1. Introduction To Distributed Systems
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2. Introduction
Computer revolution started from the year 1945 (modern era)
During the year 1985 large and expensive computers were available
As time progressed new developments took place and two new technologies were developed. They are
1. Powerful microprocessor (8 bit, 16 bit, 32 bit, 64 bit)
2. High speed networks (100 Gbits/sec LAN – , 10 Gbits/sec WAN)
The result of above two technologies were;
1. flexibility
2. easy to access and put together all the computing systems
connected together with high speed network.
The above result leads to distributed systems…. Than previous centralized systems.
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3. Definition of a Distributed System (1)
A distributed system is:
A collection of independent(autonomous ) computers that appears to its users as a single coherent system.
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4. Definition of a Distributed System (2)
1.1
A distributed system organized as middleware. Note that the middleware layer extends over multiple machines.
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5. Definition means distributed systems consists of components (computers) that are autonomous (independent, self-directing) 2. users they are dealing with single system i.e all the autonomous computers need to collaborate with each others through passing messages. when autonomous computers are collaborated we get distributed systems.
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6. Characteristic of distributed systems 1
Characteristic of distributed systems 1. Resource sharing (h/w, s/w, data, files, should have resource manger) 2. Openness 3. Concurrency 4. Scalability 5. Fault Tolerance 6. Transparency
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7. 1. Resources Sharing
Access resources and share them in a controlled and efficient way.
Printers, computers, storage facilities, data, files, Web pages, and networks, …
Connecting users and resources also makes it easier to collaborate and exchange information.
Internet for exchanging files, mail, documents, audio, and video
Security is becoming increasingly important
Little protection against eavesdropping or intrusion on communication
Tracking communication to build up a preference profile of a specific user
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8. 2. Openness
Goal: Open distributed system -- able to interact with services from other open systems, irrespective of the underlying environment:
System should conform to well-defined interface
Systems should support portability of applications
Systems should easily interoperate
Achieving openness: At least make the distributed system independent from heterogeneity of the underlying environment:
Hardware
Platforms
Languages
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9. 3. Concurrency Concurrency refers when the two or more
transaction are executed simultaneously.
Ex. Multiprogramming/Parallel programming and Multiprocessing..
Parallel executions in distributed systems
1. Many users using the same resources, application interactions
2. Many servers responding to client requests
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10. 4. Scalability Scalability: At least three components:
How the system handles growth
The current Internet is an example of scalable system
Scalability: At least three components:
Number of users and/or processes (size scalability)
Maximum distance between nodes (geographical scalability)
Number of administrative domains (administrative scalability)– (it can still be easy to manage even if it spans many independent administrative organizations.)
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11. 5. Fault Tolerance
Hardware, Software, network may fail at any point of time.
Fault tolerance can be achieved by….
Hardware redundancy
Software recovery scheme (i.e. Self Healing S/W)
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12. 6. Transparency
It is an important goal of the distributed system to hide the fact that its processes and resources are distributed across the multiple computers.
A distributed system that is able to present itself to user and application as if it were only a single computer system is said to be transparent.
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13. Transparency in a Distributed System
Description
Access
Hide differences in data representation and how a resource is accessed
Location
Hide where a resource is located
Migration
Hide that a resource may move to another location
Relocation
Hide that a resource may be moved to another location while in use
Replication
Hide that a resource may be shared by several competitive users
Concurrency
Failure
Hide the failure and recovery of a resource
Persistence
Hide whether a (software) resource is in memory or on disk
Different forms of transparency in a distributed system.
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14. Advantages of Distributed Systems
Performance: very often a collection of processors can provide higher performance (and better price/performance ratio) than a centralized computer.
Distribution: many applications involve, by their nature, spatially separated machines (banking, commercial, automotive system).
3. Reliability (fault tolerance): if some of the machines crash, the system can survive.
Incremental growth: as requirements on processing power grow, new machines can be added incrementally.
5. Sharing of data/resources: shared data is essential to many applications (banking, computer supported cooperative work, reservation systems); other resources can be also shared (e.g. expensive printers).
6. Communication: facilitates human-to-human communication.
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15. Disadvantages of Distributed Systems
Difficulties of developing distributed software: how
should operating systems, programming languages and applications look like?
Networking problems: several problems are created by the network infrastructure, which have to be dealt with: loss of messages, overloading, ...
Security problems: sharing generates the problem of
data security.
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16. References
1. – 2. www0.cs.ucl.ac.uk/staff/ucacwxe/lectures/ds98-99/dsee3.pdf Distributed Systems, Principles and Paradigms, 2nd edition, by Andrew S. Tanenbaum and Maarten Van Steen
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