Characterization of Distributed System

1. DISTRIBUTED SYSTEMS
B. Tech IV/IV, II Semester
COMPUTER SCIENCE & ENGINEERING
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2. Course Organization
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Distributed Systems
Foundation Middleware Support Systems
Algorithms &
Shared Data
Characterization
of Distributed
Systems
System Models
Inter Process
Communication
Distributed
Objects
Remote
Invocation
Operating
System
Distributed
File System
Coordination &
Agreement
Transactions &
Replications
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3. Table of Contents
1. Why to have a Distributed System?
2. What is a Distributed System?
3. Introduction to a Distributed Systems
4. Examples of Distributed Systems
5. Trends of a Distributed System
6. Focus on resource sharing
7. Design issues of a Distributed System
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4. Why to have a Distributed
System
• The need to share data across remote geographies
– Online Encyclopedia Britannica is accessed by users all over the world
– Computer users in different geographies send messages to each other
• Replication of processing power
– Independent processors working on the same task
– Distributed systems consisting of collections of microcomputers may
have processing power of a large supercomputer
• Use of heterogeneous components
– Compute-intensive sub-tasks of a problem are run on powerful
computers
– Less resource-demanding sub-tasks run on less powerful computers
– More efficient use of resources
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5. Why to have a Distributed
System
• Cost of hardware and management
– A collection of cheap computers may be less expensive than one large
supercomputer
– Small simple computers may be easier to manage than one large one
• Resilience to failures
– If one component fails, others can proceed with work on the task
• Scalability
– The system can be extended by adding more components (i.e., WWW)
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6. Introduction
• Computer hardware prices falling, power increasing
• Network connectivity increasing
– Everyone is connected with fat pipes
• It is easy to connect hardware together
Definition:
A Distributed System is one in which components located at
networked computers communicate and coordinate their
actions only by passing messages
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7. Introduction
Typical Layering in a Distributed System
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8. Introduction
• In building a Distributed System we need to address the
following consequences:
1. Concurrency
2. No Global Clock
3. Independent Failures
• The prime motivation for constructing and using distributed
systems stems from a desire to share resources
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9. Examples of Distributed Systems
1. Web Search
Nearly 3 Billion search requests hit Google servers everyday
About 276 Million search requests hit Yahoo servers everyday
• Web search is one of the largest and most complex
distributed systems installations in the history of computing
• Its infrastructure comprises of
– Underlying physical infrastructure with very large numbers of
networked computers located at data centre’s around the world
– A distributed file system designed to support very large files
– An associated structured distributed storage system that offers fast
access to very large datasets
– A lock service that offers distributed system functions such as
distributed locking and agreement
– Programming model supporting very large parallel and distributed
computations
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10. Examples of Distributed Systems
2. Massively Multiplayer Online Games
– World of Warcraft is one of the Most played games with over 10
Million customers.
– According to the predictions, MMOG are estimated to rise to 60% by
2015 to $3.1 Billion.
– Challenges include
• Fast Response Times
• Real time propagation of events
– Solutions for the design of a MMOG
• Client – server model where Server is formed through a dedicated cluster
• Distributed architectures spread across globally (Example: EverQuest)
• Completely decentralized approach based on peer-to-peer technology
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11. Examples of Distributed Systems
3. Financial Trading
– Foreign exchange markets averaged $5.3 Trillion per day in April 2013
– Events in Distributed Systems involve communication & Processing of
items of interest
– Such systems typically employ Distributed event-based Systems.
EXAMPLE OF FINANCIAL TRADING SYSTEM
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12. Other Examples of Distributed
Systems
• Flight management system in an aircraft
• Automotive control systems (50+ embedded processors in a
Mercedes S-class)
• Distributed file systems (NFS, Samba)
• P2P file sharing
• The World Wide Web!!!
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13. Trends of a Distributed System!
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How is the requirement of a Distributed System
evolved?
What are the influential trends responsible for this
paradigm shift
1. Emergence of Pervasive networking technologies
2. Emergence of Ubiquitous computing supporting mobility
3. Growing demand for Multimedia services
4. Computing as an Utility

14. Trends of a Distributed System!
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1. Pervasive networking & the modern Internet
• Inter connecting varied range of computer networks

15. Trends of a Distributed System!
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2. Mobile & Ubiquitous Computing
• Device miniaturization and wireless networking led to the
integration of portable computing devices into distributed systems
• Devices like Laptops, PDA’s, Smart phones, Tablets, Wearable
devices, Embedded devices
• Ubiquitous computing includes the need to deal with variable
connectivity and indeed disconnection

16. Trends of a Distributed System!
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3. Distributed Multimedia Systems
• Distributed Multimedia Systems
• Teleconferencing
• Distance learning
• Cellular phone systems
• IP Telephony
• Webcasting is an application of distributed multimedia technology

17. Trends of a Distributed System!
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4. Distributed Computing as a Utility
• Why shouldn’t computing resources be delivered as a Utility
• Similar to how we use Electricity & Water, as and when Needed?
• Example: Computing requirements like
• Configured environment (Type of OS, Processing power, RAM,
Hard disk, Networking capabilities) with custom requirements
of user
• Scalable storage/compute/networking needs
• Cloud Computing is used to showcase computing as a Utility
• Cluster computers provide a range of cloud services
• Grid Computing is also a form of Cloud Computing primarily focused
towards support for scientific applications

18. Focus on Resource Sharing
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• Sharing Printers, Files was very helpful 
• In the earlier slide we have introduced sharing of compute,
storage & network abilities
• To book a Tatkal ticket, users must share the common
reservation database.
• Its all about Reliability & Consistency
• WWW, Email, Networked printers are example of
Distributed Systems.
• Not all, but often, resource sharing is achieved using Client-
Server models.
• A complete interaction between a client and a server, from
the point when the client sends its request to when it
receives the server’s response, is called a Remote Invocation.

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WE NEED A
DISTIBUTED
SYSTEM 
WHAT ARE
THE
CHALLENGES

WE KNOW
WHAT IT IS


20. Design issues of a Distributed
System
Heterogeneity
Openness
Security
Synchronization
Absence of Global Clock
Partial Failures
Scalability
Transparency
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21. Challenges of DS: Heterogeneity
• Different network infrastructures (Ethernet, 802.11 –
wireless)
• Hardware and software (e.g., operating systems,
processors): how can an Intel/Windows system understand
messages sent by an Macintosh OS X system?
• Programming languages – how can a Java program and a C
program communicate?
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22. Challenges of DS: Openness
• The openness of a computer system is the characteristic that
determines whether the system can be extended and
reimplemented in various ways.
• The challenge to designers is to tackle the complexity of
distributed systems consisting of many components engineered
by different people.
• Open systems are characterized by the fact that their key
interfaces are published.
• Open distributed systems are based on the provision of a
uniform communication mechanism and published interfaces for
access to shared resources.
• Open distributed systems can be constructed from
heterogeneous hardware and software, possibly from different
vendors
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23. Challenges of DS: Security
• Shared data must be protected
– Privacy – avoid unintentional disclosure of private
data
– Security – data is not revealed to unauthorized
parties
– Integrity – protect data and system state from
corruption
• Denial of service attacks – put significant load
on the system, prevent users from accessing
it
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24. Challenges of DS:
Synchronization
• Concurrent cooperating tasks need to synchronize
– When accessing shared data
– When performing a common task
• Synchronization must be done correctly to prevent data
corruption:
– Example: two account owners; one deposits the money, the other
one withdraws; they act concurrently
– How to ensure that the bank account is in “correct” state after
these actions?
• Synchronization implies communication
• Communication can take a long time
• Excessive synchronization can limit effectiveness and scalability
of a distributed system
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25. Challenges of DS: Absence of
Global Clock
• Cooperating task need to agree on the order of
events
• Each task has its own notion of time
• Clocks cannot be perfectly synchronized
• How to determine which even occurred first?
• Example: Bank account, starting balance = $100
– Client at bank machine A makes a deposit of $150
– Client at bank machine B makes a withdrawal of
$100
– Which event happened first?
– Should the bank charge the overdraft fee?
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26. Challenges of DS: Partial Failures
• Detection of failures – may be impossible
– Has a component crashed? Or is it just slow?
– Is the network down? Or is it just slow?
– If it’s slow – how long should we wait?
• Handling of failures
– Retransmission
– Tolerance for failures
– Roll back partially completed task
• Redundancy against failures
– Duplicate network routes
– Replicated databases
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27. Challenges of DS: Scalability
• Does the system remain effective as if
grows?
• As you add more components:
– More synchronization
– More communication –> the system runs slowly.
• Avoiding performance bottlenecks:
– Everyone is waiting for a single shared
resource
– In a centrally coordinated system, everyone
waits for the co-ordinator
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28. Challenges of DS: Transparency
• Concealing the heterogeneous and distributed nature
of the system so that it appears to the user like one
system
• Transparency categories
– Access: access local and remote resources using identical
operations (NFS or Samba-mounted file systems)
– Location: access without knowledge of location of a resource
(URL’s, e-mail)
– Concurrency: allow several processes to operate
concurrently using shared resources in a consistent fashion
(two users simultaneously accessing the bank account)
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29. Challenges of DS: Transparency
• Transparency categories (continued)
– Replication: use replicated resource as if there was just one
instance
– Failure: allow programs to complete their task despite
failures
– Mobility: allow resources to move around
– Performance: adaption of the system to varying load
situations without the user noticing it
– Scaling: allow system and applications to expand without
need to change structure of applications or algorithms
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30. Advantages of distributed systems
over centralized systems
Economics Microprocessors offer a better
price/performance than mainframes
Speed A distributed system may have more total
computing power than a mainframe
Inherent
distribution
Some applications involve spatially separated
machines
Reliability If one machine crashes, the system as a
whole can still survive
Incremental
growth
Computing power can be added in small
increments
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31. Advantages of distributed systems
over isolated (personal) computers
Data sharing Allow many users access to a common data
base
Device sharing Allow many users to share expensive
peripherals like color printers
Communication Make human-to-human communication easier,
for example, by electronic mail
Flexibility Spread the workload over the available
machines in the most cost effective way
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32. Disadvantages of Distributed Systems
Software Little software exists at present for
distributed systems
Networking The network can saturate or cause other
problems
Security Easy access also applies to secret data
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33. Glossary
Distributed System: A distributed system is one in which components
located at networked computers communicate and coordinate their
actions only by passing messages
Cloud Computing: A model for enabling ubiquitous, convenient, on-
demand network access to a shared pool of configurable computing
resources (e.g., networks, servers, storage, applications and
services) that can be rapidly provisioned and released with minimal
management effort or service provider interaction.
Middleware: A software layer that provides a programming
abstraction as well as masking the heterogeneity of the underlying
networks, hardware, operating systems and programming languages
Cluster computing: is a set of interconnected computers that
cooperate closely to provide a single, integrated high performance
computing capability
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34. QUIZ
1. Which of these layers are not a part of Distributed system
architecture
(a) Middleware (b) Operating System (c) Network (d) Storage
2. What is the main motivating factor for constructing distributed
systems
(a) Resource Sharing (b) Concurrency (c) Global Clock (d) Security
3. Pick the odd man out
(a) Web Search (b) MMOG (c) Utility Computing (d) Financial Trading
4. __________ in Distributed Systems involve communication &
Processing of items of interest.
(a) (a) Networks (b) Events (c) Mobile Computing (d) Webcasting
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35. QUIZ
5. _____ is used to capture this vision of computing as a Utility
(a) Grid Computing (b) Cloud Computing (c) Cluster Computing (d)
None
6. Focus of resource sharing is on
(a) Reliability (b) Efficiency only (c) Consistency (d) A&B
7. Which layer masks the heterogeneity of the underlying Hardware,
Network and OS
(a) Application (b) Virtual Machine (c) Middleware (d) None
8. Which of the following is an advantage of a Distributed System
(a) Security (b) Reliability (c) Networking (d) A&C
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36. QUIZ
9. Which of the following would make certain aspects of distribution
invisible to the application programmer
(a) Transperency (b) Concurrency (c) Openness (d) Scalability
10. _________ a set of interconnected computers that cooperate
closely to provide a single, integrated high performance computing
capability
(a) Distributed System (b) Cloud Computing (c) Grid Computing (d) All
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37. KEY
1. D
2. A
3. C
4. B
5. B
6. D
7. C
8. B
9. A
10. C
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