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CHAPTER using none toassign none in web,windows free 2d barcode encoder 1.1 Definition interleaved 2 of 5 Introduction A distributed system is a c none none ollection of independent entities that cooperate to solve a problem that cannot be individually solved. Distributed systems have been in existence since the start of the universe. From a school of fish to a flock of birds and entire ecosystems of microorganisms, there is communication among mobile intelligent agents in nature.

With the widespread proliferation of the Internet and the emerging global village, the notion of distributed computing systems as a useful and widely deployed tool is becoming a reality. For computing systems, a distributed system has been characterized in one of several ways: You know you are using one when the crash of a computer you have never heard of prevents you from doing work [23]. A collection of computers that do not share common memory or a common physical clock, that communicate by a messages passing over a communication network, and where each computer has its own memory and runs its own operating system.

Typically the computers are semi-autonomous and are loosely coupled while they cooperate to address a problem collectively [29]. A collection of independent computers that appears to the users of the system as a single coherent computer [33]. A term that describes a wide range of computers, from weakly coupled systems such as wide-area networks, to strongly coupled systems such as local area networks, to very strongly coupled systems such as multiprocessor systems [19].

A distributed system can be characterized as a collection of mostly autonomous processors communicating over a communication network and having the following features: No common physical clock This is an important assumption because it introduces the element of distribution in the system and gives rise to the inherent asynchrony amongst the processors.. Introduction No shared memory This is none for none a key feature that requires message-passing for communication. This feature implies the absence of the common physical clock. It may be noted that a distributed system may still provide the abstraction of a common address space via the distributed shared memory abstraction.

Several aspects of shared memory multiprocessor systems have also been studied in the distributed computing literature. Geographical separation The geographically wider apart that the processors are, the more representative is the system of a distributed system. However, it is not necessary for the processors to be on a wide-area network (WAN).

Recently, the network/cluster of workstations (NOW/COW) configuration connecting processors on a LAN is also being increasingly regarded as a small distributed system. This NOW configuration is becoming popular because of the low-cost high-speed off-the-shelf processors now available. The Google search engine is based on the NOW architecture.

Autonomy and heterogeneity The processors are loosely coupled in that they have different speeds and each can be running a different operating system. They are usually not part of a dedicated system, but cooperate with one another by offering services or solving a problem jointly..

1.2 Relation to computer system components A typical distributed syste none for none m is shown in Figure 1.1. Each computer has a memory-processing unit and the computers are connected by a communication network.

Figure 1.2 shows the relationships of the software components that run on each of the computers and use the local operating system and network protocol stack for functioning. The distributed software is also termed as middleware.

A distributed execution is the execution of processes across the distributed system to collaboratively achieve a common goal. An execution is also sometimes termed a computation or a run. The distributed system uses a layered architecture to break down the complexity of system design.

The middleware is the distributed software that. Figure 1.1 A distributed system connects processors by a communication network. Communication network (WAN/ LAN) P M P M P M P M P processor(s) M memory ban k(s). 1.3 Motivation Figure 1.2 Interaction of the software components at each processor. Distributed application Extent of distributed protocols Network protocol stack Distributed software (middl none none eware libraries) Application layer Operating system Transport layer Network layer Data link layer. drives the distributed syst em, while providing transparency of heterogeneity at the platform level [24]. Figure 1.2 schematically shows the interaction of this software with these system components at each processor.

Here we assume that the middleware layer does not contain the traditional application layer functions of the network protocol stack, such as http, mail, ftp, and telnet. Various primitives and calls to functions defined in various libraries of the middleware layer are embedded in the user program code. There exist several libraries to choose from to invoke primitives for the more common functions such as reliable and ordered multicasting of the middleware layer.

There are several standards such as Object Management Group s (OMG) common object request broker architecture (CORBA) [36], and the remote procedure call (RPC) mechanism [1, 11]. The RPC mechanism conceptually works like a local procedure call, with the difference that the procedure code may reside on a remote machine, and the RPC software sends a message across the network to invoke the remote procedure. It then awaits a reply, after which the procedure call completes from the perspective of the program that invoked it.

Currently deployed commercial versions of middleware often use CORBA, DCOM (distributed component object model), Java, and RMI (remote method invocation) [7] technologies. The message-passing interface (MPI) [20, 30] developed in the research community is an example of an interface for various communication functions..

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