, and the subscribe operation allows a process to listen for such

multicast.

Each message is then 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. The middleware message system distributes the message to all its

subscribers.

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PUBLISH/SUBSCRIBE CONT…

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PUBLISH/SUBSCRIBE CONT…Decoupling Between providers and consumers Increase scalability

No dependencies No coordination & synchronization

Create highly dynamic, decentralized systems

Dimensions Of Decoupling Space Time Synchronization (flow)

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PUBLISH/SUBSCRIBE CONT… Space decoupling

No need to hold references or even know each other

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PUBLISH/SUBSCRIBE CONT… Time decoupling

No need to be available at the same time

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PUBLISH/SUBSCRIBE CONT… Synchronization decoupling

Control flow is not blocked by the interaction

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MESSAGE PASSING

Message passing is the most fundamental paradigm for distributed applications.

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.

In turn, the reply may trigger a further request, which leads to a subsequent reply, and so forth.

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MESSAGE PASSING CONT..

Pro ce s s APro ce s s B

a m es s ag e

M e s s a g e pa s s in g

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MESSAGE PASSING CONT..

The basic operations required to support the basic message passing paradigm are send, and receive.

For connection-oriented communication, the operations connect and disconnect are also required.

With the abstraction provided by this model, the interconnected processes perform input and output to each other, in a manner similar to file I/O. The I/O operations encapsulate the detail of network communication at the operating-system level.

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POINT-TO-POINT/MESSAGE QUEUING

In this model, 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 depository, and allows the sending and the receiving to be decoupled.

Via the middleware, a sender deposits a message in the message queue of the receiving process.

A receiving process extracts the messages from its message queue, and handles each one accordingly.

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POINT-TO-POINT/MESSAGE QUEUING

Messages stored in FIFO queue. Producers append messages asynchronously Consumers dequeue them synchronously Coupled in sync (consumer side)

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SHARED SPACES

Here the nodes share a global address space and communicate by writing to a global space.

A typical representative of this model is a bulletin board where groups of people can share their ideas by posting information at a common shared location.

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SHARED SPACES CONT… Producers insert data asynchronously Consumers read data synchronously Coupled in sync (consumer side)

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TUPLE SPACE-BASE SYSTEM

Tuple space model, processes communication by inserting and removing tuples from a shared tuple space.

Which act like a globally shared memory.

Tuple communication decoupled in time because process can insert and retract tuple independently.

Interaction model provide time & space decoupling.

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TUPLE SPACE-BASE SYSTEM CONT..

An in-based interaction: implement one-to-n semantics(only one consumer reads given tuple)

Read based interaction: implement one-to-n message delivery(a given tuple can be read by all consumer)

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TUPLE SPACE-BASE SYSTEM CONT.. There are three operation perform in tuple space

Out() :- To export a tuple into tuple space in() :- To import a tuple from tuple space Read():-To read a tuple from tuple space(without removing)

Tuple space has originally integrated at the language level in Linda. The original Linda model is Sun microsystem’s java spaces(1999) IBM Tspaces(2001)

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REMOTE INVOCATION/RPC

As applications grew increasingly complex, it became desirable to have a paradigm which allows distributed applications to be programmed in a manner similar to conventional applications which run on a single processor.

The Remote Procedure Call (RPC) model provides such an abstraction. Using this model, inter-process communications proceed as procedure, or function, calls, which are familiar to application program.

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REMOTE INVOCATION/RPC CONT… 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.

As in the case of local procedure calls, a remote procedure call triggers a predefined action in a procedure provided by process B.

At the completion of the procedure, process B returns a value to process A.

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REMOTE INVOCATION/RPC CONT…

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RPC MARSHALLING

Marshalling is the packing of procedure parameters into a message packet

The RPC stubs call type-specific procedures to marshal (or unmarshal ) the parameters to a call The client stub marshals the parameters into a message The server stub unmarshals parameters from the message and uses

them to call the server procedure

On Return The server stub marshals the return parameters The client stub unmarshals return parameters and returns them to the

client program

RPC BINDING Binding is the process of connecting the client to the server The server, when it starts up, exports its interface

Identifies itself to a network name server Tells RPC runtime it’s alive and ready to accept calls

The client, before issuing any calls, imports the server RPC runtime uses the name server to find the location of a

server and establish a connection

The import and export operations are explicit in the server and client programs

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REFERENCES

Priya Venkitakrishnan, Sharan Raman, Smita Hegde & Sreekanth Narasimhan, “Message Passing in Mobile Environments,” Project Paper for Advanced Operating Systems Course, Spring 2002.

P.Th. Eugster, P.A. Felber, R. Guerraoui, A.-M. Kermarrec, ‘The Many Faces of Publish/Subscribe’, Microsoft Research Limited.

Eiko Y oneki, ‘Mobile Applications with a Middleware System in Publish-Subscribe Paradigm.’

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REFERENCES CONT...

George Coulouris, Jean Dollimore, Tim Kindberg, “Distributed Systems Concepts and Design”, Addison Wesley Publishers Limited, Third Edition.

Deepak G, Dr. Pradeep B S,“Challenging Issues and Limitations of Mobile Computing”, IJCTA | JAN-FEB 2012.

Yongqiang Huang, Hector Garcia-Molina, "Publish/Subscribe in a Mobile Environment."