1. Clock Synchronization and algorithm

2. Synchronization Concentrate on how process can synchronize
Not simultaneously access a shared resources.
Multiple process can agree on the ordering of event/access the shared resources;
E.g: process p1 should send message m1 prior to pcs p2 and message m2
Synchronization in DS is much more difficult rather that in uniprocessor/multiprocessor system

3. Use of time in distributed systems:

5. Clock Synchronization make example
When each machine has its own clock, an event that occurred after another event may nevertheless be assigned an earlier time.

6. Clock Synchronization Algorithms
! Centralized Algorithms
! Cristian’s Algorithm (1989)
! Berkeley Algorithm (1989)
! Decentralized Algorithms
! Averaging Algorithms (e.g. NTP)
! Multiple External Time Sources

7. Cristian’s Algorithm Assume one machine (the time server) has
a WWV receiver and all other machines are
to stay synchronized with it.
Every specific seconds, each machine sends a message to the time server asking for the current time.
! Time server responds with message containing current time, CUTC.

8. Cristian's Algorithm
Getting the current time from a time server.

9. Cristian's Algorithm
A major problem – the sender clock/client is fast " arriving value of CUTC from the time server will be smaller than client’s current time, C.
! What to do?
! One needs to gradually slow down client clock by adding less time per tick.
Normally each interrupt add 10msec
=>
9msec per tick. or
add 11 msec per tick to advance the time

10. Cristian’s Algorithm Minor problem What to do?
– the one-way delay from the server to client is “significant” and may vary considerably.
What to do?
Measure this delay and add it to CUTC.
The best estimate of delay is (T1 – T0)/2 for the message propagation time.
Can subtract off I (the server interrupt handling time).
one way propagation time = (T1 – T0-I)/2 !

11. The Berkeley Algorithm
The time daemon asks all the other machines for their clock values
The machines answer
The time daemon tells everyone how to adjust their clock

12. Averaging Algorithm
Every R seconds, each machine broadcasts its current time.
! The local machine collects all other
broadcast time samples during some
time interval, S.
! The simple algorithm algorithm
the new local time is set as the average of the value received from all other machines.

13. Averaging Algorithms
! A slightly more sophisticated algorithm :: Discard
the m highest and m lowest to reduce the effect of a
set of faulty clocks. – Average the rest.
! Another improved algorithm :: Correct each
message by adding to the received time an estimate
of the propagation time from the source.
! One of the most widely used algorithms in the
Internet is the Network Time Protocol (NTP).

14. Logical Clocks All machine must agree with one time/clock.
Logical clock: no matter its not the same to the real time.
Lamport “ all process must agree with the sequence of event occurs”
Either input.c is older or newer then input.o

15. Logical Clock and Lamport Timestamp
Logical clocks
Order of events matters more than absolute time
E.g. UNIX make: input.c input.o
Lamport timestamp
Synchronize logical clocks
Happens-before relation
A -> B : A happens before B
Two cases which determine “happens-before”
A and B are in same process, and A occurs before B: a -> b
A is send-event of message M, and B is receive-event of same message M
Transitive relation
If A -> B and B -> C, then A-> C
Concurrent events
Neither A -> B nor B -> A is true

16. Lamport Algorithm Assign time value C(A) such that Lamport Algorithm
If a happens before b in the same process, C(a) < C(b)
If a and b represent the sending and receiving of a message, C(a) < C(b)
Lamport Algorithm
Each process increments local clock between any two successive events
Message contains a timestamp
Upon receiving a message, if received timestamp is ahead, receiver fast forward it clock to be one more than sending time
Extension for total ordering
Requirement: For all distinctive events a and b, C(a)  C(b)
Solution: Break tie between concurrent events using process number

17. Lamport Timestamp Example
Clocks run at different rate 6 12 18 24 30 36 42 48 54 60 8 16 32 40 56 64 72 80 10 20 50 70 90
100 A B C D

18. Correct clocks using Lamport’s Algorithm
Solutions
Message C From process 2 leaves at 60 > must arrive at 61 or later 6 12 18 24 30 36 42 48 70 76 8 16 32 40 61 69 77 85 10 20 50 60 80 90
100 A B C D
Correct clocks using Lamport’s Algorithm

19. Example: Totally-Ordered Multicast
Application of Lamport timestamps
Scenario
Replicated accounts in New York(NY) and San Francisco(SF)
Two transactions occur at the same time and multicast
Current balance: $1,000
Add $100 at SF
Add interest of 1% at NY
Possible results ??

20. Inconsistent State $1000*.01+1000=1010 $1000+$100=1100 1010+100
=$1110
1100* =$1111

21. Totally Ordered Multicast
Use Lamport timestamps
Algorithm
Message is time stamped with sender’s logical time
Message is multicast (including sender itself)
When message is received
It is put into local queue
Ordered according to timestamp
Multicast acknowledgement
Message is delivered to applications only when
It is at head of queue
It has been acknowledged by all involved processes
Lamport algorithm (extended) ensures total ordering of events
All processes will eventually have the same copy
of the local queue # consistent global ordering.

22. Global state local state of each process
eg: database records (not temporary records)
with messages are in transit ( have been sent but not delivered)

23. Distributed Snapshot: Intro
Reflects the state in which a system might have been
Chandy and Lamport (1985)
If it is recorded that Q recd a msg from P
then it should also be recorded that P sent it
However, If P’s sending is recorded, but not that of Q receiving it, that’s allowed
Assumption: processes are connected to each other via uni-directional point-to-point channels
Any process can initiate the algorithm
Use a marker with the message to initiate communication

24. Global State (1)
A consistent cut
An inconsistent cut

25. Global State (2)
Organization of a process and channels for a distributed snapshot

26. ALGORITHM Any process can initiate the algorithm.
2. Initiating process P starts by recording its own local state. Then it sends a marker
along each of its outgoing channels.
3. When a process Q receives a marker through an incoming channel C:
• If Q hasn’t already saved its local state,
Q first records its local state and then sends a marker along each of its own outgoing channels.
• If Q has already recorded its state earlier, the marker on channel C is an indicator that Q
should record the state of the channel.
4. A process is done when it has received a marker on each of its coming channels. The
local state of the process and the state of each of its incoming channels are sent to the
initiating process.

27. Global State (3)
Process Q receives a marker for the first time and records its local state
Q records all incoming message
Q receives a marker for its incoming channel and finishes recording the state of the incoming channel

28. A DONE message is sent if
When Q finishes its role in the snapshot it can send one of the two messages to it predecessor
DONE or CONTINUE
A DONE message is sent if
All of Q’s successors have returned a “DONE”
Q has not received any message
ElSE
CONTINUE message will sent to its predecessor

29. Election Algorithms Need to find one process that is the coordinator
Assume
Each process has a unique identifier
network address for example
One process per machine
Every process knows the process number of every other process
Processes don’t know which processes are down and which ones are still running
End result of the algorithm: all processes agree on who is the new coordinator/leader
Bully algorithm & Ring Algorithm

30. Bully Algorithm (Garcia-Molina)
A process notices that coordinator is not responding
it starts an election (any process can start one)
Election algorithm
P sends an ELECTION message to processes with higher numbers
If no one responds, P wins the election
If some process with higher process number responds
P’s job is done, that process takes over
the receiver sends an OK message to P
receiver starts an election process
Eventually all processes give up, except one
This process sends out a message saying that it is the new “COORDINATOR”
A process that was down, when it comes back up starts a new election of its own

31. The Bully Algorithm (1)
The bully election algorithm; pcs 7 as coordinator=> crashed
Process 4 is the first on noticed the crashed >> send ELECTION process to 5, 6 and 7 (higher pcs)
Process 5 and 6 respond, telling 4 to stop
Now 5 and 6 each hold an election

32. The Bully Algorithm (2) Process 6 tells 5 to stop
Process 6 wins and tells everyone

33. Ring Algorithm (1) Does NOT use a token Assume
processes are ordered
each process knows its successor
and the successor’s successor, and so on (needed in case of failures)
Process P detects that the coordinator is dead
sends an ELECTION message to its successor
includes its process number in the message
each process that receives it
adds its own process number and then forwards it to its successor
eventually it gets back that message
now what does it do?

34. Ring Algorithm (2)
The process that initiated it, then sends out a message saying “COORDINATOR”
the process with highest number in list is the leader
when this comes back, then process P deletes it

35. A Ring Algorithm(1) Election algorithm using a ring.
We start with 6 processes,     connected in a logical ring.
Process 6 is the leader,     as it has the highest number.

36. A Ring Algorithm(2)
Process 6 fails.

37. A Ring Algorithm(3) Process 3 notices that Process 6 does not respond
So it starts an election, sending a message containing its id to the next node in the ring.

38. A Ring Algorithm(4)
Process 5 passes the message on,     adding its own id to the message

39. A Ring Algorithm(5)
Process 0 passes the message on,     adding its own id to the message.

40. A Ring Algorithm(6)
Process 1 passes the message on,     adding its own id to the message.

41. A Ring Algorithm(7)
Process 4 passes the message on,     adding its own id to the message

42. A Ring Algorithm(8)
When Process 3 receives the message back,     it knows the message has gone around the ring,         as its own id is in the list.
Picking the highest id in the list,     it starts the coordinator message         "5 is the leader" around the ring

43. A Ring Algorithm(9)
Process 5 passes on the coordinator message

44. A Ring Algorithm(10)
Process 0 passes on the coordinator message.

45. A Ring Algorithm(11)
Process 1 passes on the coordinator message.