Distributed System Concepts and Architectures Services

Overview  Service provider?  Who are involved in providing the services?  Types of services Basic services Fundamental services  References

Service provider? Operating system is the service provider. An operating system is a program that acts as an intermediary between the user of a computer and the computer hardware.

Continued… The purpose of an operating system is to provide an environment in which a user can execute programs in a convenient and efficient manner. Thus we can say that: An OS is 

Continued….  Tool to make programmer's job easy  Resource allocator Must be fair; not partial to any process, specially for process in the same class Must discriminate between different class of jobs with different service requirements  Control program  Layered architecture

Layer structure of a computer system

Overview  Service provider?  Who are involved in providing the services?  Types of services Basic services Fundamental services  References

Who are involved in providing the services? Since I am describing all the things here taking Unix as standard operating system. I would like to discuss here Shell and Kernel.

Shell  Unix command interpreter Interprets the first word of a command line as a command name  Is a user program and not part of the kernel  Redirection of input and output  Background jobs

Kernel  Permanently resides in the main memory  Controls the execution of processes by allowing their creation, termination or suspension, and communication  Schedules processes fairly for execution on the CPU  Allocates main memory for an executing process

Continued…  File system maintenance  Allows processes controlled access to peripheral devices such as terminals, tape drives, disk drives, and network devices.

Overview  Service provider?  Who are involved in providing the services?  Types of services Basic services Fundamental services  References

Basic Services  Program execution  I/O operations  File-system manipulation  Communications  Error detection

Program execution The system must be able to load a program into memory and to run that program. The program must be able to end its execution, either normally or abnormally (indicating error).

I/O operations A running program may require I/O. This I/0 may involve a file or an I/O device. For specific devices, special functions may be desired (such as to rewind a tape drive, or to blank a CRT screen). For efficiency and protection, users usually cannot control I/0 devices directly.

Continued… Therefore, the operating system must provide a means to do I/O.

File-system manipulation The file system is of particular interest. Obviously, programs need to read and write files. Programs also need to create and delete files by name.

Communications In many circumstances, one process needs to exchange information with another process. Such communication can occur in two major ways. The first takes place between processes that are executing on the same computer;

Continued… the second takes place between processes that are executing on different computer systems that are tied together by a computer network. Communications may be implemented via shared memory,

Continued… or by the technique of message passing, in which packets of information are moved between processes by the operating system.

Error detection The operating system constantly needs to be aware of possible errors. Errors may occur in the CPU and memory hardware, in I/O devices and in the user program. For each type of error, the operating system should take the appropriate action to ensure correct and consistent computing.

Overview  Service provider?  Who are involved in providing the services?  Types of services Basic services Fundamental services  References

Fundamental services  Primitive services  Services by the system servers  Value added services

Primitive services Primitive services are those that must be implemented on kernel of each node in the system. In Distributed system when communication is done through message passing, a set of primitives for send and receive must be defined and implemented. Send and receive can be

Continued…. either synchronous or asynchronous. Synchronous communication, in addition to serving communication purposes, carries the side benefit of achieving internodes inter process synchronization.

Services by the system servers The services which can be implemented anywhere in the system and performs the functions which is basic to the operation of a distributed system. To achieve transparency in Distributed System server mappes physical objects with logical system names.

Continued….  Name server Used to locate users, processes and machines.  Directory Server It is used in conjunction with files and communication ports.  Network server Name server translation, selection of paths, actual routing of information is done by network server.

Value added services Services that are not needed in the implementation of a Distributed System But useful in supporting Distributed Application. It is Used To increase the computational performance and enhancement of fault tolerance or by need for cooperative activities.

Continued….  Group Server It manages the creation and termination of group activities. It is also responsible for Group addressing, communication, contain information about membership, member privileges and admission policies. Web server is also the good example of a value added server.

References  Distributed operating systems & Algorithms, By Randy chow, Theodore Johnson  Operating System Concepts 6th ed - Silberschatz Galvin, ch -3 operating System structure  Advanced UNIX- A Programmer’s Guide, By Stephen Prata  Computer science illuminated, second edition, By Nell dale, John Lewis  ber=138323

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