Distributed System

Distributed System

• Definition: A distributed system is a collection of autonomous hosts that are connected through a computer network.

• A distributed system is a collection of independent computers that appears to its users as a single coherent system. Each host executes components and operates a distribution middleware.

• Middleware enables the components to coordinate their activities. Users perceive the system as a single, integrated computing facility.

• A distributed computer system consists of multiple software components that are on multiple computers, but run as a single system. The computers that are in a distributed system can be physically close together and connected by a local network, or they can be geographically distant and connected by a wide area network.

• A distributed system can consist of any number of possible configurations, such as

mainframes, personal computers, workstations, minicomputers and so on, such a

• Distributed operating systems depend on networking for their operation. Distributed OS runs on and controls the resources of multiple machines. It sts provides resource sharing across the boundaries of a single computer system. It looks to users like a single machine OS. 

• Distributing OS owns the whole network and makes it look like a virtual otauni-processor or may be a virtual multiprocessor.

• Definition: A distributed operating system is one that looks to its users like an ordinary operating system but runs on multiple, independent CPU.

• Distributed systems depend on networking for their functionality. Fig. 1.6.1 shows the distributed system.

• Examples of distributed operating system are Amoeba, chrous, mach and v-system

• A Local Area Network (LAN) is a network that is confined to a relatively small area. It is generally limited to a geographic area such as a college, lab or building.  

• WAN provides long distance transmission of data and voice. Computers connected to a wide-area network are often connected through public networks, such as the telephone system. They can also be connected through leased lines or satellites.

• A MAN typically covers an area of between 5 and 50 km diameter. Many MANS cover an area the size of a city, although in some cases MANS may be as small as a group of buildings.

• A MAN often acts as a high speed network to allow sharing of regional resources. MAN provides the transfer rates from 34 to 150 Mbps.

Advantages and Disadvantages of DS

a) Advantages of distributed OS:

1. Resource sharing: Sharing of software resources such as software libraries, database and hardware resources such as hard disks, printers and CDROM can also be done in a very effective way among all the computers and the users.

2. Higher reliability: Reliability refers to the degree of tolerance against errors and component failures. Availability is one of the important aspects of reliability. Availability refers to the fraction of time for which a system is available for use.

3. Better price performance ratio: Reduction in the price of microprocessor and increasing computing power gives good price-performance ratio.

4. Shorter responses times and higher throughput.

5. Incremental growth: To extend power and functionality of a system by simply adding additional resources to the system.

b) Disadvantages :

1. There are no current commercially successful examples.

2. Protocol overhead can dominate computation costs.

3. Hard to build well.

4. Probably impossible to build at the scale of the internet.

Client-Server Computing

• The system is structured as a set of processes, called servers that offer services to the users, called clients.

• Server systems are of two types: Compute servers and file servers.

• Compute-server system: It provides an interface to which a client can send a request to perform an action. In response, the server performs some operation and sends the results to the client. Server contains database.

• File-server system: Client can performs various operation like create, read, update and delete file on file server. Web server is best example of this type.

• Fig. 1.6.2 shows the client server model.

• The client-server model is usually based on a simple request/reply protocol, implemented with send/receive primitives or using Remote Procedure Calls (RPC) or Remote Method Invocation (RMI) :

1. The client sends a request (invocation) message to the server asking for some service;

2. The server does the work and returns a result (e.g. the data requested) or an error code if the work could not be performed.

• Server is a process; Client is a process. Clients invoke servers, servers send results to clients. Servers can be clients of other servers.

• HTTP (Web) server is a server process to its client processes (web browsers).

• HTTP server may be a client of a database server. Service may be provided by multiple servers, as is most often the case within a large enterprise.

• Cache is a repository of recently accessed objects (files, graphics) that is physically closer to the client than the server from which it originated. Proxy server sits in between clients and servers and can play many mitigation roles. 

Advantages:

1. Simple to implement.

2. Provides good security.

3. All files are stored in a central location.

Disadvantages:

1. Single point of failure.

2. A specialist network operating system is needed.

Peer-to-Peer System

• All processes (objects) play similar role. Do not require a server process. Processes (objects) interact without particular distinction between clients and servers.

• The pattern of communication depends on the particular application. Fig. 1.6.3 shows the peer-to-peer model. (See Fig. 1.6.3 on next page.)

• Processing and communication loads for access to objects are distributed across many computers and access links. This is the most general and flexible model.

• A group of computers connected together to combine their computing and 'processing abilities to search the Internet or solve very complex problems

• Problems with peer-to-peer : High complexity due to,

1. Cleverly place individual objects

2. Retrieve the objects

3. Maintain potentially large number of replicas.

Distinguish between Client Server and Peer-to-Peer Model