Synchronisation

1
Synchronisation

• Chapter 5

2
Clock Synchronisation

• When each machine has its own clock, an event
  that occurred after another event may
  nevertheless be assigned an earlier time.

3
Physical Clocks (1)

• Computation of the mean solar day.

4
Physical Clocks (2)

• TAI seconds are of constant length, unlike solar
  seconds. Leap seconds are introduced when
  necessary to keep in phase with the sun.

5
Clock Synchronization Algorithms

• The relation between clock time and UTC when
  clocks tick at different rates.

6
Cristian's Algorithm

• Getting the current time from a time server.

7
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

8
Lamport Timestamps

• Three processes, each with its own clock. The
  clocks run at different rates.
  
• Lamport's algorithm corrects the clocks.

9
Example Totally-Ordered Multicasting

• Updating a replicated database and leaving it in
  an inconsistent state.

10
Global State (1)

• A consistent cut
• An inconsistent cut

11
Global State (2)

• Organisation of a process and channels for a
  distributed snapshot

12
Global State (3)

• Process Q receives a marker for the first time
  and records its local state (and sends out a
  marker on its outgoing channels)
  
• Q records all incoming messages
• Q receives a marker for its incoming channel and
  finishes recording the state of the incoming
  channel
  

13
The Bully Algorithm (1)

• The bully election algorithm to elect a
  coordinator
  
• Process 4 holds an election
• Process 5 and 6 respond, telling 4 to stop
• Now 5 and 6 each hold an election

14
Global State (3)

• Process 6 tells 5 to stop
• Process 6 wins and tells everyone

15
A Ring Algorithm

• Election algorithm using a ring.

16
Mutual Exclusion A Centralised Algorithm

• Process 1 asks the coordinator for permission to
  enter a critical region. Permission is granted
  
• Process 2 then asks permission to enter the same
  critical region. The coordinator does not
  reply.
  
• When process 1 exits the critical region, it
  tells the coordinator, which then replies to 2

17
A Distributed Algorithm

• Two processes (0 and 2) want to enter the same
  critical region at the same moment.
  
• Process 0 has the lowest timestamp, so it wins.
• When process 0 is done, it sends an OK also, so 2
  can now enter the critical region.

18
A Token Ring Algorithm

• An unordered group of processes on a network.
• A logical ring constructed in software.

19
Comparison

• A comparison of three mutual exclusion algorithms.

20
The Transaction Model (1)

• Updating a master tape is fault tolerant.

21
The Transaction Model (2)

• Examples of primitives for transactions.

22
The Transaction Model (3)

• Transaction to reserve three flights commits
• Transaction aborts when third flight is
  unavailable

23
Transaction Properties

• Atomic
• To the outside world, the transaction happens
  indivisibly
  
• Consistent
• The transaction does not violate the system
  invariants
  
• Isolated
• Concurrent transactions do not interfere
• Transactions are serializable
• Durable
• Once a transaction commits, the changes are
  permanent

24
Distributed Transactions

• A nested transaction
• A distributed transaction

25
Private Workspace

• The file index and disk blocks for a three-block
  file
  
• The situation after a transaction has modified
  block 0 and appended block 3 (copy on write)
  
• After committing

26
Writeahead Log

• a) A transaction
• b) d) The log before each statement is executed

27
Concurrency Control (1)

• General organisation of managers for handling
  transactions.

28
Concurrency Control (2)

• General organization of managers for handling
  distributed transactions.

29
Serializability
(d)

• a) c) Three transactions T1, T2, and T3
• d) Possible schedules

30
Two-Phase Locking (1)

• Two-phase locking.

31
Two-Phase Locking (2)

• Strict two-phase locking.

32
Pessimistic Timestamp Ordering

• Concurrency control using timestamps.