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Chapter: Distributed Systems : Communication in Distributed System

Introduction to System Models

Systems that are intended for use in real-world environments should be designed to function correctly in the widest possible range of circumstances and in the face of many possible difficulties and threats .



Systems that are intended for use in real-world environments should be designed to function correctly in the widest possible range of circumstances and in the face of many possible difficulties and threats .


Each type of model is intended to provide an abstract, simplified but consistent description of a relevant aspect of distributed system design:


Physical models are the most explicit way in which to describe a system; they capture the hardware composition of a system in terms of the computers (and other devices, such as mobile phones) and their interconnecting networks.


Architectural models describe a system in terms of the computational and communication tasks performed by its computational elements; the computational elements being individual computers or aggregates of them supported by appropriate network interconnections.


Fundamental models take an abstract perspective in order to examine individual aspects of a distributed system. The fundamental models that examine three important aspects of distributed systems: interaction models, which consider the structure and sequencing of the communication between the elements of the system; failure models, which consider the ways in which a system may fail to operate correctly and; security models, which consider how the system is protected against attempts to interfere with its correct operation or to steal its data.



Architectural models


The architecture of a system is its structure in terms of separately specified components and their interrelationships. The overall goal is to ensure that the structure will meet present and likely future demands on it. Major concerns are to make the system reliable, manageable, adaptable and cost-effective. The architectural design of a building has similar aspects – it determines not only its appearance but also its general structure and architectural style (gothic, neo-classical, modern) and provides a consistent frame of reference for the design.


Software layers


The concept of layering is a familiar one and is closely related to abstraction. In a layered approach, a complex system is partitioned into a number of layers, with a given layer making use of the services offered by the layer below. A given layer therefore offers a software abstraction, with higher layers being unaware of implementation details, or indeed of any other layers beneath them.


In terms of distributed systems, this equates to a vertical organization of services into service layers. A distributed service can be provided by one or more server processes, interacting with each other and with client processes in order to maintain a consistent system-wide view of the service’s resources. For example, a network time service is implemented on the Internet based on the Network Time Protocol (NTP) by server processes running on hosts throughout the Internet that supply the current time to any client that requests it and adjust their version of the current time as a result of interactions with each other. Given the complexity of distributed systems, it is often helpful to organize such services into layers. the important terms platform and middleware, which define as follows:


The important terms platform and middleware, which is defined as follows:


A platform for distributed systems and applications consists of the lowest-level hardware and software layers. These low-level layers provide services to the layers above them, which are implemented independently in each computer, bringing the system’s programming interface up to a level that facilitates communication and coordination between processes. Intel x86/Windows, Intel x86/Solaris, Intel x86/Mac OS X, Intel x86/Linux and ARM/Symbian are major examples.



o   Remote Procedure Calls – Client programs call procedures in server programs


o   Remote Method Invocation – Objects invoke methods of objects on distributed hosts


o   Event-based Programming Model – Objects receive notice of events in other objects in which they have interest




o   Middleware: software that allows a level of programming beyond processes and message passing


·        Uses protocols based on messages between processes to provide its higher-level abstractions such as remote invocation and events


·        Supports location transparency

·        Usually uses an interface definition language (IDL) to define interfaces



Interfaces in Programming Languages

·        Current PL allow programs to be developed as a set of modules that communicate with each other. Permitted interact ions between modules are defined by interfaces


·        A specified interface can be implemented by different modules without the need to modify other modules using the interface


Interfaces in Distributed Systems


When modules are in different processes or on different hosts there are limitations on the interactions that can occur. Only actions with parameters that are fully specified and understood can communicate effectively to request or provide services to modules in another process


·        A service interface allows a client to request and a server to provide particular services


·        A remote interface allows objects to be passed as arguments to and results from distributed modules


Object Interfaces


An interface defines the signatures of a set of methods, including arguments, argument types, return values and exceptions. Implementation details are not included in an interface.


A class may implement an interface by specifying behavior for each method in the interface. Interfaces do not have constructors.

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