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Distributed Systems > 11.Time & Clocks > Flashcards

11.Time & Clocks Flashcards

1
Q

What is “clock drift”?

A

The DIFFICULTY of maintaining synchronization once achieved

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2
Q

What is Christian’s Clock Synchronization?

A

“Time server”
-Client set their own time = reply + rtt/2
(assume time-out = time-back)

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3
Q

What is the Berkeley algorithm for calculating time?

A
  • Master processor
  • polls slaves
  • average results +own time
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4
Q

What do the slaves receive after Berkeley calculation?

A

it’s own delta (+/-)

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5
Q

What happens if master in Berkeley algorithm fails?

A

Distributed election algorithm to elect replacement from slaves

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6
Q

What kind of networks are Cristian’s and Berkeley algorithm used in ?

A

Intranet

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7
Q

What is the protocol used for calculating time in a larger scale internet?

A

Network Time Protocol (NTP)

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8
Q

What two types of servers are the in a network with NTP?

A
  • Primary (UTC clock)

- Secondary(Synchronized with primary)

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9
Q

Ways to synchronise server in NTP network?

A
  • Multicast mode
  • Procedure call mode
  • Symmetric mode
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10
Q

What method of synchronization is used on high-speed LAN, how does it work?

A

Multicast

  • Server broadcasts time to all secondaries at once
  • Each clock resets (+small delay)
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11
Q

How does the procedure call mode for synchronization work?

A
  • Christians Algorithm

- server replies with time

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12
Q

When is procedure call synchronzation used?

A
  • when multicast unsupported

- for higher accuracy

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13
Q

What synchronization mode do we use when the highest accuracy is required?

A

Symmetric mode

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14
Q

How does symmetric mode for synchronization work?

A
  • Messages exchanged
  • data built up
  • improve sync over time
  • each message contains timing info about the previous message received
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15
Q

What can we calculate with the timing info in symmetric mode sync messages?

A

OFFSET (between 2 clocks)->
oi -di /2 <=o<=oi+di/2
where di: transmission time of two messages

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16
Q

How do NTP servers measure the reliability of the other server?

A

Identify lowest d value my filtering successive (o,d) values

17
Q

How can every event in a single processor be uniquely ordered in time?

A

using the local clock

18
Q

Why not use local clock to order events in distributed system?

A

synchronisation is not sufficiently good

19
Q

What are some simple principles for deducing the order of events?

A

-Two events happen in the same process? The occur in the order given by the process
-If message is sent from one process to another?
Sending before receiving

20
Q

What is a logical clock?

A

a monotonically increasing software counter

21
Q

Where is a logical clock L?

A

Within a process

22
Q

What is a logical clock L used for?

A

timestamping events

23
Q

How is the logical clock L used?

A
  • L++ before event
  • each message contains current L (t)
  • Each message received set : = max(L,t)+1
24
Q

What can we deduce about the logical clocks L(e1),L(e2) if e1 happens before e2? (Lamport’s clock)

A

L(e1)

25
Q

Can we deduce the ordering of events from their timestamps? (Lamport’s clock)

A

nope

26
Q

What is a vector clock in a system with n processes?

A

an array of n integers

27
Q

Where does a vector clock exist?

A

Each process has it’s own

28
Q

What are the initial values in a vector clock?

A

all zeroes

29
Q

What happens when an event occurs in process i (Vector clocks)?

A

V[i]= V[i]+1

30
Q

How do the other processes know an event has occurred in a different process?

A
  • they shall receive message from the process
  • message contains their vector clock
  • merge it with processes vector clock
31
Q

What do Vector clocks capture that Lamport’s do not?

A

causality

32
Q

What can we deduce about the vector clocks V(e1),V(e2) if e1 happens before e2? (Vector clock)

A

V(e1)

33
Q

Can we deduce the ordering of events from their timestamps? (Vector clock)

A

YA

34
Q

What is an advantage of vector clocks?

A

We don’t end up with arbitrary, unnecessary orders

35
Q

What is the cost of vector clocks?

A

extra amount of data in a timestamp

Distributed Systems (6 decks)

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