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tem

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l Hau

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& D

avid

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ome

figur

es ©

200

1 V

eris

sím

o an

d R

odrig

ues 11

Distr

ibut

ed S

yst

ems

Fou

ndation

sP

rof.

Car

l Hau

ser

Cpt

. S 4

64/5

64 L

ectu

reC

hapt

er 1

from

Tex

tA

ugus

t 26

& 3

1 20

04

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trib

uted

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tem

s F

ound

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ns:

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l Hau

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& D

avid

E. B

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n. S

ome

figur

es ©

200

1 V

eris

sím

o an

d R

odrig

ues 22

Hou

sekeeping

Ite

ms

Intr

oduc

tions

–N

ame,

cla

ss (

juni

or, s

enio

r, g

rad,

etc

.), e

mpl

oym

ent

(cur

rent

or

form

er)

•R

oll c

all

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200

1 V

eris

sím

o an

d R

odrig

ues 33

Age

nda

•In

trod

uce

som

e te

rms

and

the

big

pict

ure

–W

hat i

s a

dist

ribut

ed s

yste

m?

–W

hen

are

DS

war

rant

ed?

–H

ow d

id D

S e

volv

e?–

Wha

t are

som

e ar

chite

ctur

es fo

r D

S?

•R

evie

w s

ome

conc

epts

from

oth

er c

ours

es a

long

the

way

•P

urpo

se: p

rovi

de th

e fo

unda

tion

for

the

inqu

iry a

nd

inve

stig

atio

n th

at g

oes

on th

roug

hout

this

cla

ss

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 44

What

is a

Distr

ibut

ed S

yst

em? (1.1

)•

Man

y di

ffere

nt k

inds

of D

S,

so o

ne d

efin

ition

is n

ot

adeq

uate

!•

Muc

h ea

sier

to ta

lk a

bout

ch

arac

teris

tics/

attr

ibut

es•

Pre

senc

e of

a n

etw

ork

is

nece

ssar

y•

DS

has

mor

e th

an ju

st a

ne

twor

k: s

et o

f dis

trib

uted

p

roto

cols

to a

ssis

t the

di

strib

uted

app

licat

ions

and

se

rvic

es–

Wha

t is

a pr

otoc

ol?

“An

agre

emen

t tha

t spe

cifie

s th

e fo

rmat

[, ti

min

g] a

nd m

eani

ng

of m

essa

ges

[that

] com

pute

rs

exch

ange

” [C

omer

2001

]–

Wha

t tim

e is

it p

roto

col?

Hi?

Yea

h, it

’s 4

:45

You

’re

wel

com

e

Hi

Got

the

time?

Tha

nks

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es ©

200

1 V

eris

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o an

d R

odrig

ues 55

DS A

ttribut

es

(con

t.)

•D

S h

as p

roce

sses

that

coo

pera

te to

ach

ieve

som

e co

mm

on g

oal

•Is

the

Inte

rnet

a d

istr

ibut

ed s

yste

m: m

any

com

pute

rs

act a

lone

or

inte

ract

onl

y tr

ivia

lly–

No:

Inte

rnet

con

tain

s pr

otoc

ols

and

serv

ices

that

are

ne

eded

for

build

ing

DS

s, b

ut is

not

, its

elf,

a D

S; a

ttach

ing

a co

mpu

ter

to th

e In

tern

et d

oes

not a

utom

atic

ally

mak

e it

part

of a

dis

trib

uted

sys

tem

–Y

es: t

he In

tern

et c

onta

ins

proc

esse

s su

ch a

s ro

uter

s an

d na

me

serv

ers

that

use

dis

trib

uted

pro

toco

ls to

ach

ieve

a

com

mon

pur

pose

(al

beit

a lo

w-le

vel o

ne)

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es ©

200

1 V

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o an

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odrig

ues 66

DS A

ttribut

es

(con

t.)

•S

choe

der’s

defin

ition

of a

DS

–M

ultip

le c

ompu

ters

–In

terc

onne

cted

by

a ne

twor

k–

Sha

ring

som

e st

ate

•W

hat i

s st

ate?

•S

harin

g ac

com

plis

hed

acro

ss ti

me

and

spac

e

•La

mpo

rt’s

defin

ition

–A

DS

is th

e on

e th

at p

reve

nts

you

from

wor

king

be

caus

e of

the

failu

re o

f a m

achi

ne th

at y

ou n

ever

he

ard

of

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es ©

200

1 V

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o an

d R

odrig

ues 77

DS A

ttribut

es

(con

t.)

•T

anen

baum

and

van

Ste

en’s

def

initi

on–

A c

olle

ctio

n of

inde

pend

ent c

ompu

ters

that

app

ears

to it

s us

ers

as a

sin

gle

cohe

rent

sys

tem

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200

1 V

eris

sím

o an

d R

odrig

ues 88

DS A

ttribut

es

(con

t.):

•C

ompu

ters

/CP

Us

fail

inde

pend

ently

•C

omm

unic

atio

n –

Is u

nrel

iabl

e–

Unr

elia

ble

how

?–

Has

mod

erat

e la

tenc

y–

Wha

t is

late

ncy?

Wha

t is

mod

erat

e?–

Has

var

iabl

e la

tenc

y–

Has

mod

erat

e ba

ndw

idth

–W

hat i

s ba

ndw

idth

? W

hat i

s m

oder

ate?

•H

ard

to d

eter

min

e th

e re

lativ

e or

derin

g of

eve

nts

–C

lock

s no

t syn

chro

nize

d pe

rfec

tly -

exam

ple

–C

omm

unic

atio

n la

tenc

y–

Res

ult:

only

par

tial o

rder

ing

on e

vent

s is

kno

wn

•V

ery

hard

to a

sses

s th

e gl

obal

sta

te a

t any

one

tim

e–

Mes

sage

s ca

n be

in tr

ansi

t–

Clo

cks

are

not s

ynch

roni

zed

perf

ectly

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 99

Tra

nspare

ncy?

•H

ide

diffe

renc

es in

dat

a re

pres

enta

tion

(Acc

ess

Tra

nspa

renc

y)•

Hid

e w

here

a r

esou

rce

is lo

cate

d (L

ocat

ion

T.)

•H

ide

that

a r

esou

rce

may

mov

e (M

igra

tion

T.)

•H

ide

that

a r

esou

rce

may

mov

e w

hile

in u

se

(Rel

ocat

ion

T.)

•H

ide

that

a r

esou

rce

is r

eplic

ated

(R

eloc

atio

n T

.)•

Hid

e th

at a

res

ourc

e is

sha

red

(Con

curr

ency

T.)

•H

ide

failu

re a

nd r

ecov

ery

of a

res

ourc

e (F

ailu

re T

.)•

Hid

e w

heth

er a

res

ourc

e is

in m

emor

y or

on

disk

(P

ersi

sten

ce T

.)

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tem

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ns:

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l Hau

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& D

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E. B

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n. S

ome

figur

es ©

200

1 V

eris

sím

o an

d R

odrig

ues 1

010

How

muc

h t

rans

pare

ncy is

pos

sible?

•Lo

catio

n –

dist

ance

mat

ters

•F

ailu

re –

can

affe

ct ti

me

take

n•

Rep

licat

ion

–ca

n af

fect

tim

e ta

ken

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avid

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n. S

ome

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 1

111

Cent

ralized v

s. D

istr

ibut

ed S

ys.

Sec

urity

-pe

rmea

ting

Sec

urity

-bo

unda

ry

Gra

cefu

l deg

rada

tion/

Par

tial f

ailu

reA

ll or

non

e

Sha

ring

Con

sist

ency

Sca

labi

lity

Mod

ular

ityM

anag

eabi

lity

Het

erog

enei

tyH

omog

enei

ty

Geo

grap

hic

scop

e/re

sour

ces

Loca

l sco

pe a

nd

reso

urce

s

Dis

trib

ute

d S

yste

ms

Cen

tral

ized

Sys

tem

s

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 1

212

When

to D

istr

ibut

e?

•If

you

don’

t nee

d a

DS

, the

n do

n’t d

istr

ibut

e!!!

•B

e ab

le to

ans

wer

“W

hy d

o w

e ne

ed a

dis

trib

uted

sy

stem

to s

olve

this

pro

blem

?”•

Mai

n re

ason

s: d

istr

ibut

e w

hen

1.P

robl

em h

as a

dec

entr

aliz

ed n

atur

e2.

Dis

trib

utio

n te

chni

ques

pro

vide

use

ful s

ervi

ces

even

if

prob

lem

isn’

t nat

ural

ly d

ecen

tral

ized

3.P

robl

em r

equi

res

adap

ting

to c

hang

es a

nd e

volu

tion

in

the

activ

ity a

nd lo

catio

n of

org

aniz

atio

ns

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tem

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ns:

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avid

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n. S

ome

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 1

313

When

to D

istr

ibut

e? (co

nt.)

•#1

: Pro

blem

has

a d

ecen

tral

ized

nat

ure

•M

any

loca

tions

col

labo

ratin

g on

a p

robl

em (

ex?)

–In

vent

ory/

orde

ring

for

a la

rge

reta

iler

–F

inan

cial

rep

ortin

g–

Dis

trib

uted

ast

rono

my

(coo

rdin

ated

use

of r

adio

te

lesc

opes

)–

Fed

erat

ed c

ompu

tatio

nal r

esou

rces

SE

TI@

hom

e, …

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 1

414

When

to D

istr

ibut

e (co

nt.)

•#2

: DS

tech

niqu

es c

an p

rovi

de u

sefu

l ser

vice

s–

Som

etim

es n

atur

ally

cen

tral

ized

: e.g

. ban

k da

taba

se–

Som

etim

es c

entr

aliz

ed b

usin

ess

mod

el–

But

eve

n so

, som

etim

es d

istr

ibut

ion

can

help

…

•B

ank

acco

unt d

atab

ase

(DB

) ex

ampl

e–

Nat

ural

ly c

entr

aliz

ed: e

asy

cons

iste

ncy

and

perf

orm

ance

–F

ragm

ent D

B a

mon

g re

gion

s: e

xplo

it lo

calit

y of

ref

eren

ce,

red

uce

rel

ian

ce o

n n

etw

ork

fo

r re

mo

te a

cces

s–

Rep

licat

e ea

ch fr

agm

ent f

or f

ault

to

lera

nce

•N

ow n

eed

DS

tech

niqu

es–

Rou

te r

eque

st to

rig

ht fr

agm

ent

–M

aint

ain

cons

iste

ncy

of fr

agm

ents

as

a w

hole

dat

abas

e–

Mai

ntai

n co

nsis

tenc

y of

eac

h fr

agm

ent’s

rep

licas

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tem

s F

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ns:

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n. S

ome

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 1

515

When

to D

istr

ibut

e (co

nt.)

•#3

: Ada

ptab

ility

to q

uick

ly c

hang

ing

busi

ness

sc

enar

io–

Allo

w a

uton

omy

in m

anag

ing

serv

ices

and

res

ourc

es

acro

ss d

omai

ns–

Allo

w in

crem

enta

l and

loca

lly-d

riven

exp

anda

bilit

y–

Muc

h ea

sier

to a

dd a

new

bra

nch

offic

e (c

reat

e an

othe

r re

plic

a, e

tc)

if al

read

y di

strib

uted

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tem

s F

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ns:

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n. S

ome

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 1

616

Serv

ices

of D

istr

ibut

ed S

yst

ems

(1.2

)•

DS

sha

ve a

set

of b

asic

ser

vice

s (T

able

1.3

)–

Ofte

n au

gmen

ted

with

oth

er s

peci

fic s

ervi

ces

usef

ul fo

r pa

rtic

ular

app

licat

ions

•N

ame

serv

ice

•R

egis

trat

ion,

Aut

hent

icat

ion,

and

Aut

horiz

atio

n S

ervi

ces

•F

ile S

ervi

ce•

Net

wor

king

Ser

vice

•R

emot

e In

voca

tion

Ser

vice

•B

roke

rage

Ser

vice

(fa

ncy

nam

e se

rvic

e)•

Tim

e S

ervi

ce•

Adm

inis

trat

ion

Ser

vice

s: m

anag

emen

t tas

ks

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tem

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ns:

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avid

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n. S

ome

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 1

717

Evo

lution

of

Distr

ibut

ed C

ompu

ting

•E

volv

ing

fast

sin

ce e

arly

197

0s–

Mile

ston

e ev

ent:

AR

PA

NE

T (

BB

N, 1

969)

, firs

t int

erne

t

–N

ot a

lwa

ys c

lear

wha

t sho

uld

and

wou

ld b

e di

strib

uted

(w

hich

re

sour

ces)

: app

s, fi

les,

mem

ory,

pro

cess

ing

–K

ey

enab

lers

: lo

cala

ndw

ide-

area

netw

orki

ng

•F

irst s

tage

–S

harin

g fil

es: f

tp

–R

emot

e ac

cess

to C

PU

s: te

lnet

•F

tp is

a p

ain,

erg

o di

strib

uted

file

sys

tem

s–

Sam

e sy

stem

cal

l (fo

pen)

wor

ks lo

cal a

nd r

emot

e

•D

istr

ibut

ed c

oncu

rren

t pro

cess

ing:

div

ide

and

conq

uer

acro

ss

mac

hine

s•

Rem

ote

exec

utio

n: c

all n

on-lo

cal p

roce

dure

/met

hod

–G

ave

birt

h to

clie

nt-s

erve

r pa

radi

gm–

Rem

ote

proc

edur

e ca

ll: R

PC

…. t

hen

rem

ote

obje

cts

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© 2

001-

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l Hau

ser

& D

avid

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akke

n. S

ome

figur

es ©

200

1 V

eris

sím

o an

d R

odrig

ues 1

818

Distr

ibut

ed S

yst

em A

rchitect

ures

(1.3

)•

Q: w

hat i

s an

arc

hite

ctur

e?

•A

rchi

tect

ure

vs. f

ram

ewor

k•

DS

arc

hite

ctur

es h

ave

evol

ved

as h

ardw

are

and

softw

are

avai

labl

e be

cam

e m

ore

soph

istic

ated

•P

rese

nted

now

in r

ough

ord

er o

f evo

lutio

n•

Hig

h-le

vel a

rchi

tect

ures

giv

en, m

any

way

s to

im

plem

ent (

we

will

stu

dy s

ome…

.)

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200

1 V

eris

sím

o an

d R

odrig

ues 1

919

Remot

e A

ccess

•T

he m

ost b

asic

form

of d

istr

ibut

ion

•G

oal:

prov

ide

dist

ribut

ed a

cces

s to

cen

tral

ized

fa

cilit

ies

(a)

term

inal

acc

ess

over

PO

TS

: mos

t prim

orda

l(b

) re

mot

e se

ssio

ns o

ver

data

net

wor

k: fi

les,

em

ail,

…

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1 V

eris

sím

o an

d R

odrig

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020

File a

nd M

emor

y D

istr

ibut

ion

•C

omin

g of

wor

ksta

tions

allo

wed

com

putin

g po

wer

to

be s

prea

d ou

t–

Add

ed u

p to

a lo

t of r

esou

rces

–F

ile a

nd m

emor

y di

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utio

n sh

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akke

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200

1 V

eris

sím

o an

d R

odrig

ues 2

121

Remot

e A

ccess

II

•O

ops:

DS

sno

t so

easy

to a

dmin

iste

r &

sha

re

perf

ectly

….

•B

ackl

ash:

re-

cent

raliz

e so

me

reso

urce

s(a

)D

iskl

ess

wor

ksta

tions

(b)

X-T

erm

inal

s

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200

1 V

eris

sím

o an

d R

odrig

ues 2

222

Client

-Serv

er

Arc

hitect

ures

•O

ops:

X-t

erm

inal

s an

d di

skle

ss W

Ss

unsa

tisfa

ctor

y•

Clie

nt-s

erve

r ar

chite

ctur

es: b

reak

up

appl

icat

ion

into

two

rem

ote

piec

es–

Mos

t wid

ely-

depl

oyed

arc

hite

ctur

e to

day

(a)

Clie

nt-S

erve

r A

rchi

tect

ure

(b)

Fat

clie

nts:

clie

nt m

achi

nes

have

cod

e fo

r ea

ch a

pp.

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200

1 V

eris

sím

o an

d R

odrig

ues 2

323

3-Tier

Client

-Serv

er

Arc

hitect

ures

•O

ops,

fat c

lient

s a

big

pain

(co

de, f

iles,

con

fig)

•O

ops,

too

muc

h di

strib

utio

n at

ser

ver

leve

l•

Thi

n cl

ient

s: r

educ

ed fu

nctio

nalit

y (b

ut m

ore

than

X

-Ter

min

als)

•3-

Tie

r ar

chite

ctur

e1.

Hum

an-c

lient

inte

rfac

e (G

UI m

achi

ne)

2.A

pplic

atio

n se

rver

3.D

atab

ase

or s

econ

dary

ser

vers

“be

hind

” ap

p se

rver

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200

1 V

eris

sím

o an

d R

odrig

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424

3-Tier

Client

-Serv

er

Arc

hitect

ures

(con

t.)

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1 V

eris

sím

o an

d R

odrig

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525

Mob

ile C

ode a

nd S

ite A

rchitect

ures

•O

ops:

–

thin

clie

nts

too

limite

d–

non-

dedi

cate

d cy

cles

•M

obile

cod

e: s

hip

code

whe

n ne

eded

to c

lient

(a)

Por

tabl

e an

d m

obile

cod

e(b

)M

obile

Site

Arc

hite

ctur

es: c

lient

s an

d se

rver

s ca

n m

ove

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1 V

eris

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o an

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odrig

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626

Mob

ile S

ite A

rchitect

ures

(con

t.)

•Lo

ts o

f pos

sibl

e ap

plic

atio

ns–

mili

tary

–em

erge

ncy

netw

orks

–

•V

aria

tions

–O

nly

allo

w c

lient

to m

ove

(mob

ile c

ell p

hone

net

wor

ks)

–O

nly

allo

w s

ites

to m

ove

whi

le o

fflin

e, th

en r

eapp

ear

(“no

mad

ic c

ompu

ting”

)… v

ery

popu

lar

toda

y, g

row

ing

wild

ly fa

st…

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l Hau

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avid

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ome

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200

1 V

eris

sím

o an

d R

odrig

ues 2

727

Eve

nt-Base

d A

rchitect

ures

•O

ops:

clie

nt-s

erve

r pa

radi

gm d

oes

not r

epre

sent

all

kind

s of

use

ful d

istr

ibut

ed a

pplic

atio

ns•

Som

e ar

e m

ore

peer

-pee

r•

Som

e re

quire

han

dlin

g un

solic

ited

even

ts•

Exa

mpl

e: c

lient

ser

ver

–cl

ient

: ini

tiate

s re

ques

t, ge

ts r

eply

–se

rver

: mai

ntai

ns s

tate

, get

s re

ques

t, se

nds

repl

y ba

sed

on

stat

e

•E

vent

-bas

ed (

AK

A m

essa

ge-b

ased

) ar

chite

ctur

es–

Ser

ver

gene

rally

mai

ntai

ns s

tate

, but

can

initi

ate

even

ts /

mes

sage

s / i

nvoc

atio

ns–

E.g

., w

hen

its s

tate

cro

sses

a th

resh

old

–E

xam

ples

?

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tem

s F

ound

atio

ns:

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001-

2003

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l Hau

ser

& D

avid

E. B

akke

n. S

ome

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 2

828

Eve

nt-Base

d A

rchitect

ures

(con

t.)

•E

vent

-bas

ed c

omm

unic

atio

n of

ten

mul

ticas

t/gro

up•

Can

run

mul

tipee

rco

nver

satio

ns: d

irect

ly s

endi

ng

mes

sage

s to

ano

ther

•A

sym

met

ric e

xam

ple:

pro

duce

r-co

nsum

er•

Exa

mpl

e ap

ps: e

mai

l, U

SE

NE

T n

ews

•“P

ush”

tech

nolo

gies

are

pro

duce

r-co

nsum

er•

Enh

ance

men

t: le

t pro

duce

rs a

nd c

onsu

mer

s no

t ha

ve b

e on

line

at s

ame

time:

mes

sage

que

ues

•E

nhan

cem

ent:

let c

onsu

mer

s sp

ecify

(su

bscr

ibe)

on

ly w

hat

(typ

e,va

lue)

of m

essa

ges

they

rec

eive

: “p

ublis

h-su

bscr

ibe”

(A

KA

“m

essa

ge b

us”)

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tem

s F

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001-

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l Hau

ser

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avid

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n. S

ome

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 2

929

Eve

nt-Base

d A

rchitect

ures

(con

t.)

•F

igur

es(a

)M

ultip

eer

(b)

Pub

lish-

subs

crib

e

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200

1 V

eris

sím

o an

d R

odrig

ues 3

030

For

mal Not

ions

for

DSs

(1.4

): D

S M

odels

•W

arni

ng: h

eavy

not

atio

n ah

ead

–O

nly

a sm

all p

art o

f the

cla

ss!

•D

Ss

mod

eled

as

set o

f Npr

oces

ses

or p

artic

ipan

ts p

re

sidi

ng o

n M

proc

esso

rs o

r si

tes

deno

ted

s.•

Site

s in

terc

onne

cted

by

som

e ne

twor

k, to

polo

gies

va

ry•

Evo

lutio

n of

a s

yste

m m

odel

ed b

y ev

ents

ei p

–ith

even

t of t

imel

ine

of e

ach

proc

ess

p–

Om

it su

bscr

ipt o

r su

pers

crip

ts w

hen

not a

mbi

guou

s

•W

hen

need

ed: a

ssoc

iate

phy

sica

l tim

esta

mps

with

ev

ents

: t(e

)is

the

real

tim

e in

stan

t whe

n e

took

pla

ce–

Alte

rnat

ely,

t 0, t

a, t b

, …

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avid

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n. S

ome

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 3

131

DS M

odels (co

nt.)

•S

tate

of a

pro

cess

, S, i

s m

odifi

ed u

pon

the

occc

uren

ceof

eac

h ev

ent

•E

volu

tion

of s

tate

of S

is it

s hi

stor

y, H

•H

is a

n or

dere

d se

t of t

uple

s: <

even

t, ev

ent s

tate

>•

A r

unis

an

orde

red

set o

f eve

nts

in a

pro

cess

ex

ecut

ion,

des

crib

ed b

y a

hist

ory

•A

dis

trib

uted

run

is a

par

tially

ord

ered

set

of e

vent

s in

the

exec

utio

n of

sev

eral

pro

cess

es

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tem

s F

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l Hau

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ome

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200

1 V

eris

sím

o an

d R

odrig

ues 3

232

Repr

ese

nting

Distr

ibut

ed C

omput

ation

s•

Eve

nts

at a

pro

cess

can

be

–ex

ecut

ion

even

ts: i

nter

nal c

ompu

tatio

ns–

send

eve

nts:

sen

ding

a m

essa

ge to

ano

ther

pro

cess

–re

ceiv

e ev

ents

: rec

eivi

ng a

mes

sage

from

ano

ther

pro

cess

•M

essa

ge e

xcha

nges

dep

icte

d w

ith ti

mel

ines

: e.g

.

p 1 p 2 p 3m

ab

c

de

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all 2

004

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trib

uted

Sys

tem

s F

ound

atio

ns:

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001-

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l Hau

ser

& D

avid

E. B

akke

n. S

ome

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 3

333

Global Sta

tes

•S

omet

imes

ver

y us

eful

to g

et a

glo

bal “

pict

ure”

of a

di

strib

uted

sys

tem

•G

loba

l sta

te(G

S)

of a

DS

at a

ny p

oint

is a

vec

tor

of

its in

divi

dual

pro

cess

sta

tes:

S =

{S

1, S

2, …

, SM}

•T

wo

view

poin

ts o

f sys

tem

evo

lutio

n:–

Inte

rleav

ing

view

: sys

tem

goe

s th

roug

h a

succ

essi

on o

f st

ates

–S

pace

-tim

e vi

ew: s

yste

m g

oes

thro

ugh

a pa

rtia

lly o

rder

ed

set o

f eve

nts

occu

ring

in s

ever

al p

roce

sses

in th

e sy

stem

•A

cut

(in s

pace

-tim

e vi

ew)

is a

seg

men

t int

erse

ctin

g th

e tim

elin

es o

f all

proc

esse

s.

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004

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uted

Sys

tem

s F

ound

atio

ns:

© 2

001-

2003

Car

l Hau

ser

& D

avid

E. B

akke

n. S

ome

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es ©

200

1 V

eris

sím

o an

d R

odrig

ues 3

434

Global Sta

tes

(con

t.)

•S

tron

gly

cons

iste

nt c

ut (

SC

C):

faith

fully

rep

rese

nts

GS

of t

he s

yste

m

p 1 p 2 p 3 p 4

m3

m1

m2

m4

•In

cons

iste

nt c

ut (

IC):

giv

es in

valid

pic

ture

of G

S•

Con

sist

ent c

ut (

CC

): g

ives

val

id b

ut p

ossi

bly

inco

mpl

ete

pict

ure

of th

e G

S o

f the

sys

tem

SC

CIC

CC

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004

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trib

uted

Sys

tem

s F

ound

atio

ns:

© 2

001-

2003

Car

l Hau

ser

& D

avid

E. B

akke

n. S

ome

figur

es ©

200

1 V

eris

sím

o an

d R

odrig

ues 3

535

DS P

ropert

ies

•G

oal:

spec

ify a

sys

tem

with

hig

h-le

vel p

rope

rtie

s•

Saf

ety

prop

ertie

s : s

omet

hing

bad

(w

rong

eve

nts)

ne

ver

take

s pl

ace

–S

peci

ficat

ion:

pre

dica

te P

will

nev

er b

e tr

ue in

the

DS

•Li

vene

sspr

oper

ties:

som

ethi

ng g

ood

(pos

itive

eve

nt)

even

tual

lyta

kes

plac

e–

Spe

cific

atio

n: p

redi

cate

Pw

ill e

vent

ually

be

true

in th

e D

S

•“a

ny d

eliv

ered

mes

sage

is d

eliv

ered

to a

ll co

rrec

t pa

rtic

ipan

ts”:

saf

ety

prop

erty

•“a

ny m

essa

ge s

ent i

s de

liver

ed to

at l

east

one

pa

rtic

ipan

t”: l

iven

ess

prop

erty

•T

imel

ines

s pr

oper

ties

rela

te th

e tim

es a

t whi

ch

even

ts o

ccur

–e.

g. e

vent

a o

ccur

s <

.1 s

econ

d af

ter

even

t b.