Distributed Operating Systems - Synchronization Chapter 11

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Questions and Answers

What is the primary challenge of clock synchronization in a Distributed System?

  • Synchronizing time with a global atomic clock
  • Maintaining an accurate physical clock
  • Achieving agreement on time across multiple computers (correct)
  • Eliminating leap seconds from time measurement

Which of the following is an example of a relative time synchronization application?

  • Air traffic control systems
  • Online reservation systems
  • Banking transaction systems
  • File sharing applications (correct)

What is Cristian's Algorithm primarily used for?

  • Measuring time accurately with an atomic clock
  • Synchronizing clocks in a decentralized environment
  • Obtaining current time from a time server (correct)
  • Ordering transactions in a centralized system

What kind of accuracy does the Internet's Network Time Protocol (NTP) achieve in synchronization?

<p>Within 1-50 msec (A)</p> Signup and view all the answers

Which statement about centralized algorithms for clock synchronization is accurate?

<p>They require periodic client requests for synchronization. (B)</p> Signup and view all the answers

What distinguishes a physical clock from a logical clock in synchronization?

<p>Logical clocks are based on the notion of time intervals rather than real time. (A)</p> Signup and view all the answers

Why are leap seconds introduced in time measurement?

<p>To account for discrepancies in solar time (B)</p> Signup and view all the answers

Which of the following statements about the Berkeley Algorithm is correct?

<p>It actively requests clock values from other machines. (D)</p> Signup and view all the answers

What is the primary feature of decentralized clock synchronization algorithms?

<p>They do not require a central time server. (C)</p> Signup and view all the answers

Which factor complicates time measurement in a distributed system?

<p>Clock skew prevents uniform time across machines. (A)</p> Signup and view all the answers

What does Θ0 indicate in the context of network time protocol?

<p>A's time is equal to B's time (D)</p> Signup and view all the answers

Which statement summarizes the concept of logical clocks in distributed systems?

<p>They agree on the ordering of events without referencing real-time. (A)</p> Signup and view all the answers

According to Lamport's logical clocks, which condition must be true for the 'happens-before' relation?

<p>If two events occur in the same process, the first must precede the second. (C)</p> Signup and view all the answers

What issue does the problem of totally-ordered multicasting address?

<p>Updating replicated databases without inconsistencies. (B)</p> Signup and view all the answers

In the scenario of updating a replicated database, what might result from incorrect ordering of updates?

<p>Potential financial discrepancies in account balances may occur. (C)</p> Signup and view all the answers

What role do vector clocks play in distributed systems?

<p>They provide a means to compare relative times of events. (D)</p> Signup and view all the answers

What is the primary significance of the 'happens-before' relation?

<p>It allows understanding of causality between events. (C)</p> Signup and view all the answers

What happens if the clock rates of different processes in Lamport’s logical clocks vary?

<p>The logical time can still be maintained effectively. (B)</p> Signup and view all the answers

Which of the following best describes how Lamport's algorithm corrects clocks?

<p>It adjusts their timestamps according to message exchanges. (C)</p> Signup and view all the answers

What is one key limitation of logical clocks compared to physical clocks?

<p>Logical clocks do not measure actual elapsed time. (B)</p> Signup and view all the answers

Flashcards

Network Time Protocol (NTP)

A time synchronization protocol used in networks where each device maintains its own local clock. It involves exchanging timestamps between devices to minimize clock drift.

Logical Clocks

A mechanism for ordering events in a distributed system, where each process keeps track of the order in which events happen, but doesn't necessarily rely on absolute time.

Lamport's Logical Clocks

A way to ensure that events in a distributed system are ordered consistently by assigning unique timestamps to them. It defines a "happens-before" relationship between events.

Totally-Ordered Multicasting

A situation in a distributed system where multiple updates to a replicated database can arrive in different orders at different nodes, leading to inconsistency.

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Vector Clocks

A technique for ensuring consistency in distributed systems by using a vector of timestamps for each event, reflecting the sequence of events in different processes.

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Happens-before relationship (within a process)

In Lamport's Logical Clocks, an event a happens before event b in the same process, if a occurs before b.

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Happens-before relationship (message sending)

In Lamport's Logical Clocks, an event a happens before event b if a is the event of sending a message and b is the event of receiving that message.

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Lamport's Logical Clocks (algorithm)

In Lamport's Logical Clocks, the algorithm assigns logical timestamps to events, ensuring that events in the same process have increasing timestamps and timestamps increase after a message is sent.

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Clock Skew

In a single-processor system, time is straightforward. But in a distributed system with multiple computers, "clock skew" means no two machines have identical 'current time'.

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Centralized Time

A centralized system can have a single source of time, even if that time isn't perfectly accurate. The order of events is maintained. For example, a bank system relies on a single time source for consistent transaction ordering.

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Clock Synchronization

The process of aligning clocks in a distributed system to ensure consistent time across all nodes.

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Physical Clocks

Physical clock synchronization involves aligning clocks to an external reference time, often from a physical clock like an atomic clock. This provides absolute time.

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Cristian's Algorithm

Cristian's Algorithm is a passive method for clock synchronization. A client periodically requests the current time from a time server and adjusts its clock based on the round-trip time.

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Berkeley Algorithm

The Berkeley Algorithm is an active method for clock synchronization. A time daemon periodically polls all machines for their clock values and calculates an average time, then broadcasts it to all machines.

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Relative Synchronization

Relative synchronization ensures a consistent view of time across all nodes in a distributed system. All nodes agree on the order of events, even if their absolute time values differ.

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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.
  • 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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