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Questions and Answers
What is the primary challenge of clock synchronization in a Distributed System?
What is the primary challenge of clock synchronization in a Distributed System?
Which of the following is an example of a relative time synchronization application?
Which of the following is an example of a relative time synchronization application?
What is Cristian's Algorithm primarily used for?
What is Cristian's Algorithm primarily used for?
What kind of accuracy does the Internet's Network Time Protocol (NTP) achieve in synchronization?
What kind of accuracy does the Internet's Network Time Protocol (NTP) achieve in synchronization?
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Which statement about centralized algorithms for clock synchronization is accurate?
Which statement about centralized algorithms for clock synchronization is accurate?
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What distinguishes a physical clock from a logical clock in synchronization?
What distinguishes a physical clock from a logical clock in synchronization?
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Why are leap seconds introduced in time measurement?
Why are leap seconds introduced in time measurement?
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Which of the following statements about the Berkeley Algorithm is correct?
Which of the following statements about the Berkeley Algorithm is correct?
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What is the primary feature of decentralized clock synchronization algorithms?
What is the primary feature of decentralized clock synchronization algorithms?
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Which factor complicates time measurement in a distributed system?
Which factor complicates time measurement in a distributed system?
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What does Θ0 indicate in the context of network time protocol?
What does Θ0 indicate in the context of network time protocol?
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Which statement summarizes the concept of logical clocks in distributed systems?
Which statement summarizes the concept of logical clocks in distributed systems?
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According to Lamport's logical clocks, which condition must be true for the 'happens-before' relation?
According to Lamport's logical clocks, which condition must be true for the 'happens-before' relation?
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What issue does the problem of totally-ordered multicasting address?
What issue does the problem of totally-ordered multicasting address?
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In the scenario of updating a replicated database, what might result from incorrect ordering of updates?
In the scenario of updating a replicated database, what might result from incorrect ordering of updates?
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What role do vector clocks play in distributed systems?
What role do vector clocks play in distributed systems?
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What is the primary significance of the 'happens-before' relation?
What is the primary significance of the 'happens-before' relation?
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What happens if the clock rates of different processes in Lamport’s logical clocks vary?
What happens if the clock rates of different processes in Lamport’s logical clocks vary?
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Which of the following best describes how Lamport's algorithm corrects clocks?
Which of the following best describes how Lamport's algorithm corrects clocks?
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What is one key limitation of logical clocks compared to physical clocks?
What is one key limitation of logical clocks compared to physical clocks?
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Study Notes
Distributed Operating Systems - Synchronization (Chapter 11)
- Synchronization in distributed systems is based on "actual time".
- Uniprocessor systems find time synchronization straightforward.
- In distributed systems, agreement on time is complex.
- Clock skew is a crucial factor; multiple computers don't have the same "current time".
- How to measure time accurately in a distributed environment?
- Centralized systems, while time is potentially ambiguous, consecutive time calls are ordered reasonably.
Clock Synchronization
- In a centralized system, time is ambiguous but still relatively ordered.
- Many systems, like bank transactions, require precise timing.
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Synchronization Techniques:
- Absolute: Required for real-time applications (e.g., online reservations).
- Relative: Needed for consistent time across all nodes (e.g., applications needing a consistent view of time).
How Do We Measure Time?
- Time is accurately measured with a global atomic clock (not local clocks).
- Algorithms, based on physical clocks, are devised for use in distributed systems.
Physical Clocks (2)
- TAI (International Atomic Time) seconds are constant length.
- Solar seconds vary; leap seconds are used to keep UTC (the civilian time standard), synchronized with the Earth's rotation.
Clock Synchronization - Cristian's Algorithm (Passive)
- Cristian's algorithm obtains time from a time server.
- Uses periodic client requests to get the current time.
- Client time = server time + (t1 - t0) / 2
- t1 = client's local time when sending request
- t0 = local time when getting back server's time
Clock Synchronization - Berkeley Algorithm (Active)
- A time daemon (a centralized process) requests clock values from other machines.
- It averages the responses and sets the clocks on participating computers.
Other Clock Synchronization Algorithms
- Cristian's and Berkeley algorithms are centralized.
- Decentralized algorithms also exist, and Network Time Protocol (NTP) is commonly used.
- NTP can synchronize clocks to within 1–50 milliseconds (msec) of accuracy.
Network Time Protocol (NTP)
- A's offset relative to B is calculated.
- Offset is expressed in seconds (positive for A faster, negative for A slower).
- Equation to calculate offset: 0 = (T3 - T4) + [(T2 – T1) + (T4 – T3)] / 2
Logical Clocks
- Synchronization based on relative time.
- There's no requirement for relation to real time.
- The focus is on agreeing on the ordering of events in a distributed system.
Lamport's Logical Clocks (1)
- The "happens-before" relation:
- An event "a" happens before event "b" if they occur in the same process, or transmission of a message.
Lamport's Logical Clocks (2)
- Three processes (P1, P2, P3) each have a local logical clock.
- Clocks run independently, providing local timestamps.
Lamport's Logical Clocks (3)
- Lamport's algorithm adjusts logical clock values to ensure consistent ordering of events.
Problem: Totally-Ordered Multicasting
- Replicated databases face inconsistency due to network delays in updating data.
- Updates may arrive in a different order on different replicas.
Vector Clocks (1)
- Vector clocks track the possible causal dependencies between events by different processes.
- Allows distributed systems to correctly order events even if they are not chronologically ordered on all processes.
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Description
This quiz explores key concepts from Chapter 11 on synchronization in distributed operating systems. It covers challenges in time synchronization, the impact of clock skew, and the differences between absolute and relative synchronization techniques. Test your understanding of how time is measured in these complex environments.
