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DISTRIBUTED SYSTEMS
B. Tech IV/IV, II Semester
COMPUTER SCIENCE & ENGINEERING
2/6/2017 © Copyright Material 1
UNIT II
2/6/2017 2
Distributed Systems
Foundation Middleware Support Systems
Algorithms &
Shared Data
Characterization
of ...
What we learnt in Unit I
1. Why to have a Distributed System?
2. What is a Distributed System?
3. Introduction to a Distri...
Topic of Contents
A. Introduction to a System Model
B. Types of System Models
Physical, Architectural, Fundamental
C. What...
Learning Objective
Upon completion of this unit, students will be able to:
• Understand the necessary components & placeme...
System Model
(A). What is a System Model & why to learn?
• Factors that effect any System behavior…
– Key entities partici...
System Model
(B). What are the types of System Models
Physical models: capture the hardware composition of a system in
ter...
Physical Model
(B). 1. What is a Physical Model
Evolution of Physical models started with a
1. Baseline physical model: an...
Architectural Model
(C). What is an Architectural Model
• Architectural model is an abstract view of a distributed
system
...
Architectural Model
(C). 1. Software Layers of a Distributed System
Any process & service oriented view can be expressed i...
Architectural Model
(C).1. Software Layers of a Distributed System
From the figure in the previous slide, we should know a...
Architectural Model
(C). 2. Types of System Architectures
CLIENT – SERVER
Client – server architecture could be of differe...
Architectural Model
(C). 2. Types of System Architectures
1. Clients invoking individual servers
Client processes interact...
Architectural Model
(C). 2. Types of System Architectures
2. Services provided by multiple servers
Partitioning the server...
Architectural Model
(C). 2. Types of System Architectures
3. Proxy servers & Cache
Web proxy servers provide a shared cach...
Architectural Model
(C). 2. Types of System Architectures
DECENTRALIZED
SYSTEM
ARCHITECUTE
PEER-TO-PEER
All participating ...
Architectural Model
(C). 3. Variations in Client-Server Model
Several variations from the client-server model can be
deriv...
Architectural Model
(C). 3. Variations in Client-Server Model
Thin Client: A thin client (sometimes also called a lean, ze...
Architectural Model
(C). 3. Variations in Client-Server Model
Mobile devices & Spontaneous networking:
2/6/2017 19© Copyri...
Architectural Model
(C). 3. Variations in Client-Server Model
Mobile devices & Spontaneous networking:
Key Features
1. Eas...
Architectural Model
(C).4 Role of Interfaces & Objects
• The term interface is used to refer to the specification
of the p...
Architectural Model
(C).5 Design Requirements for Distributed Architecture
Various objectives for creation of distributed ...
Architectural Model
(C).5 Design Requirements for Distributed Architecture
Quality of Service (QoS): Non-functional proper...
Architectural Model
(C).5 Design Requirements for Distributed Architecture
Caching and Replication
Caching is done to impr...
Fundamental Model
(D). What is a Fundamental Model?
Formal description of the properties that are common to all
architectu...
Fundamental Model
(E). Types of a Fundamental Model
The aspects of distributed systems that we wish to capture
in our fund...
Fundamental Model
(E). Types of a Fundamental Model
(E).1. Interaction Model
Concept of distributed algorithm comprises of...
Fundamental Model
(E). Types of a Fundamental Model
(E).1. Interaction Model
There are two variants of an Interaction mode...
Fundamental Model
(E). Types of a Fundamental Model
(E).1. Interaction Model
EVENT ORDERING:
Lets try to see how Logical c...
Fundamental Model
(E). Types of a Fundamental Model
(E).1. Interaction Model
EVENT ORDERING:
2/6/2017 30© Copyright Materi...
Fundamental Model
(E). Types of a Fundamental Model
(E).1. Interaction Model
EVENT ORDERING:
Lamport [1978] proposed a mod...
Fundamental Model
(E). Types of a Fundamental Model
(E).1. Interaction Model
EVENT ORDERING:
It finally drills down as
(i)...
Fundamental Model
(E). Types of a Fundamental Model
(E).2. Failure Model
It defines the ways in which failure may occur in...
Fundamental Model
(E). Types of a Fundamental Model
(E).2. Failure Model
A SYSTEM MODEL:
OMISSION FAILURE:
Process or chan...
Fundamental Model
(E). Types of a Fundamental Model
(E).2. Failure Model
ARBITRARY FAILURE:
• The term Arbitrary or Byzant...
Fundamental Model
(E). Types of a Fundamental Model
OMISSION & ARBITRARY FAILURES
2/6/2017 36© Copyright Material
Fundamental Model
(E). Types of a Fundamental Model
(E).2. Failure Model
TIMING FAILURE
• Timing failures are applicable i...
Fundamental Model
(E). Types of a Fundamental Model
(E).2. Failure Model
MASKING FAILURE
• A service masks a failure eithe...
Fundamental Model
(E). Types of a Fundamental Model
(E).2. Security Model
We must build a security model which would
• Sec...
Fundamental Model
(E). Types of a Fundamental Model
(E).3. Security Model
• Threat to processes: may receive messages sent...
SUMMARY
• Learnt the necessary components for building a
Distributed System
• Illustrated the layered structure of a distr...
QUIZ
1. What of these key Factors wont effect any System
behavior?
a. Key entities participating in the system
b. Interact...
QUIZ
3. Which model captures the hardware composition of a
system in terms of computers and other devices and
their interc...
QUIZ
6. What are the issues which would affect the performance
of a distributed architecture??
a. Responsiveness b. Throug...
QUIZ
9. The rate at which a computer clock deviates from a
perfect reference clock
a. Twist b. Skew c. Drift d. Deviation
...
QUIZ
12.If A has sent a message to B, C, D & B has replied A’s
message to C, D. Further C replied to A, B, D & finally D
c...
QUIZ
13. If loss of messages between the sending process and the
outgoing message buffer
a. Send-omission failure b. Recei...
KEY
1. D
2. B
3. A
4. C
5. B
6. D
7. A
8. A,B,D
9. C
10. B
11. A
12. C
13. A
14. C
2/6/2017 48© Copyright Material
GLOSSARY
System Model: It is intended to provide an illustrative/descriptive
properties and design choices in a single des...
Cache is a store of recently used data objects that is closer to one
client or a particular set of clients than the object...
Responsiveness: It is higher when accessing locally cached pages &
images for any web browsing client.
Throughput: The met...
Send Omission Failures Occurs when messages are lost between the
sending process and the outgoing message buffer
Receive-o...
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System Models

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System Models

  1. 1. DISTRIBUTED SYSTEMS B. Tech IV/IV, II Semester COMPUTER SCIENCE & ENGINEERING 2/6/2017 © Copyright Material 1
  2. 2. UNIT II 2/6/2017 2 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 © Copyright Material
  3. 3. What we learnt in Unit I 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 2/6/2017 3© Copyright Material
  4. 4. Topic of Contents A. Introduction to a System Model B. Types of System Models Physical, Architectural, Fundamental C. What is an Architectural Model 1. Different Layers in a Distributed System 2. Types of System Architectures 3. Variations in Client-Server model 4. Role of Interfaces & Objects 5. Design requirements for a Distributed System D. What is a Fundamental Model E. Types of Fundamental Models 1. Interaction Models 2. Failure Models 3. Security Model 2/6/2017 4© Copyright Material
  5. 5. Learning Objective Upon completion of this unit, students will be able to: • Understand the necessary components & placement for building a Distributed System • Learn the layered structure of a distributed system software • Illustrate the architectural models to determine locations & interactions of a Distributed system • Practice variants of client-server models • Describe general design requirements for a Distributed System • Understand Interaction, Failure & Security models as the common characteristics of a Fundamental Model 2/6/2017 5© Copyright Material
  6. 6. System Model (A). What is a System Model & why to learn? • Factors that effect any System behavior… – Key entities participating in the system? – Interact between these entities? – Factors that effect individual & Collective behavior? • Learning the above discussed factors, we have understood, what it takes to build a System. • Therefore, A System Model serves as a prototype for a Distributed System Definition • It provides an informal conceptual framework for organizing the discussion of distributed system design goals, issues, and interrelationships & provide some common terminology 2/6/2017 6© Copyright Material
  7. 7. System Model (B). What are the types of System Models Physical models: capture the hardware composition of a system in terms of computers and other devices and their interconnecting Network Architecture models: define the main components of the system, what their roles are and how they interact (software system), and how they are deployed in a underlying network of computers (system architecture) Fundamental models: formal description of the properties that are common to architecture models. Three fundamental models:  Interaction model  Failure model  Security model 2/6/2017 7© Copyright Material
  8. 8. Physical Model (B). 1. What is a Physical Model Evolution of Physical models started with a 1. Baseline physical model: an extensible set of computer nodes interconnected by a computer network for the required passing of messages 2. Early Distributed Systems: 10 to 100 homogeneous interconnected nodes over a LAN with limited Internet connectivity and supported a small range of services 3. Internet-scale Distributed Systems: a large set of heterogeneous nodes interconnected by a network of networks (The Internet). CORBA was one popular middleware of this 1990’s era. 4. Contemporary Distributed Systems: a physical architecture with a significant increase in the level of heterogeneity embracing, example, utilized in ubiquitous computing through to complex computational elements. 5. Distributed Systems of Systems: Example Environmental Management System For Flood Prediction 2/6/2017 8© Copyright Material
  9. 9. Architectural Model (C). What is an Architectural Model • Architectural model is an abstract view of a distributed system • The goal of it is to ensure that the structure will meet present and likely future demands • The placement of process that make up a Distributed System is also influenced by issues like – Performance – Scalability – Security – Cost 2/6/2017 9© Copyright Material
  10. 10. Architectural Model (C). 1. Software Layers of a Distributed System Any process & service oriented view can be expressed in terms of service layers 2/6/2017 10© Copyright Material PLATFORM
  11. 11. Architectural Model (C).1. Software Layers of a Distributed System From the figure in the previous slide, we should know about: PLATFORM: Lowest level hardware & Software layers, which provide services to above layers that are implemented individually in each Computer. EX: PowerPC/MacOS, Intel X86/Linux MIDDLWARE: It is concerned with building useful building blocks for the construction of software components that can work with one another in a Distributed System. 2/6/2017 11© Copyright Material
  12. 12. Architectural Model (C). 2. Types of System Architectures CLIENT – SERVER Client – server architecture could be of different variants, depending on the type of services offered, like 1. Clients invoking individual servers 2. Services provided by multiple servers 3. Proxy servers & Cache 2/6/2017 12© Copyright Material
  13. 13. Architectural Model (C). 2. Types of System Architectures 1. Clients invoking individual servers Client processes interacts with individual server processes in separate host computers to access the shared resource 2/6/2017 13© Copyright Material Server Client Client invocation result Server invocation result Process: Key: Computer:
  14. 14. Architectural Model (C). 2. Types of System Architectures 2. Services provided by multiple servers Partitioning the servers processes into separate host Computers interacting as necessary to provide a service to client process 2/6/2017 14© Copyright Material Server Server Server Service Client Client
  15. 15. Architectural Model (C). 2. Types of System Architectures 3. Proxy servers & Cache Web proxy servers provide a shared cache of web resources for the client machines at a site or across several sites, to increase the availability and performance of the service 2/6/2017 15© Copyright Material Client Proxy Web server Web server server Client
  16. 16. Architectural Model (C). 2. Types of System Architectures DECENTRALIZED SYSTEM ARCHITECUTE PEER-TO-PEER All participating processes run the same program and offer the same set of interfaces to each other. Aims to exploit the resources (H/W, S/W) in a large number of participating computers for the fulfillment of a given task or activity. 2/6/2017 16© Copyright Material Application Application Application Peer 1 Peers 5 .... N SharableObjects Application Peer 4 Peer 2 Peer 3
  17. 17. Architectural Model (C). 3. Variations in Client-Server Model Several variations from the client-server model can be derived from the consideration of the following factors • Use of Mobile Code (ex. Applets) & Mobile agent (ex. Accessing database entries) • Need for low-power computers with limited hardware & simple to manage • Add & remove mobile devices conveniently. 2/6/2017 17© Copyright Material
  18. 18. Architectural Model (C). 3. Variations in Client-Server Model Thin Client: A thin client (sometimes also called a lean, zero or slim client) is a computer or a computer program that depends heavily on some other computer (its server) to fulfill its computational roles 2/6/2017 18© Copyright Material Thin Client Application Process Network computer or PC Compute server network
  19. 19. Architectural Model (C). 3. Variations in Client-Server Model Mobile devices & Spontaneous networking: 2/6/2017 19© Copyright Material Alarm Service Music Service Camera PDA Laptop Hotel Wireless Network Discovery Service TV Gateway Internet
  20. 20. Architectural Model (C). 3. Variations in Client-Server Model Mobile devices & Spontaneous networking: Key Features 1. Easy connection to a local network: Wireless links 2. Easy integration to local processes: Automatic Discovery Design Issues 1. Challenge of supporting convenient connection & integration 2. Mobile users may experience limited connectivity 3. Security & Privacy issues for mobile users 4. A dedicated discovery service running all the time 2/6/2017 20© Copyright Material
  21. 21. Architectural Model (C).4 Role of Interfaces & Objects • The term interface is used to refer to the specification of the procedures offered by a server, defining the types of the arguments of each of the procedures. • The set of functions available for invocation in a process is specified by one or more Interface Definitions • Java RMI & CORBA offers remote object access through Interface Definitions 2/6/2017 21© Copyright Material
  22. 22. Architectural Model (C).5 Design Requirements for Distributed Architecture Various objectives for creation of distributed systems • Sharing of computational resources (e.g. cluster computing) • Sharing of data • Sharing of services Issues which would affect the Performance?? 1. Responsiveness 2. Throughput 3. Load Balancing 2/6/2017 22© Copyright Material
  23. 23. Architectural Model (C).5 Design Requirements for Distributed Architecture Quality of Service (QoS): Non-functional properties of a system that effect QoS are 1. Reliability 2. Security 3. Performance 4. Adaptability: to meet changing system configuration & resource availability 5. Timecritical data: Streams of data that are required to be processed or transferred from one process to another at a fixed rate 2/6/2017 23© Copyright Material
  24. 24. Architectural Model (C).5 Design Requirements for Distributed Architecture Caching and Replication Caching is done to improve the performance & Replication is done to achieve high availability of the System Dependability issues: 1. Correctness: Right results, Always 2. Security: Computations only on right environment 3. Fault Tolerance (Maintainability) : Reliability is achieved through Redundancy 2/6/2017 24© Copyright Material
  25. 25. Fundamental Model (D). What is a Fundamental Model? Formal description of the properties that are common to all architecture models It can also be defined as abstraction of essential properties of a natural phenomenon, for the purpose of understanding and analysis The purpose of such a model is: • To make explicit all the relevant assumptions about the systems we are modeling. • To make generalizations concerning what is possible or impossible 2/6/2017 25© Copyright Material
  26. 26. Fundamental Model (E). Types of a Fundamental Model The aspects of distributed systems that we wish to capture in our fundamental models are Interaction model – processes, messages, coordination (synchronization and ordering) – must reflect that messages are subject to delays, and that delay limits exact coordination and maintenance of global Time Failure model – defines and classifies failures that can occur in a DS – basis for analysis of effects of failures and for design of systems that are able to tolerate failures of each type Security model – defines and classifies security attacks that can occur in a DS – basis for analysis of threats to a system and for design of systems to resist them 2/6/2017 26© Copyright Material
  27. 27. Fundamental Model (E). Types of a Fundamental Model (E).1. Interaction Model Concept of distributed algorithm comprises of: – nodes having a set of data, and runs a program • state of each node is not accessible to any other node – Nodes interchange messages • assume that all activity in the system is driven by message reception (may need to consider system boundaries specially) – Unknown relative speed of nodes which needs to be assumed – Factors affecting performance of network: Latency, Bandwidth, Jitter – Each node having its own internal clock • with specific drift rate - clock synchronization is necessary 2/6/2017 27© Copyright Material
  28. 28. Fundamental Model (E). Types of a Fundamental Model (E).1. Interaction Model There are two variants of an Interaction model • Synchronous and Asynchronous SYNCHRONOUS SYSTEMS: These have a known lower and upper time bound for each execution step, for each message transmission, and for the clock drift Consequence: can introduce a pulsed execution system But, practically difficult to build, and may help in simplifying analysis ASYNCHRONOUS SYSTEMS: • Messages can arrive and be sent at any time and interval Event Ordering?? can usually assume no relative order of reception with reference to sending of messages It is assumed that the messages are sent on an order-preserving channel 2/6/2017 28© Copyright Material
  29. 29. Fundamental Model (E). Types of a Fundamental Model (E).1. Interaction Model EVENT ORDERING: Lets try to see how Logical clocks are used to synchronize clocks on distinct nodes…. 1. Users Ravi, Aarif & James are involved in accepting a Meeting Request 2. Ravi send a meeting request with the subject Meeting 3. Aarif & James reply by sending a message with the subject Re: Meeting How should it work in real time? Ravi sends  Aarif reads & replies James reads Ravi’s request, reads Aarif’s reply & composes a new reply!! But due to independent delay’s the messages might be sent like this…… 2/6/2017 29© Copyright Material
  30. 30. Fundamental Model (E). Types of a Fundamental Model (E).1. Interaction Model EVENT ORDERING: 2/6/2017 30© Copyright Material send receive send receive m1 m2 2 1 3 4 RAVI AARIF JAMES Physical time MEETING m3 receive receive send receive receive receive t1 t2 t3 receive receive m 2 m1
  31. 31. Fundamental Model (E). Types of a Fundamental Model (E).1. Interaction Model EVENT ORDERING: Lamport [1978] proposed a model of logical time that can be used to provide an ordering among the events at processes running in different computers in a distributed system using happened-before relationships 1. happens-before relation: e1 happens before e2, iff 2. e1 is executed by the same process before e2, or 3. e1 is a send operation, and e2 is the corresponding receive operation, or 4. there is an e3 such that e1 happens before e3, and e3 happens before E2 Then, logical time: Assign a number L to each event, such that L(e1) < L(e2) if E1 happens before e2 2/6/2017 31© Copyright Material
  32. 32. Fundamental Model (E). Types of a Fundamental Model (E).1. Interaction Model EVENT ORDERING: It finally drills down as (i) Ravi sends m1 before Aarif receives it (ii) Aarif sends m2 before Ravi receives it (iii)  Aarif receives m1 before sending m2  2/6/2017 32© Copyright Material
  33. 33. Fundamental Model (E). Types of a Fundamental Model (E).2. Failure Model It defines the ways in which failure may occur in order to provide an understanding of the effects of failures Definition of the failure model of a service enables construction of a new service that hides the faulty behavior of the service it builds upon Example: TCP on top of IP – TCP: reliable byte-stream service – IP: unreliable datagram service Specification of failure models requires a way to describe failures One approach is to classify failure types (Cristian, 1991) (Hadzilacos & Toueg, 1994) 1. Omission failures 2. Arbitrary failures 3. Timing failures & Masking Failures 2/6/2017 33© Copyright Material
  34. 34. Fundamental Model (E). Types of a Fundamental Model (E).2. Failure Model A SYSTEM MODEL: OMISSION FAILURE: Process or channels fails to perform an operation – process omission failure (e.g. crash failure) – fail-stop: other processes can detect crash (requires guaranteed delivery of messages) Communication omission failures (message drop) 2/6/2017 34© Copyright Material process p process q Communication channel send Outgoing message buffer Incoming message buffer receivem
  35. 35. Fundamental Model (E). Types of a Fundamental Model (E).2. Failure Model ARBITRARY FAILURE: • The term Arbitrary or Byzantine failure is used to describe the worst possible failure semantics, in which any type of error may occur • An arbitrary failure of a process is one in which it arbitrarily omits intended processing steps or takes unintended processing steps • Arbitrary failures in process cannot be detected by seeing whether the process responds to invocations, because it might arbitrarily omit to reply • Communication channels can suffer from arbitrary failures; – Ex. message contents may be corrupted, nonexistent messages may be delivered or real messages may be delivered more than once • Arbitrary failures of communication channels are rare because the communication software is able to recognize them and reject the faulty messages. Ex. Checksums 2/6/2017 35© Copyright Material
  36. 36. Fundamental Model (E). Types of a Fundamental Model OMISSION & ARBITRARY FAILURES 2/6/2017 36© Copyright Material
  37. 37. Fundamental Model (E). Types of a Fundamental Model (E).2. Failure Model TIMING FAILURE • Timing failures are applicable in synchronous distributed systems where time limits are set on process execution time, message delivery time and clock drift rate. • Asynchronous System: activities may not be completed within pulse • Real-time Systems: Have timing clocks and are tough to design. – Most general-purpose operating systems such as UNIX do not have to meet real- time constraints FEW TIMING FAILURES 2/6/2017 37© Copyright Material
  38. 38. Fundamental Model (E). Types of a Fundamental Model (E).2. Failure Model MASKING FAILURE • A service masks a failure either by hiding it altogether or by converting it into a more acceptable type of failure • A knowledge of the failure characteristics of a component can enable a new service to be designed to mask the failure of the components on which it depends. • Masking failures can be done through Retries & Error checks • Reliability of one-to-one communication is defined in terms of: – Validity (messages are eventually delivered to the receiver) – Integrity (received message identical to sent one, and no message is delivered twice) 2/6/2017 38© Copyright Material
  39. 39. Fundamental Model (E). Types of a Fundamental Model (E).2. Security Model We must build a security model which would • Secure processes and channels against unauthorized access • Protecting objects: access rights given to a principal • Assumption of an enemy (or adversary), capable of (threat model) – Sending messages to any process – reading and copying any message between a pair of processes • Enemy may operate either legitimately-connected node, or illegal node 2/6/2017 39© Copyright Material Network invocation result Client Server Principal (user) Principal (server) ObjectAccess rights Communication channel Copy of m Process P Process Qm The enemy m’
  40. 40. Fundamental Model (E). Types of a Fundamental Model (E).3. Security Model • Threat to processes: may receive messages sent by enemy – may not be able to reliably determine identity of sender – server: may not be able to identify principal – client: may fall to "spoofing“ • Threats to communication channels: enemy may – copy, alter, inject, or delete messages – gain information only intended for the communication partner • Other threats: Denial Of Service, Trojan Horses, Worms • Defeating security threats: Cryptography, Authentication, Secure Channels 2/6/2017 40© Copyright Material Secure channelProcess P Process Q
  41. 41. SUMMARY • Learnt the necessary components for building a Distributed System • Illustrated the layered structure of a distributed system software • Understood the architectural models to determine locations & interactions in a Distributed system • Emphasized variants of client-server models • Practiced the general design requirements for a Distributed System • Learnt Interaction, Failure & Security models as the common characteristics of a Fundamental Model 2/6/2017 41© Copyright Material
  42. 42. QUIZ 1. What of these key Factors wont effect any System behavior? a. Key entities participating in the system b. Interact between these entities c. Factors that effect individual & Collective behavior d. None of these 2. Which of the following is not a System Model a. Architectural Model b. Security Model c. Interaction Model d. Fundamental Model 2/6/2017 42© Copyright Material
  43. 43. QUIZ 3. Which model captures the hardware composition of a system in terms of computers and other devices and their interconnecting network a. Physical b. Informational c. Fundamental d. Interaction 4. NTP is an example of which type of service a. Application service b. Operating system service c. Distributed Service d. Network Service 5. Client Server model scales poorly, so we could use a. Thin Client b. Peer-to-Peer c. Proxy Server d. None 2/6/2017 43© Copyright Material
  44. 44. QUIZ 6. What are the issues which would affect the performance of a distributed architecture?? a. Responsiveness b. Throughput c. Load Balancing d. All 7. Which of the following approaches achieves high availability a. Replication b. Caching c. Redundancy d. Customer Support 8. The aspects which is/are covered in a Fundamental model are a. Interaction Model b. Architectural Model c. Failure Model d. Security Model 2/6/2017 44© Copyright Material
  45. 45. QUIZ 9. The rate at which a computer clock deviates from a perfect reference clock a. Twist b. Skew c. Drift d. Deviation 10. Logical clock is used to achieve synchronization in which form of Interaction model a. Synchronous b. Asynchronous c. Fundamental d. A&B 11. Lamport proposed a model of logical time that can be used to provide a. Event Ordering b. Clock Synchronization c. Failure Analysis d. Protecting access rights 2/6/2017 45© Copyright Material
  46. 46. QUIZ 12.If A has sent a message to B, C, D & B has replied A’s message to C, D. Further C replied to A, B, D & finally D concluded by responding to all requests. Which of the following situation is false according to happened-before relationship? a. C replies to B only when C gets message from B b. A gets message from B only when B gets message from A c. B replies to C before A sends the message to B d. D replies to all only when C replies sends the reply to B 2/6/2017 46© Copyright Material
  47. 47. QUIZ 13. If loss of messages between the sending process and the outgoing message buffer a. Send-omission failure b. Receive-omission failure c. Channel-omission failure d. None 14. Which of the following means would ensure data protection when the data is in transit a. Cryptography b. Authentication c. Secure Channel d. Confidentiality 2/6/2017 47© Copyright Material
  48. 48. KEY 1. D 2. B 3. A 4. C 5. B 6. D 7. A 8. A,B,D 9. C 10. B 11. A 12. C 13. A 14. C 2/6/2017 48© Copyright Material
  49. 49. GLOSSARY System Model: It is intended to provide an illustrative/descriptive properties and design choices in a single descriptive model for building a distributed system Physical models: capture the hardware composition of a system in terms of computers and other devices and their interconnecting network Distributed Systems of Systems is the emergence of Ultra-large-scale (ULS) distributed systems used to capture the complexity of modern distributed systems by referring to such (physical) architectures as systems of systems System Of Systems can be defined as a complex system consisting of a series of subsystems that are systems in their own right and that come together to perform a particular task or tasks. Distributed Service: It can be provided by one or more server processes interacting with each other, and with client processes in order to maintain a consistent system-wide view of service’s resources. 2/6/2017 49© Copyright Material
  50. 50. Cache is a store of recently used data objects that is closer to one client or a particular set of clients than the objects themselves Mobile Code: It is used to refer to program code that can be transferred from one computer to another and run at the destination Mobile agents: A mobile agent is a running program (including both code and data) that travels from one computer to another in a network carrying out a task on someone’s behalf, such as collecting information, and eventually returning with the results Mobile devices are small & portable computing devices including laptops, handheld devices like PDAs, Smart Phones, digital cameras, smart watches, smart spectacles (Google glass), and devices embedded in everyday appliances such as washing machines Spontaneous Network The form of distribution that integrates mobile devices & other devices into a given network 2/6/2017 50© Copyright Material GLOSSARY
  51. 51. Responsiveness: It is higher when accessing locally cached pages & images for any web browsing client. Throughput: The metric to measure the rate at which the computational work is done. It is also affected due to the intervening software layers apart from the network Load Balancing: It would involve distributed the workload across the networked computational devices and may also involve moving partially completed work as the loads on hosts change. Distributed Algorithm : The steps to be taken by each of the processes of which the system is composed, including the transmission of messages between them. Latency: The time taken for the first of a string of bits transmitted through a network to reach its destination. 2/6/2017 51© Copyright Material GLOSSARY
  52. 52. Send Omission Failures Occurs when messages are lost between the sending process and the outgoing message buffer Receive-omission Failures Occurs when messages are lost between the incoming message buffer and the receiving process Channel Omission Failures Occurs when messages are lost in transit Security model deals with describing the system architecture in terms of processes, potentially encapsulating higher-level abstractions such as objects, components or services, and providing access to them through interactions with other processes Access Rights specify who is allowed to perform the operations of an object – for example, who is allowed to read or to write its state. 2/6/2017 52© Copyright Material GLOSSARY

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