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![Interaction Model [Synchronous vs
Asynchronous]](https://image.slidesharecdn.com/2-190924094753/75/Distributed-Systems-Introduction-and-Importance-79-2048.jpg)
![BSS Algorithm ...
1. Process Pi increments the vector time VTpi[i], time stamps,
and broadcasts the message m. VTpi[i] - 1 denotes the number
of messages preceding m.
2. Pj != Pi receives m. m is delivered when:
a. VTpj[i] == VTm[i] – 1 [Pj has received all messages from Pi before m]
b. VTpj[k] >= VTm[k] for all k in {1,2,..n} - {i}, n is the
total number of processes. Delayed message are queued
in a sorted manner. [Pj has received all those messages received by Pi before m]
c. Concurrent messages are ordered by time of receipt.
3. When m is delivered at Pj, VTpj updated according Rule 2 of
vector clocks.
2(a) : Pj has received all Pi’s messages preceding m.
2(b): Pj has received all other messages received by Pi
before sending m.](https://image.slidesharecdn.com/2-190924094753/75/Distributed-Systems-Introduction-and-Importance-154-2048.jpg)
![• All messages are time stamped by the sending process.
[Note: This time is separate from the global time talked about in
the previous sections. Instead each element of the vector
corresponds to the number of messages sent (including this one)
to other processes.]
• A message can not be delivered until:
– All the messages before this one have been delivered locally.
– All the other messages that have been sent out from the
original processs has been accounted as delivered at the
receiving process.
• When a message is delivered, the clock is updated.
• This protocol requires that the processes communicate through
broadcast messages since this would ensure that only one
message could be received at any one time (thus concurrently
time stamped messages can be ordered).](https://image.slidesharecdn.com/2-190924094753/75/Distributed-Systems-Introduction-and-Importance-155-2048.jpg)
![Interaction Model [Synchronous vs
Asynchronous]](https://crownmelresort.com/image.slidesharecdn.com/2-190924094753/75/Distributed-Systems-Introduction-and-Importance-79-2048.jpg)
![BSS Algorithm ...
1. Process Pi increments the vector time VTpi[i], time stamps,
and broadcasts the message m. VTpi[i] - 1 denotes the number
of messages preceding m.
2. Pj != Pi receives m. m is delivered when:
a. VTpj[i] == VTm[i] – 1 [Pj has received all messages from Pi before m]
b. VTpj[k] >= VTm[k] for all k in {1,2,..n} - {i}, n is the
total number of processes. Delayed message are queued
in a sorted manner. [Pj has received all those messages received by Pi before m]
c. Concurrent messages are ordered by time of receipt.
3. When m is delivered at Pj, VTpj updated according Rule 2 of
vector clocks.
2(a) : Pj has received all Pi’s messages preceding m.
2(b): Pj has received all other messages received by Pi
before sending m.](https://crownmelresort.com/image.slidesharecdn.com/2-190924094753/75/Distributed-Systems-Introduction-and-Importance-154-2048.jpg)
![• All messages are time stamped by the sending process.
[Note: This time is separate from the global time talked about in
the previous sections. Instead each element of the vector
corresponds to the number of messages sent (including this one)
to other processes.]
• A message can not be delivered until:
– All the messages before this one have been delivered locally.
– All the other messages that have been sent out from the
original processs has been accounted as delivered at the
receiving process.
• When a message is delivered, the clock is updated.
• This protocol requires that the processes communicate through
broadcast messages since this would ensure that only one
message could be received at any one time (thus concurrently
time stamped messages can be ordered).](https://crownmelresort.com/image.slidesharecdn.com/2-190924094753/75/Distributed-Systems-Introduction-and-Importance-155-2048.jpg)
Uploaded bySHIKHA GAUTAM
Distributed Systems Introduction and Importance
The document provides a comprehensive overview of distributed systems, covering their characterization, structural models (client-server and peer-to-peer), theoretical foundations including limitations and logical clocks, and challenges such as security and transparency. It discusses various factors affecting system reliability, performance, and interaction models while highlighting essential architectural considerations. Key concepts include message passing, handling of concurrency, and strategies for ensuring robustness in real-world applications, including fault tolerance and security mechanisms.
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Distributed Systems Introduction and Importance
- 1. Distributed Systems Shikha Gautam AssistantProfessor KIET, Ghaziabad
- 2. UNIT-1 • Characterization ofDistributed Systems: Introduction, Examples of distributed Systems, Resource sharing and the Web Challenges. Architectural models, Fundamental Models. • Theoretical Foundation for Distributed System: Limitation of Distributed system, absence of global clock, shared memory, Logical clocks ,Lamport’s & vectors logical clocks. • Concepts in Message Passing System.
- 4. What is adistributed system? • A network of autonomous computers that communicate by message passing to achieve a common goal. • Consequences: – Concurrency – No global clock – Independent failures • Motivation – Resource sharing
- 5.
- 10. Why we useDistributed Systems? What are the advantages?
- 11.
- 12. Examples Of DistributedSystems
- 19. Resource Sharing andthe Web
- 26. Design Challenges ofD.S. • Heterogeneity • Openness • Security • Scalability • Failure handling • Concurrency • Transparency
- 28.
- 29.
- 32.
- 37.
- 38.
- 39.
- 41. • Systems thatare intended for use in real-world environments should be designed to function correctly in the widest possible range of circumstances and in the face of many possible difficulties and threats. • Different system models are: System Model
- 42. 1. Architectural models:Based on the architectural style, e.g., classifying the processes as server, client, and peer. 2. Fundamental model: Based on the some fundamental properties, such as characteristics, failures, and security.
- 43.
- 44. Architectural models • Architecture:structure in terms of separately specified components and how these components are placed. • Overall goal: structure will meet present and likely future demands • Major concerns: make system – Reliable – Manageable – Adaptable – Cost-effective
- 45. • Architectural model –Simplifies & abstracts functions of components – Placement of components – Interrelationships between components • Overview – Software layers – System architectures – Design requirements Architectural models
- 46. Architectural models: Software layers Applications,serv ices Computer and network hardware Platf orm Operating system Middleware
- 47. • Platform – Variousimplementations – Provides communication & cooperation between processes Architectural models: Software layers
- 48. Architectural models: Software layers •Middleware – Purpose • Mask heterogeneity • Provide convenient programming model – Raises level of communication activities • Remote method invocation: RMI, CORBA, DCOM • Group communication • Notification of events • Partitioning, replication of shared data – Provides infrastructural services • Naming, transactions, persistent storage
- 49. Architectural models: Software layers •Middleware limitations : end-to-end argument – Some aspects require support at application level
- 50. Architectural models • Architecturalmodel – Simplifies & abstracts functions of components – Placement of components – Interrelationships between components • Overview – Software layers – System architectures – Design requirements
- 51.
- 52. Various Architectural models 1.Client-server model 2. Peer-to-peer Variations of the above two: • Proxy server • Mobile code • Mobile agents • Network computers • Thin clients • Mobile devices
- 53. Architectural models: 1. Client-ServerModel • Defines roles for two interacting entities 1. Client: needs a particular service sends request to server and gets reply 2. Server: awaits requests from clients performs requested function • The client-server model is usually based on a simple request/reply protocol, implemented with send/receive primitives or using remote procedure calls (RPC) or remote method invocation (RMI). • Server can be client of another server.
- 54. Client-Server Model Serv er Client Client invocation result Serv erinv ocation result Process: Key: Computer:
- 58. Architectural models: 2. Peer-to-PeerModel All processes (objects) play similar role. • Processes interact without particular distinction between clients and servers. • The pattern of communication depends on the particular application. • A large number of data objects are shared; any individual computer holds only a small part of the application database. • Processing and communication loads for access to objects are distributed across many computers and access links. • This is the most general and flexible model.
- 59.
- 61. Proxy servers andcaches Client Proxy Web server Web server server Client + Reduce load on network & web servers - Consistency!
- 63. Mobile code – Goodinteractive response – Potential security threat a) client request results in the downloading of applet code Web server Client Web serverApplet Applet code Client b) client interacts with the applet
- 68. Architectural models • Architecturalmodel – Simplifies & abstracts functions of components – Placement of components – Interrelationships between components • Overview – Software layers – System architectures – Design requirements
- 69.
- 70. Architectural models: Design requirements •Minimal requirement: – maintain functionality of a non-distributed system • added value: – extended resource access – extended application interface for explicit sharing, fault tolerance, etc. – advanced end user applications: CSCW (computer supported cooperative work) • Quality of Service: – Reliability – Security – Performance – Adaptability
- 71. User Requirements Quality ofservice • Reliability and availability – reliability = measure of the likelihood of the system to deviate from the designed behaviour – increased by enabling failure detection and recovery – highly reliable services often worse response – fault tolerant system: detects failures and either • fails gracefully (predictably) • masks the fault
- 72. User Requirements Quality ofservice • Security: new problems – privacy and integrity of users data in network packets • by tampering the network cable • by connecting a machine to read and/or inject data packets – openness to interface with system software • not all machines are physically secure
- 73. User Requirements Quality ofservice • Performance – Responsiveness – Throughput • Processing speed at clients & servers + data transfer rate – Balancing computational load
- 74.
- 75. Fundamental Models • Systemmodel gives answers to – What are the main entities in the system? – How do they interact? – What are characteristics that affect individual & collective behavior? • Purpose of model: – Make explicit all relevant assumptions – Make generalizations concerning what is possible or impossible
- 76. Fundamental models Three fundamentalmodels: 1. Interaction Model 2. Failure Model 3. Security Model
- 77.
- 78. Performance characteristics of communicationchannels 1. Latency: The delay time. The start of message transmission from one process and the beginning of its receipt by another process. 2. Bandwidth: Total amount of information that can be transmitted over a communication channel per second. 3. Jitter: variation in the time taken to deliver a series of messages.
- 79. Interaction Model [Synchronousvs Asynchronous]
- 81. Fundamental models: Interaction model •Time is important – E.g. multimedia application requires timeliness – E.g. Event ordering problem in email Inbox Item From Subject 1 Z Re:Meeting 2 X Meeting 3 Y Re:Meeting
- 82. Fundamental models: Interaction model •No global notion of time • Synchronization of time impossible due to: – Performance variations: • Latency • Bandwidth • Processing time for messages – Computers have different clock drift rates
- 83. Fundamental models: Interaction model •Synchronous distributed systems – Upper & lower bounds for • Time to execute processing step • Message transmission • Clock drift rate – Allow • Use of timeouts to detect process failure • Guarantee of timeliness (multimedia) • Partial clock synchronisation
- 84. Fundamental models: Interaction model •Asynchronous distributed systems – No time bounds – Many systems are asynchronous • E.g. Internet • Due to sharing of processors & communication channels • Often offer the best performance (because no resources are wasted) – Consequences: • Clock synchronization impossible • No guarantee of timeliness possible
- 85. Fundamental models: Interaction model •Solution to ordering problem –With (perfect) clock synchronization no problem –In asynchronous model • Facts: – Ordering possible within a single process – Send m before receive m Event ordering possible • Implementation: logical clocks
- 87. Fundamental models • Aspectscaptured in models: – Interaction – Failure – Security
- 88.
- 90. Fundamental models: Failure model Classof failure Affects Description Fail-stop Process Process halts and remains halted. Other processes may detect this state. Crash Process Process halts and remains halted. Other processes may not be able to detect this state. Omission Channel A message inserted in an outgoing message buffer never arrives at the other end’s incoming message buffer. Send-omission Process A process completes a send,but the message is not put in its outgoing message buffer. Receive-omissionProcess A message is put in a process’s incoming message buffer, but that process does not receive it. Arbitrary (Byzantine) Process or channel Process/channel exhibits arbitrary behaviour: it may send/transmit arbitrary messages at arbitrary times, commit omissions; a process may stop or take an incorrect step.
- 91. Fundamental models: Failure model •Timing failures – Applicable in synchronous systems Class of Failure Affects Description Clock Process Process’s local clock exceeds the bounds on its rate of drift from real time. Performance Process Process exceeds the bounds on the interval between two steps. Performance Channel A message’s transmission takes longer than the stated bound.
- 92. Fundamental models: Failure model •How can distributed systems fail? – Partial failure of • processes and communication channels • Kind of failure: • Omission • Arbitrary • Timing
- 93. Fundamental models: Failure model •Omission failure = Failure to perform an action a) Processes: • Subclasses: – Crash no further execution – Fail-stop crash + detection possible • Consequences for asynchronous systems – Failure not detectable – Reaching agreement impossible
- 94. Fundamental models: Failure model •Omission failure – Communication: • Send-omission • Receive-omission • Channel-omission processp process q Communication channel send Outgoing m essage buff er Incoming message buff er receivem
- 95. Fundamental models: Failure model •Arbitrary or Byzantine failures: = Worst possible failure semantics • Any behavior possible – Processes: • Omit processing steps • Perform unintended steps – Communication • Message contents corrupted • Non-existing message delivered • Messages delivered twice • Rare: checksums, sequence numbers
- 96. Fundamental models: Failure model Examples: •Checksums: corrupted message omission failure • Retransmission of message: hide omission failure
- 97. Fundamental models • Aspectscaptured in models: – Interaction – Failure – Security
- 98. Fundamental models: Security model •Avoid unauthorized use of resources. • Secure processes and interactions. • Threat to processes & communication channels. Communication channel Copy of m Process p Process qm The enemy m’
- 99.
- 100. Protecting objects • Protectingobjects/resources by – giving access rights to users – associating with each invocation an authority (a user with access rights) who allows for the use of the object or asked for it e.g. user asks a remote process to print something on his printer,the authority here is the user. • Server checks identity of authority and checks its access rights • Works only if communication is secure.
- 101. Fundamental models: Security model •Securing processes and interactions – Threats to processes • False identification of sender of message – Threats to communication channels • Copy, alter, inject messages – Denial of service • Overload resource (channel, processor)
- 102. Fundamental models: Security model •Defeating security threats – Cryptography – Shared secrets Authentication Secure channels Principal A Secure channelProcess p Process q Principal B
- 103. Limitation of Distributedsystems
- 104. Limitation of Distributedsystems – A DS is a collection of computers that are spatially separated and do not share a common memory – Processes communicate by exchanging messages over communication channel – Messages are delivered after an arbitrary transmission delay – DS suffers some inherent limitations because of lack of common memory and a system wide common clock – How these limitations can be overcome?
- 105. 1. Absence ofglobal clock • There is no system-wide common clock in a DS. • Solutions can be: – Either having a global clock common to al the computers, or – Having synchronized clocks, one at each computer • Both of the above solutions are impractical due to following reasons: – If one global clock is provided in the DS: • Two processes will observe a global clock value at different instants due to unpredictable delays • Two processes will falsely perceive two different instants in physical time to be a single instant in physical time – if the clocks of different systems are tried to synchronize: • These clocks can drift from the physical time and the drift rate may vary from clock to clock due to technological limitations • This may also end up with the same result • We cannot have a system of perfectly synchronized clocks
- 106. Impact of theabsence of global time Temporal ordering of events is integral to the design and development of DS. – an OS is responsible for scheduling processes – A basic criterion used in scheduling is the temporal order in which requests to execute processes arrive – Hence, algorithms for DS are more difficult to design and debug – Also, the up-to-date state of the system is harder to collect
- 107. • Due tothe lack of shared memory, an up-to-date state of the entire system is not available to any individual process • It is necessary for reasoning about the system’s behavior, debugging and recovery • A process in a DS can obtain a coherent but partial view or a complete but incoherent view of the system • A view is said to be coherent if all the observations of different processes are made at the same physical time • A complete view includes the local views (local states) at all the computers and any messages that are in transit in the DS • A complete view is also referred to as global state. 2. Absence of Shared Memory
- 108. Need of SynchronizedClocks
- 109. Every computer isequipped with CMOS clock circuit. These are electronic devices that count oscillations occurring in a crystal. • Also called timer, usually a quartz crystal, oscillating at a well defined frequency. • Timer is associated with two registers: A Counter and a Holding Register, counter decreasing one at each oscillations. • When counter reaches zero, an interrupt is generated; this is the clock tick. • Clock tick have a frequency of 60-100 ticks per second. Physical Clocks
- 110. It is impossibleto guarantee that crystals in different computers all run at exactly the same frequency. This difference in time values is clock skew.
- 111.
- 112. Why do weneed global clock
- 113. Lack of GlobalTime in DS
- 114. When each machinehas its own clock, an event that occurred after another event may nevertheless be assigned an earlier time. Lack of Global Time in DS (Example)
- 117. a) Each processeswith own clock with different rates. b) Lamport's algorithm corrects the clocks. c) Can add machine ID to break ties
- 118. For many purposesit is sufficient that all machines agree on the same time. Logical clocks Often processes need to agree on the order in which events occur.
- 119. Logical Clocks • Fora certain class of algorithms, it is the internal consistency of the clocks that matters. The convention in these algorithms is to speak of logical clocks. Lamport showed clock synchronization need not be absolute. What is important is that all processes agree on the order in which events occur.
- 121.
- 126. If a happensbefore b, then if any changes happens in a that will definitely affect b. • In general, an event changes the system state, which in turn influences the occurrence and outcome of future events • past events influence future events and this influence among causally related events (those events that can be ordered by “ “ ) is referred to as causal affects.
- 128. Casually Related VsConcurrent Events
- 133. Lamport’s algorithm correctsthe clocks.
- 141. Vector clock: Inshort
- 142.
- 144.
- 149. • The purposeof causal ordering of messages is to insure that the same causal relationship for the "message send" events correspond with "message receive" events. - i.e. All the messages are processed in order that they were created.
- 151. Two protocols thatmakes use of vector clocks for the causal ordering of messages in distributed systems. 1. Birman-Schiper-Stephenson Protocol -Processes are assumed to communicate using broadcast messages. 2. Schiper-Eggli-Sendoz Protocol – Does not require processes to communicate only through broadcast messages.
- 153. BSS Algorithm • BSS:Birman-Schiper-Stephenson Protocol • Broadcast based: a message sent is received by all other processes. • Deliver a message to a process only if the message preceding it immediately, has been delivered to the process. • Otherwise, buffer the message. • Accomplished by using a vector accompanying the message. Processes are assumed to communicate using broadcast messages.
- 154. BSS Algorithm ... 1.Process Pi increments the vector time VTpi[i], time stamps, and broadcasts the message m. VTpi[i] - 1 denotes the number of messages preceding m. 2. Pj != Pi receives m. m is delivered when: a. VTpj[i] == VTm[i] – 1 [Pj has received all messages from Pi before m] b. VTpj[k] >= VTm[k] for all k in {1,2,..n} - {i}, n is the total number of processes. Delayed message are queued in a sorted manner. [Pj has received all those messages received by Pi before m] c. Concurrent messages are ordered by time of receipt. 3. When m is delivered at Pj, VTpj updated according Rule 2 of vector clocks. 2(a) : Pj has received all Pi’s messages preceding m. 2(b): Pj has received all other messages received by Pi before sending m.
- 155. • All messagesare time stamped by the sending process. [Note: This time is separate from the global time talked about in the previous sections. Instead each element of the vector corresponds to the number of messages sent (including this one) to other processes.] • A message can not be delivered until: – All the messages before this one have been delivered locally. – All the other messages that have been sent out from the original processs has been accounted as delivered at the receiving process. • When a message is delivered, the clock is updated. • This protocol requires that the processes communicate through broadcast messages since this would ensure that only one message could be received at any one time (thus concurrently time stamped messages can be ordered).
- 158. SES Algorithm • Noneed for broadcast messages. • Each process maintains a vector V_P of size N - 1, N the number of processes in the system. • V_P is a vector of tuple (P’,t): P’ the destination process id and t, a vector timestamp. • Tm: logical time of sending message m • Tpi: present logical time at pi • Initially, V_P is empty.
- 160.
- 161. $500 Communication ChannelS1:A S2:A $200(a) $450 CommunicationChannelS1:A S2:A $200(b) $500 Communication ChannelS1:A S2:A $250 (c) Global State
- 163. The ability toextract and reason about the global state of a distributed application has several other important applications: • distributed deadlock detection • distributed termination detection • distributed debugging Q: Is it possible to assemble a global state from local states in the absence of a global clock?
- 173.