13 Questions
What is the main challenge in synchronizing clocks in a distributed system?
The main challenge is that there is no global time, and agreeing on time among many systems is not trivial.
Why is it hard to synchronize clocks in a distributed system using physical clocks?
It is hard because no two timers (clocks) are exactly the same, and different clocks tend to diverge over time (clock skew).
What is an example of the problem that arises when each machine in a distributed system has its own clock?
In the UNIX make program, an event that occurred after another event may be assigned an earlier time when each machine has its own clock.
How do computer timers (clocks) in a distributed system function?
A computer timer is a quartz crystal that oscillates at a well-defined frequency and creates an interrupt (clock tick) after a number of oscillations.
What are the implications of clock synchronization issues for real-time systems?
It is important to synchronize clocks with real-world clocks and with each other to ensure accurate timing in real-time systems.
Why is there no global time in distributed systems?
There is no global time in distributed systems because agreeing on time among many systems is not trivial.
What is the precision of Universal Coordinated Time (UTC) provided as a service by satellites and shortwave radio?
Between ±1 msec and ±10 msec
What is the ideal condition for clock synchronization in the system model where each machine has a timer that causes an interrupt H times a sec?
Ideally, for all machines and times, the machine's clock value should be equal to the UTC time on the machine or the derivative of the clock value with respect to time should be 1.
What is the need for clock synchronization according to the text?
If two clocks are drifting from UTC in the opposite direction, at time δ after the last synchronization, they may be up to 2ρδ, where ρ is the maximum drift rate, and in order not to differ by more than δ, clocks must be synchronized at least every δ/2ρ seconds.
How does the Network Time Protocol account for message delays?
The Network Time Protocol accounts for message delays by having clients contact a time server, which has an accurate clock, and by adjusting the clock based on the message delays and timestamps.
How does the Berkeley algorithm handle time synchronization?
In the Berkeley algorithm, the time server polls every machine periodically, computes the average based on the answers, and tells all machines to adjust to the average time.
What is the motivation behind logical clocks according to the text?
The motivation behind logical clocks is that many applications need to agree on a current time, which doesn't need to match real time, and lack of synchronization will not be noticed by processes that do not interact.
According to Lamport's logical clocks, when can the happens-before relation be observed directly?
The happens-before relation can be observed directly in two situations: If a and b are events in the same process, and a occurs before b.
Study Notes
Challenges in Clock Synchronization
- The main challenge in synchronizing clocks in a distributed system is due to the lack of a single, global clock.
Physical Clocks in Distributed Systems
- It is hard to synchronize clocks in a distributed system using physical clocks because each machine has its own clock, which can drift apart over time.
Problem of Independent Clocks
- An example of the problem that arises when each machine in a distributed system has its own clock is that the clocks can become desynchronized, leading to inconsistencies.
Computer Timers in Distributed Systems
- Computer timers (clocks) in a distributed system function independently on each machine, which can lead to synchronization issues.
Implications for Real-Time Systems
- Clock synchronization issues can have severe implications for real-time systems, where timely responses are critical.
Global Time in Distributed Systems
- There is no global time in distributed systems, making clock synchronization a challenge.
Precision of Universal Coordinated Time (UTC)
- The precision of Universal Coordinated Time (UTC) provided as a service by satellites and shortwave radio is accurate up to a few milliseconds.
Ideal Condition for Clock Synchronization
- The ideal condition for clock synchronization in the system model is when each machine has a timer that causes an interrupt H times a second, ensuring synchronization.
Need for Clock Synchronization
- The need for clock synchronization arises because independent clocks can drift apart, leading to inconsistencies.
Network Time Protocol
- The Network Time Protocol accounts for message delays by compensating for the transmission time of messages.
Berkeley Algorithm
- The Berkeley algorithm handles time synchronization by adjusting the local clock to match the most accurate clock in the system.
Logical Clocks
- The motivation behind logical clocks is to provide a consistent view of the system's state, despite the lack of global time.
Lamport's Logical Clocks
- According to Lamport's logical clocks, the happens-before relation can be observed directly when the clock values of two events are compared.
Test your understanding of clock synchronization in distributed systems with this quiz. Explore the challenges of achieving time agreement in distributed systems and its implications for applications like the UNIX make program.
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