1. Introduction to Distributed System
Chapter 1
Introduction to Distributed System
Outlines Definition Characteristics Organization and Goals of Distributed Systems Hardware and Software Concepts The Client-server Model
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2. Chapter-1: Introduction to Distributed System
Computer systems are undergoing a revolution and modern computer is started from 1945.
At that time, computers are very large and expensive
Later, around the mid-1980s, there are two advances in technology began to change that situation.
The development of powerful microprocessors( 8, 16,32 and 64 bits)
The invention of high-speed computer networks (LANs)
A distributed system is a collection of independent computers that appears to its users as a single coherent system.
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3. Chapter-1: Introduction to Distributed System
It is used to connect multiple components together in a shorter space and the communication component was more or less based on cabling.
It is also a collection of autonomous computers linked by a computer network that appear to the users of the system as a single computer.
How to connect independent computers?
Networks and protocols
How to make them appear as a single system?
Distributed software
A distributed system enables machines involved:
Coordinate their activities
Share resources- hardware, software, data, etc.
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4. Chapter-1: Introduction to Distributed System
key features of a distributed system are:
Components in the system are concurrent. A distributed system allows resource sharing, including software by systems connected to the network at the same time.
There can be multiple components, but they will generally be autonomous in nature.
A global clock is not required in a distributed system. The systems can be spread across different geographies.
Compared to other network models, there is greater fault tolerance in a distributed model.
Price/performance ratio is much better.
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5. Examples of distributed Systems
rlogin or telnet (for remote access)
network file system, network printer, etc.
ATM (cash machine)
Distributed databases
Network computing
Retail point-of-sale terminals
Air-traffic control
Enterprise computing etc
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6. Advantages and Disadvantages of Distributed System
Economics
Speed
Inherent distribution
Reliability
Incremental growth
Disadvantages:
Software
Network
More components to fail
Security
The Makings of a Distributed System
Machine A Machine B Machine C
network
Distributed Applications
Middleware Services
Local OS
Local OS
Local OS
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7. Hardware sharing for: convenience and reduction of cost
Characteristics of Distributed system
Resource Sharing:
hardware - disks and printers
software - files, windows, and data objects
Hardware sharing for: convenience and reduction of cost
Data sharing for:
consistency - compilers and libraries
exchange of information - database
cooperative work - groupware
Resource Manager how?
Software module: that manages a set of resources
Each resource requires its own management policies and methods
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8. Characteristics of Distributed system …
Client server model - server processes act as resource managers for a set of resources and a set of clients
Object based model- resources are objects that can move and Object manager is movable.
Request for a task on an object is sent to the current manager and manager must be collocated with object.
Examples: Amoeba, migratory improvements - Arjuna, Clouds, Emerald
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9. Concurrency of components Lack of a global clock
Characteristics of Distributed system …
Concurrency of components
Lack of a global clock
Independent failure of components.
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10. Openness of distributed systems
Characteristics of Distributed system …
2. Openness
Openness of a computer system is the characteristic that determines whether the system can be extended and re-implemented in various way.
Eg. UNIX, open source software
Openness of distributed systems
determined by the degree to witch new resource sharing services can be added and be made available for use by a variety of other programs
Eg. Web.
How to deal with openness?
key interfaces are published.
Eg. RFC
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11. 2. Openness …. Characteristics of Distributed system …
Goals: interoperability, portability, flexibility
3. Scalability
A system is scalable if it will remain effective when there is a significant increase in the number of resources and the number of users
Example of scalable system: the Internet
Design challenges
Controlling the cost of physical resources
e.g., servers support users at most O(n)
Controlling the performance loss
e.g., DNS no worse than O(logn)
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12. 3. Scalability … Characteristics of Distributed system …
Preventing software resources from running out
e.g., IP address
Avoiding performance bottlenecks
e.g., partitioning name table of DNS, cache and replication
There are three dimensions:
Size :- it can easily add more users and resources to the system
Geographically scalable:-s one in which the users and resources may lie far apart.
Administratively scalable:- it can still be easy to manage even if it spans many independent administrative organizations.
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13. 3. Scalability … There are basically four techniques for scaling:
Characteristics of Distributed system …
3. Scalability …
Scalability limitations:
Scaling Techniques
There are basically four techniques for scaling:
Use only decentralized algorithms
Hiding communication latencies
Distribution
Replication
Concept
Example
Centralized services
A single server for all users
Centralized data
A single on-line telephone book
Centralized algorithms
Doing routing based on complete
information
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14. Decentralized Algorithms
Why are centralized algorithms bad?
Single point of failure
Lots of communications to one point
Centralized algorithm is different from decentralized algorithms
No machine has complete information about the system state.
Machines make decisions based only on local information.
Failure of one machine does not ruin the algorithm.
No global clock .
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15. Hiding communication latencies
It is important to achieve geographical scalability.
The difference between letting:
a server or
a client check forms as they are being filled
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16. Distribution
It involves taking a component, splitting it into smaller parts.
An example of dividing the DNS name space into zones.
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17. Replication
To solve the scalability, replicate the component across the distributed system is an important.
Replication is
Increases the availability of data
Balance the load of between components and leads better performance
Server based replication
Scalable web servers
Web proxy caching
Replicated databases
Auto replication in cloud storage
Client based replication
Caching
Prefetching
Static vs. dynamic replication
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18. GOALS of distributed system High performance Reliability Scalability
Consistency
Security
Challenges for distributed systems:
Coordination and resource sharing can be difficult if proper protocols or policies are not in place.
Process knowledge should be put in place for the administrators and users of the distributed model.
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19. Challenges for distributed systems …
Security
Confidentiality
protection against disclosure to unauthorized individuals
e.g. ACL in Unix File System
Integrity
protection against alteration or corruption
e.g. checksum
Availability
protection against interference with the means to access the resources
e.g. Denial of service(DOS).
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20. Challenges for distributed systems …
Failure Handling
Detecting failures
e.g. checksum for corrupted data
Sometimes impossible, e.g. a remote crashed server in the Internet
Masking failures
e.g. Retransmit message, standby server
Tolerating failures
e.g. a web browser cannot contact a web server
Recovery from failures
e.g. Roll back
Redundancy
e.g. IP route, replicated name table of DNS
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21. Challenges for distributed systems …
Concurrency
Correctness
ensure the operations on shared resource correct in a concurrent environment
e.g. records bids for an auction
Performance
Ensure the high performance of concurrent operations
Transparency in a Distributed System
Transparency
Description
Access
Hide differences in data representation and how a resource is accessed (local and remote resources are accessed using identical operations
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22. Challenges for distributed systems …
Transparency…
Location
Hide where a resource is located (access resources without the need to know their physical or network location such as building or IP address)
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 is replicated (multiple instances of resources can be used to increase reliability and performance without knowledge of the replicas by users or application programmers)
Concurrency
Hide that a resource may be shared by several competitive users (several processes can operate concurrently using shared resources without interference between them)
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23. Challenges for distributed systems …
Transparency…
Failure
Hide the failure and recovery of a resource (conceal faults, allowing users and application programs to complete their tasks despite the failure of hardware or software components)
Performance
allows the system to be reconfigured to improve performance as loads vary
Scaling
allows the system and applications to expand in scale without change to the system structure or the application algorithms
Persistence
hide whether a (software) resource is in memory or on disk)
Programming
hide the physical distribution from the application developers
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24. Challenges for distributed systems… Heterogeneity Networks
Ethernet, token ring, etc
Computer hardware
big endian / little endian
Operating systems
different API of Unix and Windows
Programming languages
different representations for data structures
Implementations from different developers
no application standards
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25. Hardware and Software Concepts
An operating system is primarily a resource manager
Design is tied to the hardware and software resources the operating system must manage
processors
memory
secondary storage (such as hard disks)
other I/O devices
processes
threads
files
Databases
Hardware Components
A computer’s hardware consists of: processor(s),main memory and input/output devices
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26. Hardware and Software Concepts Hardware Components …
A processor is hardware that executes machine language
CPU executes the instructions of a program
Coprocessor executes special-purpose instructions
Ex., graphics or audio coprocessors
Registers are high-speed memory located on processors
Data must be in registers before a processor can operate on it
Instruction length is the size of a machine-language instruction
Computer time is measured in cycles
One complete oscillation of an electrical signal
Provided by system clock generator
Processor speeds are measured in GHz (billions of cycles per
second)
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27. Hardware and Software Concepts Hardware Components ...
Main memory(RAM) is the lowest data referenced directly by processor
Volatile – loses its contents when the system loses power
Common forms of RAM include:
dynamic RAM (DRAM) – requires refresh circuit
static RAM (SRAM) – does not require refresh circuit
Bandwidth is the amount of data that can be transferred per unit of time
Peripheral Devices(Input/Output)
Character devices transfer data one bit at a time
Keyboards and mice
Can be attached to a computer via ports and other buses
ex. Serial ports, parallel ports, USB port etc.
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28. Hardware and Software Concepts
Software Overview
Programming languages
Some are directly understandable by computers, others require translation
Classified generally as either:
Machine language
Assembly language
High-level language (C,C++,java, C#)
Translators
Compiler , assembler , Interpreter
Middleware: is software for distributed systems that enables interactions among multiple processes running on one or more computers across a network
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29. Hardware and Software Concepts …
Middleware…
Facilitates heterogeneous distributed systems
Simplifies application programming
Example, Open DataBase Connectivity (ODBC)
Permits applications to access databases through middleware called an ODBC driver
Client-server Model:-is a distributed application structure that partitions task or workload between the providers of a resource or service, called servers, and service requesters called clients.
In a client-server architecture, when the client computer sends a request for data to the server through the internet, the server accepts the requested process and deliver the data packets requested back to the client
. Examples of Client-Server Model are , World Wide Web, etc.
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30. How the Client-Server Model works ?
Its basically the Client requesting something and the Server serving it as long as its present in the database.
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31. THANK YOU END OF Chapter -1
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