[PPT] - Distributed Snapshots 1 Goals of the lecture Consistent PowerPoint Presentation

SLIDE 1 Distributed Snapshots 1 Goals

f

the lecture

Consistent

State

Algo

rithm

Co

rrectness

Stable

Prop erties Reference: Chandy and Lamp

rt

c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 2 Distributed Snapshots 2 Case

f

the dubious dolla rs $400 $300

A

B picture tak en here ($400) Send $ 100 fr

m

A to B picture tak en here ($400) The total amount b ecomes $800 c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 3 Distributed Snapshots 3 Problem T

determine

global system state Each p ro cess can reco rd its

wn

state and messages it sends and receives. No sha red clo ck

r

memo ry Analogy: group

f

photographers c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 4 Distributed Snapshots 4 Mo del

f

a Distributed System

Finite

set

f

p ro cesses

Finite

set

f

channels

p
q
r

@ @ @ @ @ @ @ @ @ I c 3

c

4 c 1 c 2 Channel := FIF O, erro r free, and innite buer c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 5 Distributed Snapshots 5 Denition

f

a p ro cess Pro cess P = (S; I ; E )

?

H H H H H H H H j Set

f

States Initial State Set

f

Events An event can change the state

f

P and at most

ne

channel. e = h p; s; s ; m; c i

)
P

P P P P P P P P q @ @ @ R ? p

st-state

p ro cess p re-state channel message c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 6 Distributed Snapshots 6 Global state and global sequence state(D ) =

i

state(p i )

i

state(c j ): next (s; e) = global state immediately after e seq = (e i :

i
n)

is a computation

f

D i s = initial global state s i+1 = next(s i ; e i )

i
n

c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 7 Distributed Snapshots 7 Example 1

q
p

c 1 c 2 p q h h

r

ecv t q sendt q s s 1 h h

sendt

p r ecv t p s 1 s c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 8 Distributed Snapshots 8 Example 1 [Contd.]

q
p

c 1 c 2

s
s

1 hi hi state(p) = s 1

s
s

htok eni hi state(c 1 ) = htok eni state(c 2 ) = hi state(q ) = s c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 9 Distributed Snapshots 9 Global State Detection Algo rithm Sending Rule : F

r

all channels c directed a w a y from p, p sends

ne

ma rk er after p reco rds its state and b efo re it sends further messages along c. Receiving Rule : On receiving a ma rk er along c if q has not reco rded its state then reco rds its state ma rks c as empt y else state(c) = h seq

f

messages i received along c after the state w as reco rded and b efo re ma rk er is received. c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 10 Distributed Snapshots 10 Example 2

q
p

c 1 c 2 p q h h

recv

M send M B A h h

recv

M send M D C c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 11 Distributed Snapshots 11 Example 2 [contd.]

q
p

hi hi C A S

q
p

hM i hi C B S 1

q
p

hM i hM i D B S 2

q
p

hM i hi D A S 3 c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 12 Distributed Snapshots 12 Prop ert y

f

the reco rded global state S

S
S
=

snapshot

S
is

reachable from S

S
is

reachable from S

q
p

hi hM i D A Reco rded global state (S

)

c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 13 Distributed Snapshots 13 Prop ert y

f

the reco rded global state [Contd.] Theorem 1 Ther e exists a c

mputation

seq = (e i ;

i)

wher e 1. F

r

al l i, wher e i <

r

i

:

e i = e i , and 2. the subse quenc e (e i ;

i

< ) is a p ermutation

f

the subse quenc e (e i ;

i

< ), and 3. for al l i wher e i

r

i

:

S i = S i , and 4. ther e exists some k ;

k

< , such that S

=

S k . c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 14 Distributed Snapshots 14 Colo rful description (due to Dijkstra)

Each

machine, atomic action and message is either white

r

red

S

) Snapshot (SS) ) S 1 white red c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 15 Distributed Snapshots 15 Colo r Assignm ent A tomic Action : same colo r as the machine Message : same colo r as the machine that sends it Snapshot state SSS consists

f
state

when it made the transition from white to red

the

sequence

f

white messages accepted b y a red machine c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 16 Distributed Snapshots 16 Pro

f

c Vija y K. Ga rg Distributed Systems Sp ring 96

SLIDE 17 Distributed Snapshots 17 Summa ry

Beautiful

pap er Beautiful algo rithm

Example
f

generalization

f

a p roblem c Vija y K. Ga rg Distributed Systems Sp ring 96