From Coulouris, Dollimore, Kindberg and Blair Distributed Systems: Concepts and Design Edition 5, © Addison-Wesley 2012 Slides for Chapter 14: Time and

From Coulouris, Dollimore, Kindberg and Blair

Distributed Systems: Concepts and Design

Edition 5, © Addison-Wesley 2012

Slides for Chapter 14: Time and Global States

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012

Figure 14.1Skew between computer clocks in a distributed system


Figure 14.2Clock synchronization using a time server

mr

m tp Time server,S


Figure 14.3An example synchronization subnet in an NTP implementation

1

2

3

2

3 3

Note: Arrows denote synchronization control, numbers denote strata.


Figure 14.4Messages exchanged between a pair of NTP peers

Ti

Ti-1Ti-2

Ti- 3

Server B

Server A

Time

m m'

Time


Figure 14.5Events occurring at three processes


Figure 14.6Lamport timestamps for the events shown in Figure 14.5


Figure 14.7Vector timestamps for the events shown in Figure 14.5


Figure 14.8Detecting global properties


Figure 14.9Cuts

m1 m2

p1

p2Physical

time

e10

Consistent cutInconsistent cut

e 11

e 12

e 13

e 20

e 21

e 22


Figure 14.10Chandy and Lamport’s ‘snapshot’ algorithm

Marker receiving rule for process pi

On pi’s receipt of a marker message over channel c:if (pi has not yet recorded its state) itrecords its process state now;records the state of c as the empty set;turns on recording of messages arriving over other incoming channels;else pi records the state of c as the set of messages it has received over c since it saved its state.end if

Marker sending rule for process pi

After pi has recorded its state, for each outgoing channel c: pi sends one marker message over c (before it sends any other message over c).


Figure 14.11Two processes and their initial states


Figure 14.12The execution of the processes in Figure 14.11


Figure 14.13Reachability between states in the snapshot algorithm

Sinit Sfinal

Ssnap

actual execution e0,e1,...

recording recording begins ends

pre-snap: e'0,e '1,...e'R-1 post-snap: e 'R,e 'R+1,...

'


Figure 14.14Vector timestamps and variable values for the execution of Figure 14.9

m1 m2

p1

p2Physical

time

Cut C1

(1,0) (2,0) (4,3)

(2,1) (2,2) (2,3)

(3,0)

x1= 1 x1= 100 x1= 105

x2= 100 x2= 95 x2= 90

x1= 90

Cut C 2


Figure 14.15The lattice of global states for the execution of Figure 14.14

Sij = global state after i events at process 1 and j events at process 2

S00

S10

S20

S21S30

S31

S32

S22

S23

S33

S43

Level 0

1

2

3

4

5

6

7


Figure 14.16Algorithms to evaluate possibly φ and definitely φ


Figure 14.17Evaluating definitely φ

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From Coulouris, Dollimore, Kindberg and Blair Distributed Systems: Concepts and Design Edition 5, © Addison-Wesley 2012 Slides for Chapter 14: Time and