01- Introduction to Distributed Systems

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Distributed Systems Peter Arnold [email protected]

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Introduction

A distributed system is a collection of autonomous hosts connected through a computer network. Hosts

communicate through message passing to facilitate cooperation. A distributed system can also be

referred to as a collection of independent computers that appears to its users as a single coherent

system. Hosts on a distributed system work jointly to perform a single task or provide a common service.

Examples of distributed systems

1.

The Web

If you buy a flight on-line, your computer interacts with a distant machine.

Shared Resources such as Documents

Users interested in the documents are also distributed.

The documents are also distributed.

2.

Civil Aviation

Modern planes have replicated flight control computers (Airbus A330/A340 planes have 5 for

in

3.

Peer-to-Peer (P2P) file sharing networks

Each user contributes to the network

Auto-organization, no centralized entity

4.

Bank ATMs :

Clients at different ATMs may access the same accounts simultaneously

ATMs use nearest central office

Each office acts as a backup of the other

Clients may access their accounts from ATM machines.

z xm b m s depicted in the figure

below.


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Components of a Distributed System

Data component - Concerned with structures for information retention and manipulation

Processing component - Concerned with processing data objects

Presentation component - Makes data visible to users and handling user interaction

Communication channel - Pipelines between processes. This usually refers to inter-platform network

MessageData sent between processes over communication channel

Characteristics of a Distributed System

Multiple Computers

A distributed system usually consists of more than one physical computer, each consisting of CPUs, local

memory, and possibly stable storage, and Input/output paths to connect it with the environment.

Interconnections

Refers to mechanisms for communicating with other nodes on a network.

Availability

A distributed system should be permanently available (even though parts of it may be unavailable at

times.

Shared State

If a subset of nodes cooperates to provide a service, then a shared state is maintained by these nodes.

The shared state is distributed or replicated among the participants.

Scalability

Should be easy to expand and scale.

the rest of

email server

Web server

Desktopcompu ters

File server

router/firewall

print and other servers

other servers

print

Local area

network

email server

the Internet

F 1 z


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Transparency

The separation of the components of a distributed system is concealed from the user and the

application programmer.

Fault tolerance

The ability to continue in operation after a fault has occurred.

Processes interact via non-shared local memory.

Goals of Distributed Systems

To ensure availability i.e. to make sure that a user requesting for a service gets it.

To achieve consistency

To achieve reliability

To improve efficiency

To improve performance

To ensure scalability

Advantages of Distributed Systems

For many applications there are a number of economic and technical reasons or motivations that make

distributed systems much more attractive than their centralized counterparts. The advantages of

distributed systems include:

1. Sharing of resourcesDistributed systems allow sharing of resources e.g. storage drives, printers,

databases, files etc.

2. Better Performance - By using the combined processing and storage capacity of many nodes,

performance levels can be improved.

3. Scalability - Resources such as processing and storage capacity can be increased incrementally.

4. Inherent distribution - Some applications, such as email and the Web are naturally distributed. This

includes cases where users are geographically dispersed as well as when single resources (e.g., printers,

data) need to be shared.

5. Enhanced Reliability - By having redundant components the impact of hardware and software faults

on users can be reduced.


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Disadvantages of Distributed Systems

The advantages associated with distributed systems are often offset by several problems encountered

during the use and development of the same systems. These limitations include the following.

New components lower network performance

Networks are needed to connect independent nodes and are subject to performance limitations.

Besides these limitations, networks also constitute new potential points of failure.

Weak security

Because a distributed system consists of multiple components, there are more elements that can be

compromised and must, therefore, be secured. This leads to a compromise of security in the same

systems.

Software complexity

Distributed systems software is more complex and harder to develop than conventional software,

hence, it is more expensive to develop and there is a greater chance of introducing errors.

Management complexity

More effort is required for distributed systems management.

Distributed Systems Concepts

Resources- Refers to services, components or servers in the context of providers of information e.g.

web servers. A resource is anything that can be shared or used on a distributed network. Examples of

resources that can be shared on a distributed system include computers, file storage systems, RAM,

CPU, files, communication channels, databases, servers e.g. a web server and applications.

Transparency- Refers to the property of hiding aspects of distribution from the user, e.g. location and

access transparency.

Distributed paradigms- Refers to models, for describing distribution and communication. Some of these

paradigms are: client/server model, peer to peer model.

Client- Computer which acts as a consumer of information in a distributed system.

Server- Computer which act as a provider of information. A server provides a service e.g. a web server.

Node- Used to represent any computer on a distributed network.

Services- Services are analogous to function calls. Servers receive a request (i.e. the arguments to a

function call) and return a response (i.e. the returned function data or data updated within a pass by

reference argument).


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A Peer- A special kind of node in which runs both a client and a server. A computer is both a consumer

and provider of information.

Rules of the Thumb

Trade-offs

At a certain point, trade-offs must be made i.e. a choice must be made about which requirement orproperty is more important and how far to go when fulfilling that requirement. For instance availability

holds only in the face of certain problems, but not others?

Separation of Concerns

When tackling a large, complex, problem (such as designing a distributed system), it is useful to split the

problem up into separate concerns and address each concern individually. Approaching the design of a

distributed system in this particular way leads to highly modular or layered systems, which helps to

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Keep It Simple, Stupid (KISS)

Overly, complex systems are error prone and difficult to use. If possible , solutions to problems and

resulting architectures should be simple rather than mind-numbingly complex.

Policy versus Mechanism

Separation of policy and mechanism helps to build flexible and extensible systems, and is, therefore,

important to follow when designing distributed systems e.g. different mechanisms can be used to

implement one policy.

End-to-End Argument

When building layered systems some functions can only be reliably implemented at the application

level. End to end functionality refers to functionality required at the application level. Implementing

functionality at the wrong level forces everyone to use that mechanism. It also renders the mechanism

less useful than when it is implemented at a higher (application) level.

01- Introduction to Distributed Systems

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