1. Overview of Distributed Systems

ITE 3108 – Internet Programming and Web Services 1. Overview of Distributed Systems Daminda Herath B.Sc(Col), MBCS, M.Sc(Col), MCSSL, MIEEE

Distributed Systems A Distributed System is a collection of individual computing devices that can communicate with each other (i.e. a collection of independent computers, interconnected via a network, capable of collaborating on a task). This general definition encompasses a wide range of modern day computer systems, ranging from a VLSI chip to a tightly coupled shared memory microprocessor, to a local area cluster of workstations, to the internet. Distributed Systems are ubiquitous today throughout business, academia, government and the home. Typically they provide means to share resources, for instance, special purpose equipment and to share data, crucial for our information based economy.

Ubiquitous computing (ubicomp) Ubiquitous computing (ubicomp) is an advanced computing concept where computing is made to appear everywhere and anywhere. In contrast to desktop computing, ubiquitous computing can occur using any device, in any location, and in any format. A user interacts with the computer, which can exist in many different forms, including laptop computers, tablets, terminals and phones.

A Distributed System is a collection of independent computers that appears to its users as a Single coherent System.

Examples for Distributed Systems

Centralized vs. Distributed Computing The terms centralized and distributed computing is used to describe where the network processing takes place. In a centralized computing model, one system provides both the data storage and the processing power for client systems. This networking model is most often associated with computer mainframes and dumb terminals, where no processing or storage capability exists at the workstation. These network environments are rare, but they do still exist. A distributed network model has the processing power distributed between the client systems and the server. Most modern networks use the distributed network model, where client workstations share in the processing responsibilities.

Distributed System Paradigms 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.

Distributed System Paradigms Message Passing

Client Server Paradigm •The Client-Server paradigm is the most prevalent model for distributed computing protocols. •It is the basis of all distributed computing paradigms at a higher level of abstraction. •It is service-oriented, and employs a request-response protocol. •Operations required include those for a server process to listen and to accept requests, and for a client process to issue requests and accept responses. •By assigning asymmetric roles to the two sides, event synchronization is simplified: the server process waits for requests, and the client in turn waits for responses.

On the Internet, many services are Client-server applications. These services are often known by the protocol that the application implements. Well known Internet services include HTTP, FTP, DNS, etc. User applications may also be built using the client-server paradigm.

Peer to Peer Paradigm •Peer-to-peer is an architecture where computer resources and services are direct exchanged between computer systems. •These resources and services include the exchange of information, processing cycles, cache storage, and disk storage for files. •In such architecture, computers that have traditionally been used solely as clients communicate directly among themselves and can act as both clients and a servers, assuming whatever role is most efficient for the network. •In the peer-to-peer paradigm, the participating processes play equal roles, with equivalent capabilities and responsibilities (hence the term “peer”). Each participant may issue a request to another participant and receive a response.

The Message System Paradigm •The Message System or Message-Oriented Middleware (MOM) paradigm is an elaboration of the basic message-passing 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. •A sender deposits a message with the message system, which forwards it to a message queue associated with each receiver. Once a message is sent, the sender is free to move on to other tasks.

The Message System Paradigm Two subtypes of message system models exist The Point-To-Point Message Model The Publish/Subscribe Message Model

The Point-To-Point Message Model • 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. • Compared to the basic message-passing model, this paradigm provides the additional abstraction for asynchronous operations. To achieve the same effect with basic message-passing, a developer will have to make use of threads or child processes.

The 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. The middleware message system distributes the message to all its subscribers. •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.

The Publish/Subscribe Message Model •In the publish/subscribe domain, message producers are called publishers and message consumers are called subscribers. They exchange messages by means of a destination called a topic: publishers produce messages to a topic; subscribers subscribe to a topic and consume messages from a topic

The Publish/Subscribe Message Model

Remote Procedure Call (RPC) •As applications grew increasingly complex, it became desirable to have a paradigm which allows distributed software 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 programmers. • 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.

Collaborative Applications In this model, processes participate in a collaborative session as a group. Each participating process may contribute input to part or the entire group. Processes may do so using: •Multicasting to send data to all or part of the group, or •They may use a Virtual sketchpads or whiteboards which allows each participant to read and write data to a shared display.

Mobile Agents •An agent is “an independent software program which runs on behalf of a network user”. •A mobile agent is a program which, once it is launched by a user, can travel from node to node autonomously, and can continue to function even if the user is disconnected from the network. •In this model, an agent is launched from an originating host. •The agent travels from host to host according to an itinerary that it carries. •At each stop, the agent accesses the necessary resources or services, and performs the necessary tasks to accomplish its mission.

•An agent server process runs on each participating host. •Participating hosts are networked through links that can be low- bandwidth and unreliable. •An agent is a serializable object whose execution state can be frozen for transportation and reconstituted upon arrival at a remote site •The paradigm offers the abstraction for a transportable program or object. •In lieu of message exchanges, data is carried by the program/object as the program is transported among the participants.

Advantages of Mobile Agents: •They allow efficient and economical use of communication channels which may have low bandwidth, high latency, and may be error-prone. •They enable the use of portable, low-cost, personal communications devices to perform complex tasks even when the device is disconnected from the network. •They allow asynchronous operations and true decentralization

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