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H PHN TN

Chng 1 : Tng quan v h phn tn.1.1 nh ngha.C nhiu nh ngha v h phn tnnh ngha 1: H phn tn l tp hp cc my tnh t tr c kt ni vi nhau bi mtmng my tnh v c ci t phn mm h phn tn.nh ngha 2: H phn tn l mt h thng c chc nng v d liu phn tn trn cctrm (my tnh) c kt ni vi nhau bi mt mng my tnh.nh ngha 3: H phn tn l mt tp cc my tnh c lp giao tip vi ngi dngnh mt h thng thng nht, ton vn.

Nh vy, c th ni : H phn tn = mng my tnh + phn mm h phn tn.Phn loi h phn tn:Trc y, h phn tn c chia thnh ba loi : h iu hnh h phn tn, c s dliu h phn tn v cc h thng tnh ton h phn tn.

Ngy nay, h phn tn c phn chia nh sau:- H phn tn mang tnh h thng: h iu hnh phn tn.

- H phn tn mang tnh ng dng: cc h thng truyn tin phn tn.1.2 Mc tiu ca h phn tn.a. Kt ni ngi s dng v ti nguynGii quyt bi ton chia s ti nguyn trong h thng (resource sharing).

b. Tnh trong sutn giu s ri rc v nhng nhc im nu c ca h phn tn i vi ngi s dng(end-user ) v nhng nh lp trnh ng dng (application programmer).Theo tiu chun ISO cho h phn tn ISO / IS / 10746 tn l "Open distributed

processing reference model" 1995 c th ha tm dng trong sut:Trong sut truy cp (Access transparency): che giu s khc bit v cch biu din vcch truy cp ti nguyn.

Trong sut v v tr (Location transparency): che giu v tr ca ti nguyn. Hai dngtrong sut va trnh by c gi chung l trong sut mng (network transparency).Trong sut di tr (Migration transparency): che giu kh nng chuyn v tr ca tinguyn.Trong sut v vic nh v li (Relocation transparency): che giu vic di chuyn cati nguyn khi ang c s dng.Trong sut nhn bn (Replication transparency): che giu tnh trng tnh trng s dng

bn sao ca ti nguyn.Che giu s che s ti nguyn tng tranh (Concurency transparency).Trong sut s c (Failure transparency): che giu li h thng nu c.Trong sut kh nng di chuyn ti nguyn (Persistence transparency): che giu vic di

chuyn ti nguyn t b nh ngoi vo b nh trong v ngc li.c. Tnh m (Openness).

H phn tn c gi l m nu n cung cp cc dch v theo cc quy tc chun m tc php v ng ngha ca dch v .

Thng thng trong h phn tn cc dch v thng c t qua cc giao din bngngn ng c t giao din (Interface Definition Language- IDL). V th ch quan tm

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n c php. N cho php cc d ch v khc nhau cng chung sng. Nu cc giao dinca h phn tn c c t y v ng n.

Xt hai khi nim ca h phn tn l khi nim lin tc (Interroperability) v khi nimchuyn mang (portability).

Lin tc: cc ci t ca cc h thng hoc thnh phn h thng t cc nh sn xutkhc nhau c th lm vic vi nhau thng qua lin tc.

Chuyn mang: nh chuyn mang m mt ng dng c pht trin cho h phn tn Ac th thc hin khng cn thay i g trn mt h phn tn B khc, vi iu kin cci cng giao din nh A

d. Tnh co gin (Scalability)

Mt h phn tn c gi l c tnh co gin nu n thch nghi vi s thay i quy mca h thng. Th hin trn cc kha cnh sau:

- D b sung ngi s dng v ti nguyn h thng

- Khi h thng thay i quy m v mt a l dn n s thay i v v tr a l cangi s dng v cc ti nguyn.

- H thng c thay i quy m v qun tr.

Nu h phn tn c tnh co gin thng nh hng n hiu nng ca h thng (hiunng ca h thng l hiu qu nng lc hot ng ca i tng).

C ba gii php ph dng gii quyt vn co gin ca h phn tn:

- n giu

- Phn tn: phn nh thnh phn h thng v phn b chng trn phm vi ca h thng(qun l phn cp). V d DNS xc nh theo cch phn cp min ln thnh cc mincon. Vi phng php ny s gii quyt c vn khi thm ngi dng hay tinguyn vo h thng.

- Nhn bn: nhn bn mt thnh phn no ca h thng. V d ti nguyn d liut ti cc v tr khc nhau trong h thng.

1.3 Cc khi nim phn cng.

a. Phn loi my tnh.

C hai loi my tnh:

- Cc loi my tnh c chia s b nh (Shared memory): cc loi my a x l(multiproccessor).

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- Cc my tnh khng chia s b nh (Private memory): cc h thng multicomputors

Trong mi loi li chia tip theo mng kt ni bus - based ch c mt ng kt ni vswitch - base c nhiu ng kt ni t my ny sang my khc

Hnh 1: Hai cch t chc vi x l v b nh trong h my tnh phn tn.

b. H thun nht / h khng thun nht.

H thng thun nht: mng my tnh cng s dng mt cng ngh, cc b x l l nhnhau, truy cp n cng mt b nh ging nhau. Thng dng trong h thng c tnhton song song.

H khng thun nht: nhng my tnh khc nhau kt ni vi nhau.

1.4 Cc khi nim phn mm.

a. DOS (distributed OS).

L h iu hnh cho cc h multiproccessor v cc h homogenous multicomputer.

Mc tiu l n giu v cung cp cc dch v qun tr ti nguyn.

c im l cc dch v c th c thc hin bi cc li triu gi t xa.

Hnh 2. Cu trc chung ca DOS

b. NOS (Network OS).

L h iu hnh cho cc h thng heterogenous multicomputer (LAN, WAN).

Mc tiu ca NOS l cung cp cc dch v t xa.

Hnh 3 . Cu trc chung ca NOS

c. Middleware.

L tng ph nm gia tng dch v ca NOS v tng ng dng phn tn.

Hnh 4. Cu trc chung ca mt h middleware

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1.5 M hnh client - server

a. Tng quan v m hnh Client - server.

M hnh client - server trong mt h phn tn c phn chia thnh hai nhm chnh lnhm cc server v nhm cc client. Nhm cc server cha cc dch v c bit.

Nhm cc client l nhm gi yu cu n server c s dng cc dch v trnserver.

M hnh tng tc tng qut gia client v server:

Hnh 5 M hnh tng tc chung gia mt client v mt server

b Phn tng cc ng dng.

Vic phn nh rch ri chc nng ca client v server n gi cng rt khc bit vkhng thun nht. Do ngi ta a ra tng l chia thnh ba mc chc nng:

User - interface level: bao gm cc chng trnh cung cp giao din cho php ngi sdng tng tc vi chng trnh ng dng.

Processing level: lm nhim v x l cc tc v ca ngi dng trn c s d liu

Data level: gm cc chng trnh duy tr cc d liu m cc chng trnh ng dng xl.

Chng 2: Truyn thng.

(Communication)

2.1 Cc giao thc phn tng (Layered protocols).

Mt trong nhng m hnh phn tng thng dng nht hin nay l m hnh OSI 7 tng.Mi tng c cc giao thc ring cho n.

- Tng ng dng.

- Tng trnh din.

- Tng phin.

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- Tng vn chuyn.

- Tng mng.

- Tng lin kt d liu.

- Tng vt l.

Mt ci tin trong h phn tn l gp tng trnh din v tng phin thnh mt tng mil tng middle ware. Do ta cng phi xy dng cc giao thc tng ng cho tngmiddleware ny.

C 4 m hnh dch v middleware m ta s xt ln lt sau y:

- Gi th tc t xa RPC (Remote Procedure Call).

- Triu gi i tng t xa (Remote Object Invocation)- Middleware hng thng ip (Message - oriented Middleware)

- Middleware hng dng (Stream - oriented Middleware)

2.2 Gi th tc t xa (Remote procedure call - RPC).

2.2.1 Tng quan v RPC.

Khi mt tin trnh trn my A mun thc hin mt th tc no nm trn mt my B

khc th n s thc hin mt li gi th tc t xa ti my B. Th tc s c thchin my B da trn cc tham s c truyn n t my A v kt qu s ctruyn tr li cho my A tng ng.

Trong m hnh client - server th li gi th tc t xa c thc hin qua cc bc sau:

- Tin trnh mun thc hin th tc my client s gi client stub.

- Client stub s to mt bn tin v c li gi n h iu hnh ca client .

- H iu hnh ca my client s gi bn tin ti h iu hnh ca my server.

- H iu hnh ca server s gi bn tin ti server stub.

- Server stub ly cc thng tin ca gi tin v gi server tng ng.

- Server thc hin cng vic c yu cu v tr kt qu v cho server stub.

- Server stub ng gi kt qu vo bn tin ri gi h iu hnh ca server .

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- H iu hnh ca my server ny s gi bn tin kt qu h iu hnh ca myclient.

- H iu hnh ca my client s gi bn tin cho client stub.

- Client stub s m gi tin kt qu v tr v cho client.

Trong , client stub v server stub my client v server l thnh phn nhm gimnh cng vic cho client v server, lm cho h thng hot ng mt cch trong suthn.

Hnh 6. RPC gia mt client v server

2.2.2 Xt chi tit cc thao tc RPC.

ng gi cc tham s: vic ng gi cc tham s chun b truyn i do client stubthc hin. Client stub s sp xp cc tham s v a vo hng i v qu trnh nyc gi l parameter marshaling. Cc tham s c truyn i gip cho server hiuc cng vic mnh cn thc hin tng ng l g xc nh li gi n th tcthch hp.

Truyn tham s: Vic truyn tham s t client ti . C hai cch truyn: truyn thambin v truyn tham tr.

- Truyn tham tr: cc tham s c truyn i l cc gi tr c th. Cc th tc c gin s coi cc tham bin c truyn kiu tham tr nh l cc bin c khi to cc

b, c th thay i gi tr nhng li khng nh hng ti gi tr gc trong ln gi sau.Vn t ra khi truyn tham tr l yu cu gia cc my phi c s ng nht v vicbiu din d liu v cc kiu d liu.

- Truyn tham bin: cc tham s c truyn i l con tr hay bin cha a ch cani cha gi tr thc ca chng. Cc th tc c gi s cn c vo a ch ny tham chiu n gi tr khi tnh ton. Khi gi tr ny b thay i trong khi thc hin thtc th s c thng bo cho client v cc ln gi sau s dng gi tr mi .

2.3 Cc m hnh RPC m rng.

2.3.1 RPC d b (Asynchronous RPC).

T tng thc hin l: client gi ti server li gi th tc v ch bn tin chp nhn tserver. Pha server s gi bn tin chp nhn v cho client thng bo nhn c yucu v bt u thc hin yu cu RPC . Lc ny client s tp tc thc hin cng vicca mnh m khng ch kt qu t server nh RPC truyn thng.

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Hnh 7. RPC d b.

2.3.2 RPC ng b tr (Deferred synchronuos RPC):

Thc hin hai li gi, mt t client v mt t server.

Client gi ti server li gi th tc v ch bn tin chp nhn t server. Pha server sgi bn tin chp nhn v cho client thng bo nhn c yu cu v bt u thchin yu cu RPC . Lc ny client s tp tc thc hin cng vic ca mnh. Khi thchin th tc xong, server s thc hin li gi ti client bo nhn ly kt qu. Clientthc hin ngt, nhn kt qu v gi li cho server bn tin thng bo nhn kt quthnh cng.

Hnh 8. RPC ng b tr.

2.3.3 RPC n tuyn (one- way RPC).Sau khi thc hin li gi th tc t xa ti server, client khng ch i thng bo nhnyu cu thnh cng t server m tip tc thc hin ngay cc cng vic khc ca mnh. l RPC n tuyn.

2.4 Triu gi i tng t xa (Remote object invocation).

2.4.1 i tng phn tn (Distributed object ).

Mt i tng phn tn gm cc thnh phn sau:

- State: l cc d liu c ng gi.

- Method: l cc thao tc c th thc hin trn d liu.

- Interface: l ni giao tip vi cc phng thc ca i tng. Ni cch khc , ccphng thc sn sng thng qua interface.

Mt i tng c th thc thi nhiu interface v cng c th c nhiu i tng cngthc thi mt interface ging nhau.

S c lp gia cc interface v cc i tng thc thi interface cho php ta c th tmt interface vo mt my no trong khi chnh bn thn i tng c th c tr my khc.

Hnh 9. i tng phn tn.

2.4.2 Cc bc thc hin triu gi i tng t xa.

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Hnh 10. Triu gi i tng t xa.

Khi cn triu gi cc phng thc t xa, client s gi yu cu n proxy - mt th hinca interface.

Proxy s marshal (sp xp v a vo hng theo th t) cc phng thc c yu cuvo mt bn tin ri gi cho h iu hnh ca my client.

H iu hnh ca client s gi bn tin yu cu n h iu hnh ca server.

H iu hnh server nhn bn tin v chuyn cho skeleton (ging server stub ca RPC).

Skeleton s unmarshal bn tin nhn c gi n interface ca i tng cphng thc tng ng.

i tng thc thi phng thc ri tr kt qu v cho skeleton.

Skeleton marshal kt qu nhn c ri gi tr v cho h iu hnh ca client.

H iu hnh ca client nhn bn tin kt qu ri chuyn ti cho proxy.

Proxy unmarshal bn tin ri chuyn kt qu v cho client.

Ch l c client va server u s dng interface ging nhau.

Mt s cc i tng

Compile - time object: l cc i tng trong cc ngn ng lp trnh hng i tng.N c nh ngha nh l mt mu ca class.

Runime object

Persistent Object - i tng kin tr: l i tng vn tn ti ngay c khi n khng tnti trong khng gian a ch ca tin trnh no trn server.

Transient object - i tng tc thi: l i tng ch tn ti khi server gi n n, sau

khi dng xong n s c gii phng.

Triu gi phng thc t xa (RMI - remote method invocation)

Sau khi triu gi mt i tng t xa, client c th triu gi t xa phng thc cai tng .

C hai phng php triu gi phng thc t xa l: triu gi phng thc t xa ngv triu gi phng thc t xa tnh.

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Triu gi phng thc t xa ng: khi cn gi n mt phng thc mi xc nhinterface ang dng trong li triu gi t xa . V th khi interface thay i, ccchng trnh ng dng khng cn phi bin dch li.

Triu gi phng thc t xa tnh: cc interface c xc nh trc. Cc chng trnhng dng khng thch ng c khi interface hin hnh thay i. Nu interface hinti c s thay i th cc chng trnh ng dng phi c bin dch li mi c thhiu

2.5 Truyn thng hng thng ip (Message - oriented communication).

2.5.1 Cc loi truyn thng c bn

Truyn thng kin tr (Persistent communication): Th in t l mt v d minh har nt cho khi nim truyn thng kin tr.Khi mt trm mun gi bn tin i trnmng, n s gi bn tin n interface ca my mnh. Qua b nh m, bn tin

c truyn i trong mng cc b n mail server c b. Mail server ny tm thilu tr bn tin vo b nh m ca mnh, xc nh a ch trm ch, ri gi tiserver cc b ca trm ch tng ng (c th i qua nhiu mail server trung giankhc). Ti mail server cui cng, bn tin lc ny s c lu li trc khi pht chotrm ch tng ng.

Truyn thng nht thi (Transient communication): bn tin gi i ch c lu litrong phin trao i . Khi phin trao i hon thnh hoc khi kt ni b hy b thcc bn tin cng b hy b trn cc server. Do , v mt l do no m mtserver trung gian khng th chuyn tip bn tin i c th bn tin ny s b hy b.

Truyn thng ng b (Synchronous communication): khi trm gi gi i mt bn tinth n s trng thi kha (blocked) cho n khi nhn c thng bo bn tin n ch thnh cng.

Truyn thng d b (Asynchronous communication): khi trm gi gi i bn tin, n stip tc thc hin cng vic ca mnh. iu ny cng c ngha l bn tin c luli trn b nh m ca trm gi hoc ca server cc b.

2.5.2 Mt s loi truyn thng h hp.

Truyn thng d b, kin tr: bn tin c lu tr lu di hoc l b nh m ca

trm gi hoc l trn server truyn thng u tin m bn tin ti. V d h thng thin t.

Truyn thng ng b, kin tr: bn tin c lu tr lu di trm nhn, trm gi s trng thi blocked cho n khi bn tin c lu tr b nh m trm nhn.

Truyn thng d b, nht thi: sau khi lu tr bn tin cn gi ra b nh m ca mymnh, trm gi s tip tc thc hin cng vic ca mnh. Cng lc, bn tin s c

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truyn ti trm nhn. Khi bn tin n c trm nhn m trm nhn li khng lmvic, khi qu trnh truyn thng b hy b.

Truyn thng ng b, nht thi: bn tin khng c lu tr lu di. Khi gi i mtbn tin, trm gi s ch bn tin bo nhn thnh cng ca trm nhn gi v mi thchin tip cng vic ca mnh.

Hnh 11. Mt s dng truyn thng.

2.6 Truyn thng hng dng (stream- oriented communication).

2.6.1 Mt s khi nim c bn.

Medium (s nhiu l media) : ch cc phng tin dng truyn thng tin nh ccthit b lu tr, ng truyn, cc phng tin hin th...

Continuous media: quan h thi gian gia cc mc l yu t c bn thng dchng ng ngha thc s ca d liu.

Discrete media: quan h thi gian khng cn l yu t c bn thng dch ng dliu.

Data stream: l mt chui cc n v d liu. Vi data stream th thi gian l yu tquyt nh. kim sot thi gian ngi ta a ra ba phng thc truyn sau:

Truyn d b (asynchronous transmission mode): cc mc d liu truyn tun t v

khng c rng buc thi gian i vi vic truyn.

Truyn ng b (synchronous transmission mode): quy nh trc tr ti a chomi n v d liu trong data stream.

Truyn ng thi (isochronous transmission mode): quy nh tr ln nht v nhnht cho mi n v d liu trong data stream. Cch truyn ny ng mt vai tr quantrng trong vic trnh din audio v video.

Dng n (simple stream) l dng ch gm mt chui n v d liu.

Dng phc (complex stream): bao gm nhiu chui n v d liu khc nhau. Michui ny c gi l mt dng con (sub stream).

2.6.2 QoS - cht lng dch v.

Cht lng dch v QoS lin quan n cc vn sau:

Bng thng yu cu, tc truyn, tr...

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Loss sensitivity: kt hp cng vi loss interval cho php ta xc nh c tc mtmt thng tin c th chp nhn c.

Burst loss sensitivity: cho php xc nh bao nhiu n v d liu lin tip c th bmt.

Minimum delay noticed: xc nh gii hn thi gian tr trn ng truyn cho php bn nhn khng nhn bit c l c tr.

Maximum delay variation: xc nh tr (jitter) rung ln nht cho php.

Quality of guarantee: ch s lng cc dch v yu cu cn phi c.

2.6.3 ng b cc dng.

C hai loi ng b:

ng b n gin: thc hin ng b gia dng tr v dng lin tc. V d trong victrnh din slide c km m thanh. Dng hnh nh slide l dng tr cn dng m thanhl dng lin tc, phi ng b hai dng ny thu c kt qu trnh din nh mun.

ng b phc tap: l vic ng b gia cc dng d liu lin tc. V d trong vicxem phim trc tuyn, c dng m thanh v dng hnh nh u l cc dng lin tc cn

phi c ng b.

Cc k thut ng b: c hai k thut ng b

K thut n gin: da trn vic ng b cc thao tc c ghi trn cc dng d liu saocho ph hp vi cc yu cu thi gian cho trc v cc rng buc v ng b.

Hnh 12. ng b n gin.

K thut phc tp: ng b trn mi trng mng da trn c vic ng b gia bnnhn v bn gi.

Hnh 13. ng b phc tp

Chng 3 : Tin trnh

(Processes)

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3.1 Lung (Thread).

Tin trnh (Process) l chng trnh ang c thc hin, n coi tnh trong sut lquan trng.

Lung (Thread): l mt hay mt phn chng trnh ang thc hin, n coi hiu nng lquan trng.

Li gi h thng (System call): l tp lnh m rng do h iu hnh cung cp xc nhgiao din gia h iu hnh v cc chng trnh ngi s dng.

Blocking System call: l li gi h thng m sau khi c gi bi tin trnh ngi sdng th tin trnh ny b dng li cho n khi thc hin xong li gi h thng.

Non - Blocking System call: sau khi gi, iu khin c tr li cho tin trnh gi vtin trnh ny tip tc thc hin song song vi li gi h thng.

a lung (Multi thread): p dng cho m hnh client/server c gi l multithreadserver v multithread client. Vi m hnh ny gip n gin ha khi lp trnh choserver ng thi cng tng kh nng x l song song, lm tng hiu nng ca hthng.

C ba phng php tip cn xy dng mt server:

n lung (single - threaded server) : non - parallelism, blocking system call.

a lung (multi - threaded server) : parallelism, blocking system call.

My trng thi hu hn (Finite State Machine): parallelism, non - blocking systemcall.

3.2 Di tr m.

L do cn phi di tr m: tng hiu nng v linh hot ca h thng do vic dichuyn ca cc tin trnh ang thc hin l rt kh khn.

Mt tin trnh bao gm :

Phn m (Code Segment): cha tp cc lnh ca tin trnh ang thc hin.

Phn ti nguyn (Resource Segment): cha cc tham chiu n tt c cc ti nguynbn ngoi m tin trnh ang cn

Phn thc thi (Execution segment): cha cc trng thi thc thi hin hnh ca tintrnh.

Cc m hnh di tr m:

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Hnh14 Cc m hnh di tr m.

Weak mobility: ch truyn phn m v mt s cc d liu khi ng ca tin trnh.c tnh ca m hnh ny l mt chng trnh c truyn i lun c bt u ttrng thi khi ng, ch yu cu my ch c th thc thi yu cu (code)

Strong mobility: truyn c phn m v phn thc thi. c im ca m hnh ny lmt tin trnh ang chy c th c dng li ri chuyn n mt my khc v tip tcthc hin tip tin trnh kh thc thi hn.

Sender initiated migration (di tr c khi to t pha gi) : Di tr c khi ng tmy m phn code ca tin trnh c lu tr hoc ang thc hin. Di tr ny honthnh khi upload chng trnh.

Receiver initiated migration (di tr c khi to t pha nhn) : Di tr m ban u tmy tnh.

Di tr c khi to t pha nhn thc thi n gin hn di tr c khi to t phagi.

3.3 Tc t mm.

3.3.1 nh ngha v phn loi:

nh ngha: Tc t l mt tin trnh t tr c kh nng phn ng, trao i, cng tc vi

cc tc t khc trong mi trng ca n.

Phn loi theo khi nim di tr ha:

Tc t di ng (mobie agent): l mt tc t n gin c kh nng di chuyn gia ccmy khc nhau. Trong di tr m, cc tc t di ng thng yu cu h tr cho m hnhdi ng mnh mc d l khng cn thit. Yu cu ny n t thc t l cc tc t l ttr v c nh hng ln nhau v vi mi trng ca chng. S di chuyn mt tc tn my khc kh c th c thc hin nu khng xt n trng thi thc thi ca n.Tnh di ng l c tnh chung ca cc tc t.

Tc t thng minh (Intelligent agent): l tc t dng qun l thng tin t nhiungun khc nhau. Vic qun l thng tin bao gm vic sp xp, lc, thu thp... V cctc t ny thao tc trn thng tin t nhng ngun vt l khc nhau nn chng ng vaitr rt quan trng.

3.3.2 Cng ngh tc t.

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Hnh 15. M hnh agent flatform ca FIPA

ACL (Agent Communication Language): Truyn thng gia cc tin trnh tun ththeo giao thc truyn thng mc ng dng ACL. ACL message bao gm phn headerv ni dung. Phn header cha trng xc nh mc ch ca thng ip, cng vitrng xc nh ngi gi v ngi nhn. Cng nh cc giao thc truyn thng,

phn ni dung c tch ring. ACL khng qui nh khun dng hay ngn ng thhin ni dung thng ip.

ACC: Mt thnh phn quan trng trong nn tc t l knh truyn thng tc t - ACC.Trong hu ht cc m hnh cho h thng a tc t, cc tc t truyn thng bng cchtrao i thng ip. M hnh FIPA cng cho mt ACC qun l vic truyn thnggia cc agent flatform khc nhau. C th, ACC l nguyn nhn cho vic truyn thngim ti im vi cc nn khc mt cch xc thc.

Chng 4: nh danh.(Naming)

4.1 Cc thc th nh danh (Naming Entities).

4.1.1 Tn, nh danh v a ch.

Tn (name): l xu cc bit hoc k t dng tham chiu n mt thc th trong hphn tn.

a ch (address): khi truy cp n thc th ta s dng im truy cp (access point).Cc im truy cp ny cng phi c t tn v tn chnh l a ch ca n. Nhvy a ch ca thc th chnh l tn ca im truy cp thc th tng ng.

nh danh (Identifiers): y cng l mt kiu tn c bit. Vic nh danh mt tn phitha mn ba tnh cht sau:

- Mi thc th ch c tham chiu bi duy nht mt nh danh ID

- Mi ID tham chiu ti mt thc th.

- ID khng c gn cho mt thc th khc.

Khng gian tn (Name space): dng biu din tt c cc tn. Nu xt v mt hnhhc th y l mt th c hng, gm cc nt v cc cung, gi l th tn (naminggraph). th c cu trc: Mi nt l miu t mt mt thc th. Mi nt directory gnvi nhiu nt khc; lu tr trong bng directory, bng ny l tp cc cp(label,indetifier).

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Tn thn thin (Human-friendly name): l cc tn c t mt cch d hiu, thnthuc vi con ngi.

4.1.2 phn gii tn.

Khng gian tn a ra k thut lu tr v tm kim cc tn trn n mt cch d dng.Mt trong nhng phng php hay dng l s dng ng dn tn (path name). Qutrnh tm kim tn trong khng gian tn c gi l phn gii tn (name resolution).Qu trnh phn gii tn tr v nh danh mt nt.

Closure machanism: l k thut cho ta bit qu trnh tm kim tn c bt u nh thno v bt u u.

Linking: k thut ny s dng b danh (alias) - tn ging vi tn ca thc th. Vi kthut ny cho php nhiu ng dn cng tham chiu n cng mt nt trn th tn.

Mt cch tip cn khc l dng mt nt l khng phi lu tr a ch hay trng thica thc th m lu tr ng dn tuyt i ti thc th .

Mounting: l k thut c thc hin khi tm kim trn hai khng gian tn. Mt nt thmc c gi l mt mount point (im gn kt) lu gi id (hoc cc thng tin cnthit cho vic xc nh v truy nhp) mt nt th mc bn pha khng gian tn cn gnkt c gi l mounting point.

Hnh 16. Mouting mt khng gian tn t xa nh mt giao thc truy cp

Thng thng, nu 2 khng gian tn NS1, NS2 - gn kt mt thc th bn ngoitrong h phn tn, chng ta cn ti thiu nhng thng tin sau:

- Tn ca giao thc truy nhp ( c xc nh thc hin giao thc truyn thng)

- Tn ca server (x nh a ch server)

- Tn ca mounting point (xc nh id ca nt trong khng gian tn bn ngoi)

4.1.3 Thc hin mt khng gian tn.

Phn phi khng gian tn

Trong h phn tn, vic qun l tn c thc hin bng cch phn thnh cc mc:

Mc Global: Cha nhng nt th mc mc cao ( gc v con ca n). Trong lp nycc nt th mc t thay i.Kh nng sn sng lp Global c yu cu cao hn sovi cc lp cn li. Nu name server ca lp ny b li th vic phn gii tn khng ththc hin.

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Mc Administrational: Cha nhng nt th mc mc trung gian, n c th cnhm thnh cc nhm, v mi nhm c th c chia cho nhng khu vc qun trkhc nhau. Cc nt trong nhm ny cng t khi thay i. Kh nng sn sng caname server trong lp administrational l rt quan trng i vi cc client do nameserver qun l. V nu server ny li th c rt nhiu cc ti nguyn khng th truy cp

Mc Managerial: Cha nhng nt th mc mc thp. Cc nt trong mc ny thayi kh thng xuyn. V d nh cc host trong mt mng LAN. Yu cu i tnh snsng ca name server ca lp managerial t kht khe hn so vi 2 lp trn. Song vhiu nng th yu cu i vi lp ny cao hn do phi thng xuyn cp nht cc thayi.

Hnh17 . Phn phi khng gian tn

Thc hin phn gii tn

Cch 1: phn gii tn tng tc (interactive name sesolution),vic phn gii tn thchin bng cch truyn v nhn qua li gia client v cc name server cc mc khcnhau. Theo cch ny th cc server khng trao i trc tip vi nhau, mi server ch

phn gii nhn tng ng vi lp xc nh a ch ca server tip theo, kt qu tr

li cho client l a ch ca name server tip theo, v vic lin kt vi server tip theol do client m nhim.

Hnh 18. Phn gii tn tng tc

Cch 2: phn gii tn quy (recursive name resolution), theo cch ny th mi nameserver s gi kt qu n name server tip theo m n tm thy. V c nh vy cho nkhi hon thnh phn gii ton b ng dn.

4.2 nh v cc thc th di ng.

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4.2.1 Tn v vic nh v cc thc th.

Mi thc th u c tn v a ch tng ng, vic nh x t tn n a ch ca thcth c thc hin theo hai phng php: theo m hnh mt lp v theo m hnh hailp.

Theo m hnh mt lp: ch c mt mc nh x gia tn v thc th. Mi ln thc ththay i v tr, nh x cn phi c thay i theo

Theo m hnh hai lp: phn bit tn v a ch nh Entity ID. Gm hai qu trnh: qutrnh tm Entity ID tng ng t tn ca thc th c thc hin bng dch v tn(naming service) v qu trnh xc nh v tr ca thc th t ID c thc hin bi dchv nh v (Location service).

Hnh 20 (a) .M hnh mt lp (b). M hnh hai lp.

4.2.2 Cc gii php nh v thc th .

Broadcasting v multicasting: gi ID cn tm ti tt c cc my. My no c thc th th gi li mt thng bo cha a ch ca access point. Vi phng php ny, yucu tt c cc tin trnh u lng nghe yu cu gi n.

Dng con tr (forwarding pointer): vi mt thc th di ng ri khi v tr A ca nn v tr B th n s li mt tham chiu ti v tr mi ca n. Nh , khi nh vc thc th, client c th xc nh ngay c a ch hin ti ca thc th ny nhvt a ch .

Home-based approaches: cp pht cho mi thc th mt v tr gc (home)

Vi phng php ny s to ra mt home location lu gi a ch hin ti ca ccthc th (thng l ni thc th c to ra ).

a ch ca home c ng k ti naming service.

Home ng k a ch ngoi ca cc thc th

Client lun n home trc tin, v sau tip tc vi cc v tr bn ngoi.

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Hnh .

Hierarchical approaches: xy dng mt cy tm kim phn cp v thc hin phn min cc mc khc nhau. Mi domain hnh dung nh mt nt th mc ring bit dir(d).

Nt gc bit tt c cc thc th. Mi thc th trong mt domain D tng ng vi mtlocation record trong nt th mc dir(D), n l a ch hin ti ca thc th hoc mtcon tr.

Hnh 22 .Hierarchical approaches

a ch ca mt thc th c lu trong mt nt l, hoc mt nt trung gian. Nt trunggian cha mt con tr n mt nt con nu v ch nu cy con nm ti nt con lu trmt a ch ca thc th. Mt thc th c th c nhiu a ch (v d trong trng hpto bn sao).

Hnh 23 . Cu trc nt

Nguyn l c bn: Bt u tm kim cc nt l cc b. Nu nt bit thc th, tiptheo s i xung pha di theo con tr, ngc li i ln trn. Tm kim ln mc caonht l root

Hnh 24 .Nguyn l tm kim

4.3 Xa b cc thc th khng cn c tham chiu (Unreferenced Entities).

4.3.1 m cc tham chiu (Reference Counting).

Mi ln client to(xa) mt tham chiu n mt i tng O, mt b m tham chius tng thm (gim i).

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4.3.2 Ln danh sch cc tham chiu (Reference Listing).

Skeleton duy tr mt danh sch tt c cc proxy tr n n.

y a ra khi nim Idempotent operation l mt thao tc n c th lp i lp linhiu ln m khng nh hng n kt qu ( v d 1*1=1).

Thng ip thm/xa mt proxy ca danh sch cng gn ging nh tng/gim bm tham chiu.

Cc thc th chuyn tham chiu cho cc thc th khc nhng khng th ly c troot.

Tp hp loi b da trn c s truy nguyn: kim tra nhng phng thc c th lyc t root v remove

Chng 5 : ng b ha

(Synchronization)

5.1 ng b ha ng h (Clock Synchronization).

Trong h phn tn,mi my tnh l mt ng h nn vic ng b cc ng h ny lrt cn thit v rt kh khn.

5.1.1 ng h vt l (Physical Clock).

Chng ta c nhiu cch xc nh thi gian.Ph bin nht l cc h m thi giantheo thin vn v y l mt tri.C 23h mt ngy v 3600 giy.Mt giy mt tric tnh l 1/8600 ca mt ngy mt tri.Mt trong nhng m hnh tnh thi gianp dng phng php trn l Internatinal Atomic Time vit tt l TAI. Tuy nhin, TAIli c mt vn l c 86400TAIs s c 3ms chm hn so vi ng h mt tri.

thng nht thi gian vt l ngi ta a ra khi nim thi gian phi hp ton cuUCT (Universal Coordinate Time). Vin chun quc gia M lp ra trm pht radiosng ngn W W V gi UTC khi cn hoc nh k.

5.1.2 Cc gii thut ng b ha vt l (Clock synchronization algorithm).

Nu tt c cc my tnh u c WWV Receiver th vic ng b chng l d dng vtt c u cng ng b vi gi chun quc t UTC.Tuy nhin khi khng c WWV thvic ng b c thc hin bng cc gii thut ng b sau.

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a. Gii thut Cristian

Gi s trong h phn tn c mt my c WWV (gi l Time server ) v chng ta stin hnh ng b cc my khc vi my ny.Trong khong thi gian /2p mi my sgi mt thng ip n my ch hi thi gian hin ti. My ch nhanh s phn hi

bng mt thng ip mang gi tr thi gian C(utc).Bn gi nhn c phn hi n sthit lp li clock thnh C(uct).

Hnh 25 . Xc nh thi gian trong time server

nh gi: gii thut ny c 2 vn :

- Mt l nu clock bn gi chy nhanh th lc ny C(uct) s nh hn thi gian hin tiC ca bn gi..C th gii quyt bng cch thay i nhp ngt li nhanh hn hocchm hn cho n lc khp nhau.

- Hai l s chnh lch t lc C(uct) c gi cho n lc nhn c c th gy li.Giiquyt bng cch ghi nhn khon thi gian gia lc gi v nhn

b. Gii thut Berkeley.

T tng ca gii thut:

Server s ch ng cho cc my khc bit thi gian chun ca mnh CUTC sau syu cu thng tin v thi gian ca cc client.

Client s tr li khong thi gian chnh lch gia n v server.

Server s tnh khong thi gian m cc client so vi thi gian chun ca server lc v gi cho cc my khch cch iu chnh thi gian cho ph hp.

Hnh 26 . ng b theo gii thut Berkeley

c. Gii thut trung bnh

Gii thut ny thc hin chia thi gian thnh nhng khong ng b c nh. Khongthi gian I s bt u t thi im (To + i.R) v chy n khi To+(i+1)R vi To l thiim xc nh trc v R l mt bin h thng .

Vo thi im bt u ca mi ln ng b tt c cc my ca mng s broadcast thigian ca mnh .

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Sau khi broadcast n s bt u thu thp thi gian m cc my khc gi n trongkhong thi gian S. Sau b i gi tr ln nht v nh nht ri tnh trung bnh cacc gi tr thi gian cn li.

5.2 ng h logic (Logical Clock)

Trong nhiu trng hp, gia cc tin trnh khng nht thit phi ph hp theo thigian thc t m ch cn khp vi nhau v thi gian. Do ngi ta a ra khi nimng h

logic.

5.2.1 Nhn thi gian Lamport (Lamport timestamps).

Lamport a ra m hnh ng h logic u tin cng vi khi nim nhn thi gian.

a. Xt nh ngha mi quan h "xy ra trc" ()

Khi c A B : A xy ra trc B th tt c cc tin trnh trong h phn tn tha thun skin A xy ra trc ri n s kin B.

A v B l hai s kin ca cng mt tin trnh. Nu A xy ra trc B th AB l ng.

Nu A l s kin bn tin c gi bi mt tin trnh no , cn B l s kin bn tin c nhn bi mt tin trnh khc th quan h A B l ng.

Quan h xy ra trc c tnh bc cu: A B , B C th A C.

b. Tem thi gian (Time Stamps)

o thi gian tng ng vi 4 s kin x th ta gn mt gi tr C(x) cho s kin vtha mn cc iu kin sau:

Nu A B trong cng mt tin trnh th C(A) < C(B).

Nu A v B biu din tng ng vic gi v nhn mt thng ip th ta c C(A)< C(B)

Vi mi s kin phn bit (khng c lin quan) th C(A)C(B)

5.2.2 Vector thi gian (Vector Timestamps)

Gii thut vector timestamp a ra mt vetor timestamp VT(a) gn cho s kin a cthuc tnh l nu Vtt(a) < VT(b) th s kin l nguyn nhn ca b.

Trong vector thi gian mi tin trnh Pi lu gi mt Vi vi gi tr N (cc tin trnhkhc nhau th N khc nhau)

- Vi[i] l s cc s kin xy ra ti Pi

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- Nu Vi[j] = k ngha l Pi bit c k s kin xy ra ti Pj

Yu cu: mi khi c s kin mi xy ra tin trnh Pi th phi tng Vi[i] v phi mbo vector ny c gi cng thng ip sut trong qu trnh.

Nh bn nhn s bit c c bao nhiu s kin xy ra ti Pi .Quan trng hnpha nhn s bo cho bit l c bao nhiu s kin cc tin trnh khc xy ratrc khi Pi gi thng ip m.Ni cch khc timestamp VT ca n ni cho bn nhn

bit bao nhiu s kin xy ra trong cc tin trnh khc trc m.

Lut cp nht vector

- Thit lp Vi[j] =0 vi mi j,i

- S kin xy ra Pi l nguyn nhn tng Vi[i]

- Pi gn mt timestamp t=V[i] vo mi thng ip gi i- Khi Pi nhn c mt thng ip c t n s thit lp

Vi[j]=Max(Vi[j] ,t[j]) v tng Vi[i]

5.3 Trng thi tng th (Global sate).

Vic xc nh trng thi tng th ca h thng rt c ch. Mt trong nhng phngphp c a ra l Chp Nhanh Phn Tn (Distributed Snapshort) cng khi nim ltct (cut).

Hnh 28 .(a) Lt ct nht qu. (b) Lt ct khng nht qun

Mt lt ct nht qun c biu din l ng chm gch trong hnh a. Lt ct m ts kin cui cng m s kin ny c ghi li cho mi tin trnh. Bng cch ny n cth kim tra li rng tt c cc thng ip nhn u tng ng vi cc thng ip gic ghi li trn ng ct. Ngc li l lt ct khng nht qun nh hnh v b: Thiim tin trnh P3 nhn thng ip m2 c ghi vo lt ct nhng vic ghi li nykhng tng ng vi s kin gi.

5.4 Cc gii thut bu chn (Election Algorithm).

Khi tin trnh iu phi gp li th s phi c qu trnh bu chn chn ra mt tintrnh khc lm iu phi thay cho n. C hai gii thut bu chn hay c s dng l:

5.4.1 Gii thut p o (Bully Algorithm)

Vi gi thit:

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Mi mt tin trnh u c mt ID duy nht.Tt c cc tin trnh khc u c th bitc s ID v a ch ca mi tin trnh trong h thng.

Chn mt tin trnh c ID cao nht lm kha.Tin trnh s khi ng vic bu chnnu nh n khi phc li sau qu trnh xy ra li hoc tin trnh iu phi b trc trc.

Cc bc ca gii thut:

1.P gi thng ip ELEC n tt c cc tin trnh c ID cao hn

2.Nu khng c tin trnh no phn hi th P s tr thnh tin trnh iu phi

3.Nu c mt tin trnh c ID cao hn phn hi th n s m nhim vai tr iu phi.

Hnh 29 .V d theo gii thut p o

5.4.2 Gii thut vng (Ring Algorithm)

Vi gi thit :

Cc tin trnh c mt ID duy nht v c sp xp trn 1 vng trn Logic. Mi mttin trnh c th nhn bit c tin trnh bn cnh mnh.

Cc bc thut ton:

Mt tin trnh bt u gi thng ip ELEC ti cc nt cn tn ti gn nht, qu trnhgi theo 1 hng nht nh. Thm d lin tip trn vng cho n khi tm c 1 ntcn tn ti.

Mi mt tin trnh s gn ID ca mnh vo thng ip gi.

Cui cng s chn ra 1 tin trnh c ID cao nht trong s cc tin trnh cn hot ngv gi thng ip iu phi cho tin trnh .

5.5 Loi tr nhau (Mutual Exclusion).

T chc cc "vng ti hn" (critial section region).

C nhiu gii thut c xy dng ci t c ch loi tr nhau thng qua cc vngti hn. C ba gii thut ph bin l:

5.5.1 Gii thut tp trung (Centralized Algorithm)

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Gi thit: mi tin trnh c mt s ID duy nht. Tin trnh c bu chn lm iuphi l tin trnh c s hiu ID cao nht.

Ni dung thut ton: Khi mt tin trnh no cn vo vng gii hn n s gi mtthng ip xin cp quyn .Nu khng c mt tin trnh no ang trong vng gii hnth tin trnh iu phi s gi phn hi cho php. Cn nu c mt tin trnh khc ang trong vn ti hn ri th tin trnh iu phi s gi thng ip t chi v a tintrnh ny vo hng i cho n khi khng c tin trnh no trong vng ti hn na.

Khi tin trnh mt tin trnh ri khi vng gii hn n s gi mt thng ip n tintrnh iu phi thng bo tr li quyn truy cp.Lc ny tin trnh iu phi s giquyn truy cp cho tin trnh u tin trong hng i truy cp.

nh gi : Thut ton ny c m bo s tn ti duy nht mt tin trnh trong vng tihn v ch cn 3 thng ip thit lp l: Request -Grant - Release .Nhc im duynht l nu tin trnh iu phi b hng th h thng s sp .V nu mt tin trnhang trong trng thi Block n s khng th bit c tin trnh iu phi c b DEAD

hay khng .Trong mt h thng ln nu ch c mt tin trnh iu phi s xut hinhin tng tht c chai

Hnh 30 .v d theo gii thut tp trung

5.5.2 Gii thut phn tn (Distributed Algorithm)

Khi mt tin trnh mun vo vng gii hn, trc ht n s to ra mt nhn thi gianv gi cng vi mt thng ip n tt c cc tin trnh khc. Cc tin trnh khc saukhi nhn c thng ip ny s xy ra ba tnh hung:

Nu bn nhn khng trong vng gii hn v cng khng mun vo vng gii hn thn s gi thng ip OK cho bn gi

Nu bn nhn ang trong vng gii hn thay v tr li n s cho vo hng i yucu ny.

Nu bn nhn cng mun vo hng i th n s so snh timestamp ai thp hn sthng.

Sau khi gi i thng ip yu cu vo vng gii hn tin trnh s i cho n khi c tr

li cng sm cng tt .Khi vo vng gii hn ri th n s gi thng ip OK ntt c cc tin trnh khc v xa cc tin trnh trong hng i i.

Hnh 31 . V d theo gii thut phn tn

5.5.3 Gii thut vng vi th bi (TokenRing Algorithm).

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Gi thit tt c cc tin trnh c sp xp trn mt vng trn logic, cc tin trnh uc nh s v u bit n cc tin trnh cnh n.

Bt u qu trnh truyn, tin trnh 0 s c trao mt th bi. Th bi ny c th luhnh xung quanh vng trn logic. N c chuyn t tin trnh k n tin trnh (k+1)

bng cch truyn thng ip im - im. Khi mt tin trnh ginh c th bi t tintrnh bn cnh n s kim tra xem c th vo vng ti hn hay khng. Nu khng ctin trnh khc trong vng ti hn n s vo vng ti hn. Sau khi hon thnh phnvic ca mnh n s nh th bi ra, th bi c th di chuyn t do trong vng trn. Nu1 tin trnh mun vo vng ti hn th n s gi ly th bi, nu khng n s choth bi truyn qua. Vn ln nht trong thut ton truyn th bi l th bi c th bmt, khi chng ta phi sinh li th bi bi v vic d tm li th bi l rt kh.

Hnh 32. V d theo gii thut vng vi th bi

5.6 Cc giao tc phn tn (Distributed Transactions).

Bn tnh cht ca giao tc i vi th gii bn ngoi: ACID

Tnh nguyn t (Atomic): mi giao tc din ra khng th phn chia c.

Tnh nht qun (Consistent): giao tc khng xm phm cc bt bin ca h thng.

Tnh c lp (Isolated): cc giao tc ng thi khng gy tr ngi cho nhau.

Tnh lu bn (Durable): khi giao tc cam kt th cc thay i i vi n khng phil tm thi m l ko di.

5.6.1 Phn loi cc giao tc

Giao tc c chia thnh cc loi sau:

Limition of Flat Transaction.

Nested Transaction

Distributed Transaction.

5.6.2 iu khin tng tranh:

L qu trnh cho php nhiu giao tc thc hin ng thi m khng sy ra s tranhchp gia cc giao tc. C hai loi tng tranh:

Tng tranh bi quan.

Tng tranh lc quan.

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Chng 6 : Nht qun v nhn bn

(Consisstency & replication)

6.1 t vn .

C hai l do s dng bn sao:

Dng bn sao tng tin cy v tnh sn sng ca h thng: khi d liu b li hay vmt nguyn nhn no m khng th dng c, ta c th dng ngay bn sao d liu h thng khng phi dng li v trnh c tnh trng s dng cc d liu khngchnh xc.

Dng bn sao tng hiu nng ca h thng: c th tng quy m h thng c v slng ln phm vi a l.

Tuy nhin vic s dng nhn bn cng phi tr gi, l tnh nht qun d liu ca hthng b suy gim. Do s dng bn sao nn c th xy ra trng hp c s thay itrn mt d liu m khng cp nht trn cc bn sao ca n. iu ny s gy ra cc saist trong h thng. Do phi tn nhiu cng sc xy dng cc m hnh m botnh nht qun ca d liu.

6.2 Cc m hnh nht qun ly d liu lm trung tm.

6.2.1 M hnh nht qun cht (Strict consistency).

L m hnh tha mn iu kin sau: Thao tc c bt k trn mc d liu x u tr vmt gi tr tng ng vi kt qu ca thao tc ghi gn nht trn x .

S dng khi nim thi gian tuyt i. Thi gian tuyt i ny l tng th cho c hthng xc nh ng khi nim "gn nht". iu ny l kh kh thi vi h phn tn.

Cc k hiu:

- Wi(x)a: thao tc ghi c thc hin bi tin trnh P(i) ln mc d liu x vi gi tr a.

- Ri(x)b: thao tc c c thc hin bi tin trnh P(i) ln mc d liu x cho kt qu

b.

Gi thit x c gi tr ban u l null.

Hnh 33 (a). M hnh nht qun cht. (b) Khng phi l m hnh nht qun cht

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Do vic lan truyn cc b ca P1 cha ti P2 nn P2 c d liu x vn l gi tr nullban u.

M hnh ny l khng kh thi nn a ra m hnh gim nh hn.

6.2.2 M hnh nht qun tun t v m hnh nht qun tuyn tnh.

a. M hnh nht qun tun t.

L m hnh lng lo hn, yu hn m hnh nht qun cht. N tha mn cc yu cusau:

Kt qu ca s thc hin bt k l nh nhau nu thao tc c v ghi do cc tin trnhthc hin trn mc d liu mt cch tun t v cc thao tc ca mi tin trnh xuthin trong chui thao tc ny ch ra bi chng trnh ca n.

Khi cc tin trnh chy ng thi trn cc my khc nhau th cho php s an xen ca

cc thao tc nhng tt c cc tin trnh u phi nhn bit c s an xen ca ccthao tc l nh nhau.

Hnh 34 (a). M hnh nht qun tun t. (b) Khng l m hnh nht qun tun t.

b. M hnh nht qun tuyn tnh.

L m hnh yu hn m hnh nht qun cht nhng mnh hn m hnh nht qun tunt. M hnh ny tha mn iu kin sau: "Kt qu ca bt k s thc hin no l nh

nhau nu cc thao tc (c v ghi) ca tt c cc tin trnh ln d liu c thc hinmt cch tun t v cc thao tc ca mi tin trnh xut hin trong chui thao tc nyphi theo th t c ch ra trong chng trnh ca n. Thm vo , nu tsop1(x)< tsop2(y) th thao tc op1(x) phi c thc hin trc op2(y) trong chui thao tc"

6.2.3 M hnh nht qun nhn qu.

y l m hnh lng lo hn m hnh nht qun tun t. M hnh ny phn bit cc skin c quan h nhn qu v cc s kin khng c quan h nhn qu.

Nu s kin b c gy ra hoc b tc ng bi mt s kin a xy ra sm hn th tnh

nhn qu i hi mi thc th khc phi "nhn" thy a trc ri mi thy b sau.

M hnh nht qun nhn qu tha mn cc iu kin sau: cc thao tc ghi c quan hnhn qu tim nng phi c nhn bit bi tt c cc tin trnh khc trong cng mtth t. Cc thao tc ghi ng thi c th nhn bit c theo th t khc nhau trn ccmy khc nhau.

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Hnh 35 M hnh nht qun nhn qu.

6.2.4 M hnh nht qun FIFO .

Nht qun FIFO cn c gi l nht qun PRAM. y l m hnh yu nht v mhnh ny b qua gii hn v trt t ca bt k thao tc ng thi no. Nht qun FIFOtha mn : "Cc thao tc ghi bi mt tin trnh n phi c tt c cc tin trnh khcnhn thy theo cng mt trt t m chng ra. Nhng thao tc ghi bi nhiu tin trnhkhc nhau c th c thy theo nhng trt t khc nhau bi cc tin trnh khc nhau".

Hnh 36 M hnh nht qun FIFO.

6.2.5 M hnh nht qun yu (Weak consistency).

M hnh nht qun yu khng tp trung vo cc thao tc trn d liu nh cc m hnh

trn m chng quan tm n trt t cc nhm lnh bng vic s dng cc bin cng b.

M hnh nht qun yu c ba c tnh sau:

Vic truy cp n mt bin ng b ha c kt hp vi kho d liu l mt nhtqun tun t.

Khng c thao tc no ln cc bin ng b ha c php thc hin cho n khi ttc cc thao tc ghi trc c hon thnh mi ni.

Khng c thao tc c hay ghi d liu ln cc mc d liu no c php thc hincho n khi tt c cc thao tc trc ln cc bin ng b ha c thc hin.

Hnh 37 (a) M hnh nht qun yu. (b) Khng l m hnh nht qun yu

6.2.6 M hnh nht qun i ra (Release consistency).

S dng thm hai lnh: lnh acquired bo mun vo vng ti hn (critial region) vlnh release bo gii phng vng ti hn. Hai lnh ny cng c hai cch thc thikhc nhau nh: bng mt bin hoc bng mt lnh c bit. Hai thao tc ny ch thc

hin vi cc d liu dng chung ch khng p dng cho tt c cc d liu.

M hnh nht qun i ra tha mn cc iu kin sau:

- Trc khi thc hin mt thao tc c hay ghi ln d liu chia s th tt c cc thao tcacquire do tin trnh ny thc hin trc phi hon tt.

- Trc khi mt thao tc release c php thc hin th tt c cc thao tc c v ghido tin trnh ny thc hin trc phi c hon tt.

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- Truy cp vo cc bin ng b ha l nht qun FIFO (Khng yu cu nht quntun t).

Hnh 38 Trnh t s kin theo m hnh nht qun i ra

6.2.7 M hnh nht qun i vo (Entry consistency).

Cng ging m hnh nht qun i ra, m hnh nht qun i vo cng s dng hai lnhacquired v release khi mun s dng vo vng ti hn. Nhng cc lnh ny thao tctrn tng mc d liu ca vng d liu chia s. Tin trnh no mun s dng mc dliu th phi i cho tt c cc tin trnh khc gii phng mc d liu .

Hnh 39 .Trnh t s kin theo m hnh nht qun i vo. ghi ln mt mc d liu, client phi c c bin ng b ho ca mc trongch dnh ring. iu c ngha l khng client no khc c th s dng bin .Khi client cp nht xong mc d liu, th n gii phng bin .

Khi client mun c mt mc d liu no , n phi c c bin ng b ha kthp ch khng dnh ring. Nhiu client c th gi mt bin ng b ha ch khng dnh ring.

Khi thc hin mt thao tc acquire, client ly v phin bn mi nht ca mc d liu

t tin trnh cui cng thc hin thao tc acquire trn bin .

Nht qun i vo phi tha mn cc iu kin sau:

- Mt thao tc acquire truy cp vo mt bin ng b ha khng c php thchin trong mt tin trnh cho n khi tt c cc cp nht ln mc d liu trong tintrnh c thc hin.

- Trc khi mt truy cp trong ch dnh ring ca mt tin trnh ti mt bin ngb ha c php thc hin th khng tin trnh no khc cn c gi cc bin ngb ha, trong ch khng dnh ring th khng c n yu cu nh vy.

- Sau khi mt truy cp trong ch dnh ring ln mt bin ng b ha c thchin th bt k s truy cp ca tin trnh no khc trong ch khng dnh ring ln

bin cng khng c thc hin cho n khi ch nhn ca bin ng b thc hinxong vic truy cp ca mnh.

6.3 Cc m hnh nht qun ly client lm trung tm.

6.3.1 Nht qun cui cng (eventual consistency).

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Khi mt d liu c nhiu bn sao th yu cu a ra l sau cc thao tc cp nht th ttc cc bn sao cui cng l phi bng nhau. Yu cu ny s c thc hin tt nu miclient lun chu kh cp nht cho cc bn sao.

Nu cc client l di ng th vic thc hin yu cu trn gp kh khn hn. Phi lunm bo rng ngay c khi client thay i v v tr vt l th vic s dng cc bn saocng phi chnh xc. Tc l cc bn sao lun lun l nht qun.

6.3.2 Nht qun c u (monotonic - write consistency).

M hnh nht qun c u phi m bo iu kin sau:

Mt tin trnh thc hin thao tc c trn mt mc d liu th phi m bo bt k thaotc c no cng u cho cng mt kt qu hay kt qu gn y nht.

M hnh nht qun c u m bo rng mt client s lun nhn thy nhng d liu

mi hn v khng bao gi phi nhn thy nhng d liu c hn nhng g m mnh c trc . iu c ngha l khi mt client thc hin mt thao tc c trn mtbn sao ri tip theo li c trn mt bn sao khc th bn sao th hai kia t nht cngphi c ghi ging vi bn sao u tin.

Hnh 41 (a) Kho d liu theo m hnh nht qun c u (b) Kho d liu khng theom hnh nht qun c u

V bn cht th m hnh ny l phin bn hng ngi dng ca m hnh nht qun

FIFO (im khc bit ch n ch p dng i vi mt client).

6.3.3 Nht qun ghi u (monotonic - read consistency).

M hnh nht qun c u phi m bo iu kin sau:

Thao tc ghi trn mc d liu x ca mt tin trnh phi c hon thnh trc bt kmt thao tc ghi no khc trn x bi cng mt tin trnh.

Ni cch khc th cc thao tc ghi ln mt mc d liu s c sp xp mt cch ctrt t.

Hnh 42 (a) Kho d liu theo m hnh nht qun ghi u (b) Kho d liu khng theom hnh nht qun ghi u.

6.3.4 Nht qun c kt qu ghi (Read - your - write consistency)

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Trong m hnh nht qun ny, ngi dng c m bo rng s lun c nhn thynhng kt qu ghi mi nht.

"Tc ng ca mt thao tc ghi ca mt tin trnh ln mc d liu x s lun c nhnthy bi mt thao tc c ln lt trn x ca cng tin trnh ".

Hnh 43 (a) Kho d liu theo m hnh nht qun c kt qu ghi (b) Kho d liu khngtheo m hnh c kt qu ghi

6.3.5 Nht qun ghi theo sau c (write - follow - read consistency).

M hnh nht qun ny ngc vi nht qun c kt qu ghi, n m bo rng mtngi dng s lun thc hin thao tc ghi ln mt phin bn d liu m t nht cng

phi mi bng phin bn cui cng ca n.

"Tc ng bi mt thao tc ghi ca mt tin trnh ln mc d liu x s lun c nhnthy bi mt thao tc c lin tip ln x ca cng tin trnh ".

Hnh 44 (a) Kho d liu theo m hnh nht qun ghi theo sau c (b) Kho d liukhng theo m hnh ghi theo sau c.

6.4 Cc giao thc phn phi (distribution protocols).

6.4.1 Xp t cc bn sao (replica placement).

C 3 kiu bn sao:

Cc bn sao thng trc: trong tin trnh hay trn my lun c mt bn sao. S lngcc bn sao thng xuyn ny rt t, thng c tp hp li thnh nhm cc mytrm (COWs) hoc trong cc h thng phn chiu (mirrored), thng l cc Webserver hay l cc server c cha c s d liu d phng.

Bn sao khi u t server: Cc bn sao ny c s dng lm tng hiu nng. Ccbn sao ny c xp t ng da vo yu cu ca server khc. Mt v d in hnh

l chng c cc cng ty web hosting s dng nh v v tr a l ca cc bn saogn nht khi h cn.

Cc bn sao khi u t client: Cc bn sao ny c to ra t yu cu ca client,chng hn nh vic cache d liu ca mt trnh duyt. Chng c xp t ng davo yu cu ca client.

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Hnh 45 T chc logic ca cc loi bn sao.

6.4.2 Lan truyn cc cp nht

C 3 kh nng lan truyn cc cp nht

Ch thng bo l c cp nht: Thng dng trong vic cache d liu.Thng bo vvic mt hiu lc ca mt giao thc.Phng php ny tt khi t l cc thao tc c sovi thao tc ghi nh.

Truyn d liu cp nht t bn sao ny ti mt bn sao khc.Thc hin tt khi cnhiu thao tc c. Ghi li cc thay i v tp hp cc cp nht li truyn i (chtruyn i cc thay i ch khng truyn c d liu b thay i, v th tit kim c

bng thng).

Lan truyn cc thao tc cp nht ti cc bn sao khc (nhn bn ch ng). Tn t bngthng nhng i hi nng lc x l cao v trong nhiu trng hp th cc thao tc l

rt phc tp.Cc giao thc ko v y

y cp nht: l giao thc do server khi to, trong giao thc ny cc cp nht clan truyn mi khi c mt server khc yu cu.

Ko cp nht: l giao thc do client khi to khi client mun c cp nht.

6.4.3 Cc giao thc bnh dch (epidemic protocol).

y l mt giao thc c th dng thc hin m hnh nht qun sau cng.

tng c bn ca thut ton bnh dch l:

- Gi s rng khng xy ra xung t gia cc thao tc ghi - ghi.

- Cc thao tc cp nht ban u c thc hin ch trn mt hay mt vi bn sao (cngt cng tt).

- Mt bn sao ch gi cc cp nht ca n ti mt s hu hn cc hng xm.

- Vic lan truyn cc cp nht xy ra chm chp v khng phi ngay lp tc.

- Cui cng th mi cp nht cng n c tng bn sao.

Da trn thut ton bnh dch m c cc m hnh lan truyn cp nht. iu ng lutm trong m hnh ny l cc cp nht c lan truyn ti cc bn sao vi cng tthng ip cng tt v cng nhiu bn sao b "nhim" cc lan truyn cng nhanh thcng tt. n cui cng nu bn sao no m khng lan truyn c cp nht ca mnhth n s b loi b. Tuy nhin vic loi b c th s khng d dng.

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Mt trong nhng m hnh lan truyn cp nht c gi l: anti entropy. Trong m hnhny, mi bn sao c nh k li chn ngu nhin mt bn sao khc v trao i nhngtrng thi khc nhau ca mnh, sau mt thi gian th c 2 pha s c nhng trng thiging ht nhau.

Mt m hnh khc l gossiping. Trong m hnh ny mt bn sao c cp nht s"k" cho mt s bn sao khc v nhng cp nht ca mnh v th s lm cho nhng bnsao b nhim nhng cp nht ca mnh.

6.5 Cc giao thc nht qun.

6.5.1 Giao thc Primary-based

a. Cc giao thc ghi t xa

Vi giao thc ny, tt c cc thao tc ghi c thc hin ch trn mt server t xa.

Giao thc ny thng c kt hp vi cc h thng ch khch truyn thng.Mt dng giao thc ghi t xa l giao thc Primary-Backup.

Nhc im ca giao thc ny l vn hiu nng. Tt c cc thao tc ghi trong giaothc u chim kh nhiu thi gian, c bit l khi c giao thm giao thc ghi theokhi c s dng.

u im ca giao thc ny l

- S dng giao thc ghi khng theo khi x l cc cp nht.

- Tt c cc thao tc ghi c th c gi n cc bn sao d phng theo cng mt tht, iu ny to iu kin thun li khi thc thi m hnh nht qun tun t.

b. Cc giao thc ghi cc b

Trong giao thc ny mt bn sao ca mc d liu c duy tr. Khi c yu cu thaotc ghi, mc d liu c nhn bn t server xa chuyn n server cc b. Vic nyc gi l tip cn theo kiu di tr hon ton.

Mt vn c t ra cho cc tin trnh s dng giao thc ny c hoc ghi ln

cc mc d liu l: thi gian tht s nh v c mt mc d liu c th cn lnhn c thi gian tin trnh s dng n.

Mt dng ca giao thc ghi cc b l l giao thc Primary-backup. Trong giao thcny bn chnh c di tr n tin trnh ang mun thc hin vic cp nht, ri sau

bn d phng s c cp nht.

6.5. 2. Cc giao thc Replicated-write.

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Trong cc giao thc ny, thao tc ghi c th c tin hnh ti bt k bn sao no.

V d:

Mt tin trnh c bit s mang cc thao tc cp nht ti tng bn sao. Mt tem thigian Lamport c th c s dng ly cc thao tc v, tuy nhin phng php nykhng c linh hot cho cc h phn tn.

Mt phng php khc l s dng mt b sp xp dy, l mt tin trnh gn cc sID duy nht cho mi cp nht, sau truyn cc cp nht ti tt c cc bn sao.

Giao thc nhn bn ch ng

Trong giao thc ny, cc thao tc ghi c truyn n tt c cc bn sao, trong khi ccthao tc c c thc hin cc b. Giao thc ghi c th c truyn s dng giao tip

point-to-point hay multicast. u im ca giao thc ny l tt c cc bn sao u nhnc cc thao tc cng lc v theo cng mt trt t, v n cng khng cn nh du

mt bn chnh hay phi gi tt c cc thao tc ti mt server.Tuy nhin giao thc ny li i hi phi truyn theo kiu multicast ng hoc phi cmt b sp xp dy tp trung m c 2 phng php ny u kh c th tip cn mtcch linh hot.

Trong giao thc ny c mt vn cn quan tm l "triu gi bn sao". trnh chomt bn sao b gi qun nhiu ln, mt b iu phi c gn mi bn (client vserver), iu ny m bo cho vic ch c mt li gi v mt li p c gi i.

Giao thc Quorumbased

Vi giao thc ny, cc thao tc ghi c thc hin trn mt tp nh nht cc bn sao.Khi thc hin mt thao tc c, ngi dng cng phi lin h vi mt tp cc bn sao tm ra phin bn mi nht ca d liu.

Trong giao thc ny tt c cc mc d liu c kt hp vi mt s phin bn (versionnumber). Mi ln mt mc b sa i th s phin bn ca n cng c tng ln.

Giao thc ny nh ngha ra s i biu c v s i biu ghi, hai i biu ny s xcnh s bn sao phi c lin h trc khi thc hin thao tc c v ghi. S i biuc phi ln hn tng s bn sao, v th tng ca s i biu c v ghi phi ln

hn tng s bn sao. Bng cch ny, mt ngi mun thc hin mt thao tc c thphi m bo vic lin h vi t nht mt bn sao c cha phin bn mi nht ca mcd liu. Vic la chn s lng i biu da vo t l gia thao tc c v ghi cng vicost (bandwidth...) khi thc hin phng php giao tip gia cc nhm.

Cache-coherence protocols

Cache l mt dng c bit ca nhn bn, n c iu khin bi client thay v ciu khin bi server. C nhiu gii php cho vic cache d liu.

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Vi chin lc pht hin s c kt, l chin lc xc nh khi no th s khng nhtqun tht s b pht hin v t loi b nhng d liu gy ra s khng nht qun, thc 2 gii php khc nhau.

- Vi gii php tnh, ti thi im bin dch chng trnh th nhng ch th ph s cthm vo pht hin nhng d liu khng nht qun.

- Vi gii php ng th ti thi im chy chng trnh th c nhng on m kimtra tnh khng nht qun ca d liu cache vi d liu ca server.

Vi chin lc p buc s c kt, l chin lc xc nh xem d liu cache cgi nht qun vi d liu lu trn server nh th no, th c 2 cch buc gi liu

phi c kt vi nhau. l:

- cho server gi i mt thng ip v s khng hp l mi khi d liu b thay i.

- Cp nht cc k thut lan truyn.Cc thao tc ghi d liu vo cache c tin hnh nh sau:

- Vi cache ch c ra (Read-only Cache) th cc cp nht c thc hin bi server(bng giao thc y) hoc bi client (bng giao thc ko mi khi client nhn thy dliu cache c).

- Vi cache ghi thng (Write-Through Cache) th client s thay i ni dung cacache, sau s gi cc cp nht n cho server.

- Vi cache ghi li (Write-Back Cache) th client tr hon s lan truyn cc cp nht,cho php nhiu cp nht c to ra cc b sau gi nhng cp nht mi nht choserver (vic ny c kh nng nh hng mnh n hiu nng).

Chng 7: Chu li

(Fault Tolerance)

7.1 Chu li v mt s khi nim lin quan

7.1.1 Mt s khi nim c bn.

a. Cc khi nim.

Tnh chu li lin quan nhiu ti khi nim h c th tin cy c (dependablesystem). Thut ng "c th tin cy c" bao gm cc thuc tnh sau:

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Tnh sn sng (availability): h thng c tnh sn sng l h thng lun sn sng hotng tt mi thi im.

Tnh tin cy (Reliability): mt h thng c tnh tin cy l h thng c kh nng hotng trong mt thi gian di m khng b gin on, khng xy ra li.

Tnh an ton (Safety): h thng c tnh an ton l h thng m khi xy ra li cngkhng dn ti thm ha. Cc h thng cn phi c an ton cao l cc h thng iukhin.

Kh nng bo tr (Maintainability):h thng c kh nng bo tr l h thng c khnng phc hi li c sau khi c li. Nu s phc hi ny din ra t ng th c thni h thng ny cng c tnh sn sng cao.

Tnh chu li cn c lin quan ti khi nim iu khin li (Fault control). iu khinli bao gm ngn nga li, loi b li v d bo li vi mc tiu xy dng thnh cngkh nng chu li cho h thng.

b. Phn loi li.

Li c phn chia thnh cc loi sau:

Li nht thi (Transient faults): L loi li xut hin mt ln ri bin mt. Cch khcphc: thc hin li hot ng c li ny

Li lp (Intermittent faults): L loi li m chng xut hin, ri bin mt, sau lixut hin li v c tip tc nh th. Li ny thng gy ra cc hu qu trm trng vchng rt kh xc nh c.

Cch khc phc: s dng b sa li cho h thng (fault doctor) khc phc li.

Li lu di (Permanent faults):L loi li vn tn ti ngay c khi thnh phn gy li c sa cha.

7.1.2 Cc m hnh li.

Li sp (crash failure): khi server gp li ny th n s b treo, trc server vnhot ng tt cho n khi ngng hot ng. Khi server gp li ny, n s khng thlm g c na. Mt v d hay gp li ny l h iu hnh ca cc my c nhn. Khi

h iu hnh ngng hot ng th ch cn cch duy nht l khi ng li.

Li b st (omission failure): l li m mt server khng th p ng c yu cu giti n. Ngi ta chia n thnh hai loi:

Li khi nhn thng ip gi ti: gp li ny, server khng nhn c yu cu ngay ct client gn n nht v mc d kt ni gia server vi client c thit lp. Li khinhn thng ip ch lm cho server khng nhn bit c cc thng ip gi ti n mkhng h nh hng n trng thi ca server.

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Li khi gi thng ip: server vn nhn c cc yu cu, vn hon thnh yu cu nhng v mt l do no li khng th gi kt qu ti my yu cu. Mt trongnhng l do thng gp l do b nh m gi y. Trong trng hp gp li ny,server cn chun b tnh hung clien s gi li yu cu gi .

Li thi gian (timing failure): l li xy ra khi server phn ng li qu chm, sau cthi gian cho php. Trong mt h thng lun c cc rng buc v mt thi gian. Nu

bn gi gi n bn nhn nhanh qu, b nh m ca bn nhn khng cha ths gy ra li. Tng t, server phn ng li chm qu, vt qu khong timeout quynh sn cng s gy ra li, nh hng n hiu nng chung ca h thng.

Li p ng (Response failure): l li khi server tr li khng ng. y l mt kiuli rt ngim trng v c phn chia thnh hai loi:

Li v mt gi tr: l li khi server tr li li yu cu ca client vi gi tr khng chnhxc. V d khi s dng cc my tm kim, kt qu tr v khng h lin quan g ti yu

cu ca ngi s dng.Li v chuyn trng thi: l li khi server hot ng trch hng khi lung iukhin. C ngha l server tr li cc yu cu c gi ti mt cch khng theo nhmong i.

Li bt k (Arbitrary failure): mt server c th to ra mt li bt k bt k thi gianno. y l loi li nguy him nht. C th c hai kh nng xy ra:

Th nht: mt server to ra mt kt qu sai m khng th pht hin ra c.

Th hai: server b li c lin kt vi cc server khc to ra mt kt qu sai.

Ta c th xt mt vo li bt k hay gp sau : li fail-stop, li fail-silent v li fail-safe.Vi fail-stop, server b treo, ngng hot ng v c thng bo ti cc tin trnh khc.Vi fail-silent, server t ngt hot ng chm li v th lm cho cc tin trnh khngth kt thc c, nh hng n hiu nng ca h thng. Li fail-safe l li m khiserver to ra kt qu ngu nhin nhng cc tin trnh nhn dng cc kt qu ny lkhng c gi tr.

7.2 Cc phng php che giu li.

7.2.1 Che giu li bng d tha.

C ba loi d tha: d tha thng tin, d tha thi gian v d tha vt l.

D tha thng tin : b sung thm cc bit d tha pht hin li v phc hi li. V dtrong vic truyn d liu thng thm vo cc bit kim tra chn l, m Haming,CRC... pht hin li v khi phc li.

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D tha thi gian: khi mt hot ng c thc hin, nu d tha thi gian n cth c thc hin li. K thut d tha thi gian ph hp khi li l ngn v khng lintc. V d: khi mt giao tc b hy gia chng, n c th c thc hin li m khnggy nguy hi g.

D tha vt l: b sung thm ti nguyn

7.2.2 Khi phc tin trnh.

a. Cc vn khi thit k.

Nguyn tc: t chc cc tin trnh ging nhau vo cng mt nhm.

Hot ng: khi nhm nhn c thng bo th thng bo ny s c gi ti tt c ccthnh vin trong nhm. Nu c tin trnh no trong nhm b li th s c tin trnh khcthay th .

c im: cc nhm ny c th l ng. Tnh ng th hin cc mt sau:S lng cc nhm l khng c nh: c th to thm hay hy b mt nhm.

S lng cc tin trnh trong cng mt nhm l khng c nh: mt tin trnh c thgia nhp hay ri khi nhm.

Mt tin trnh c th l thnh vin ca nhiu nhm trong cng thi im.

Do tnh ng m cn phi a ra cc c ch qun l nhm: qun l mi quan hgia cc nhm v qun l thnh vin trong mt nhm.

Phn loi nhm: da trn cu trc bn trong th nhm c phn thnh hai loi:

Nhm ngang hng:

- Tt c cc tin trnh trong nhm l ngang hng nhau.

- Khi thc hin mt cng vic no s phi c mt qu trnh bu c (vote) xcnh xem tin trnh no ph hp thc hin cng vic .

- u im: khi mt tin trnh b li th ch lm cho kch thc ca nhm gim i ch

khng nh hng n hot ng ca c nhm.

- Nhc im: do phi c qu trnh bu c nn tn thi gian (delay &overhead).

Hnh 46. Nhm ngang hng.

Nhm phn cp:

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- Trong mi nhm s c mt tin trnh gi vai tr qun l gi l coordinator, cn cctin trnh khc ng vai tr thc hin (worker). Cc tin trnh thc hin chu s iukhin ca coordinator.

- Khi c yu cu gi n nhm, yu cu ny s c gi ti coordinator. Coordinators quyt nh xem tin trnh no trong nhm m nhim cng vic mt cch phhp nht v chuyn yu cu nhn c n tin trnh .

- u im: khng b tr nh kin trc ngang hng.

- Nhc im: khi coordinator gp s c th ton b hot ng ca nhm s b dngli.

Hnh 47. Nhm ngang hng

Cc phng php qun l thnh vin trong nhm:

Phng php 1: dng mt server gi l group server

Server ny cha tt c cc thng tin v cc nhm v cc thnh vin ca tng nhm.

u im: hiu qu, d s dng

Nhc im: nu server b li th khng th qun l c ton b h thng v ccnhm c th phi xy dng li t u cc cng vic mnh thc hin.

Phng php 2: phng php phn tn.

Khi tin trnh mun gia nhp hay ri khi nhm th n phi gi bn tin thng bo titt c cc tin trnh khc.

Phng php 3: yu cu vic gia nhp/ ri khi nhm phi ng b vi bn tin gi haynhn.

Khi mt tin trnh gia nhp nhm n s nhn tt c cc bn tin t nhm .

Khi mt tin trnh ri khi nhm th n s khng c nhn bt k bn tin no t nhm na v khng mt thnh vin no ca nhm c nhn c cc bn tin t n

b. Che giu li v nhn bn.

C hai phng php nhn bn : bng giao thc primary-based v bng giao thcreplicated-write

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Bng giao thc primary-based: Cc tin trnh trong nhm t chc theo m hnh phncp. Nu coordinator ca nhm chnh dng hot ng th coordinator ca cc nhmsao lu s thc hin cc gii thut la chn nhm chnh mi (mc d n c th mnhim cng vic ).

Bng giao thc replicated-write : Cc tin trnh trong nhm t chc theo m hnhnhm ngang hng.Vn l cn nhn bn vi s lng l bao nhiu

7.2.3 Che giu li trong truyn thng client/server tin cy.

Vic che giu li trong h phn tn tp trung vo trng hp c tin trnh b li.Nhng ta cng phi xt n trng hp cc giao tip b li. Thng thng, mt knhgiao tip c th gp cc li: li sp , li b st, li thi gian v li ty . Vic xydng mt knh truyn thng tp trung vo che giu li sp v li ty .

a. Truyn thng im - im .

Trong h phn tn, truyn thng im - im tin cy c thit lp bng cch s dngcc giao thc truyn tin cy nh TCP. TCP che giu c li b st bng cch dng cch thng bo ACK/NACK v vic thc hin truyn li. TCP khng che giu c lisp . Khi xy ra li sp th kt ni TCP s b hy. Ch c mt cch che giuli sp l h thng phi c kh nng t ng to mt kt ni mi.

b. RPC khi xy ra li v cch khc phc

Vi h thng RPC, nm lp li c th xy ra l:

- Client khng th nh v c server: Nguyn nhn gy li l do server v client

dng cc phin bn khc nhau hoc do chnh server b li. Khc phc bng cch sdng cc ngoi l (exception) bt li nh ngn ng java v iu khin tn hiu(signal handle) nh ngn ng C. Hn ch ca phng php ny l khng ph i ngnng no cng h tr ngoi l hay iu khin tn hiu. Nu t vit mt ngoi l hay iukhin tn hiu th s ph hy tnh trong sut.

- B mt bn tin yu cu t client gi n server: y l loi li d x l nht: h iuhnh hay client stub kch hot mt b m thi gian (timer) khi gi i mt yu cu.Khi timer tr v gi tr 0 m khng nhn c bn tin phn hi t server th n sgi li yu cu . Nu bn client nhn thy c qu nhiu yu cu phi gi li th n sxc nhn rng server khng hot ng v s quay li thnh kiu li "khng nh v

c server"

- Server b li ngay sau khi nhn c yu cu t client: Lc ny li phn chia thnhhai loi:

Loi 1: Sau khi thc hin xong yu cu nhn c th server b li. Phng php khcphc: sau server s gi thng bo hng cho client

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Loi 2: Va nhn c yu cu t client server b li ngay. Phng php khc

phc: client ch cn truyn li yu cu cho. Vn t ra lc ny l client khng thni cho server bit yu cu no l yu cu c gi li.

Khi gp li kiu ny, pha my server s thc hin theo 3 k thut sau:

K thut 1: i n khi no server hot ng tr li, n s c thc hin yu cu nhnc trc khi li . Nh th RPC thc hin t nht mt ln.

K thut 2: server sau khi c khi phc n s khng thc hin yu cu nhn ctrc khi b li m s gi li thng bo hng cho client bit client gi li yu cu.Vi k thut ny th RPC thc hin nhiu ln nht.

K thut 3: khng thc hin g m bo c. Khi server b li, client khng h haybit g c. Kiu ny, RPC c th c thc hin nhiu ln cng c th khng thc hinln no.

Cn client th c th thc hin theo 4 chin lc sau:

Mt l: Client khng thc hin gi li cc yu cu. V th khng bit bao gi yu cu mi thc hin c hoc c th khng bao gi c thc hin.

Hai l: Client lin tc gi li yu cu: c th dn ti trng hp mt yu cu c thc

hin nhiu ln.

Ba l: Client ch gi li yu cu no khi khng nhn c bn tin ACK phn hi tserver thng bo nhn thnh cng. Trng hp ny, server dng b m thi gian.Sau mt khong thi gian xc nh trc m khng nhn c ACK th client s gili yu cu .

Bn l: Client gi li yu cu nu nhn c thng bo hng t server.

- Mt bn tin phn hi t server gi tr v client: Phng php khc phc: thit k ccyu cu c c tnh khng thay i gi tr (idempotent). Client nh s th t cho cc

yu cu, server s nhn ra c u l yu cu c gi li nh cc s t t ny.Do server s khng thc hin lp li cc yu cu. Tuy nhin server vn phi gi trv bn tin thng bo yu cu no b tht lc. Hoc ta c th s dng mt bit phnheader ca yu cu phn bit yu cu no l yu cu c gi li.

- Client b li ngay sau khi gi yu cu ti server: Client gi yu cu ti server ri bli trc khi nhn c tr li t server gi v. Cng vic m server thc hin nhngkhng c ch no i nhn c gi l mt "orphan". Nh th s gy lng ph chuk CPU.

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C 4 gii php c a ra trong trng hp ny l:

Mt l: trc khi gi i yu cu no , client stub s to ra mt bn ghi xc nh cngvic cn thc hin ny v lu li. Nh th, khi c phc hi sau khi li, client s lyli bn ghi v v vic thc hin cc orphan ang din ra s dng li. Phng phpny c nhiu nhc im: Chi ph trang b a lu li mi bn ghi cho mi RPC.Orphan c th t mnh thc hin RPC to ra mt grandorphan nn rt kh xc nh.

Hai l: chia thi gian hot ng lin tc ca client thnh cc s lin tc gi l cc thik. Mi khi cc clietn khi phc tr li th s ch thi k ny li tng ln mt n v.Lc ny clietn s gi thng bo n tt c cc my khc thng bo s thi k mi camnh. Khi nhn dc thng bo ny th cc orphan s dng li

Ba l: khi nhn c bn tin thng bo thi k mi, mi my s kim tra xem mnh cang thc hin mt tnh ton t xa no khng. Nu c, my s c xc nh xemclient no gi yu cu ny. Nu khng xc nh c th qu trnh tnh ton ny s

b hy b.Bn l: quy nh mi RPC ch c mt khong thi gian xc nh T thc hin, saukhi gp li, clietn s pho i thm mt khong thi gian T trc khi khi ng li nhn cc orphan. Vn t ra l phi la chn gi tr T nh th no cho hp l.

7.2.4 Che giu li trong truyn thng nhm tin cy (dng multicasting)

a. Multicasting tin cy c bn (Basic Reliable-multicasting).

Sau khi cc tin trnh c phn nhm th mt tin trnh khc mun thc hin

multicast tc l s gi bn tin ti tt c cc t in trnh trong nhm . Multicast tin cyl phi c c ch m bo bn tin n c tt c cc thnh vin trong nhm.Khi xy ra li th s p dng phng php sau che giu li:

Phng php: nh s cc bn tin cn gi. Cc bn tin c lu ti mt buffer ca bngi v vn lu cho n khi nhn c bn tin ACK bo v t bn nhn. Nu bnnhn xc nh l b mt mt bn tin no th n s gi v mt bn tin NACK yucu gi li. V thng thng, bn gi s t ng gi li bn tin sau trong khong thigian xc nh no m n khng nhn c bn tin ACK bo v.

Hnh 48. (a). Truyn bn tin (b). Bn tin phn hi

b. Multicast tin cy m rng.

tng hiu qu cng vc khi lm vic vi mt s lng ln cc tin trnh th a ram hnh multicast tin cy m rng. Vi m hnh ny s khng gi tr v bn tin ACK

bo nhn thnh cng m ch gi tr v cho tin trnh nhn bn tin NACK thng bo

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khi c li truyn.Vic ny c thc hin bng giao thc SRM (Scalable ReliableMulticasting).

c th thc hin multicast tin cy cho mt nhm ln cc tin trnh th thc hin tchc cc nhm theo cu trc dng cy. Cu trc ca cy :

- Gc l nhm cha tin trnh gi.

- Cc nt l cc nhm c cha tin trnh nhn.

Hnh 49. Multicast tin cy dng cy

Vic thc hin multicast c thc hin cho cc nhm nh . Vic chia thnh ccnhm nh hn ny cho php s dng cc kch bn multicast tin cy cho tng nhmnh .

Trong mi nhm nh s c mt tin trnh lm coordinator. Coodinator c kh nngiu khin vic truyn li khi nhn c thng bo truyn li. Coodinator ca minhm s c b m (history buffer) ring.

- Nu Coordinator ca mi nhm khng nhn c bn tin m th n s gi yu cutruyn li ti coordinator ca nt cha n.

- Trong kch bn truyn tin cy s dng bn tin ACK th khi coordinator nhn thnhcng mt bn tin m n s gi bn tin ACK ti coordinator ca nt cha n.

- Nu coordinator ca mt nhm nhn c bn tin ACK bo nhn thnh cng bn tinm ca tt c cc tin trnh trong nhm gi v th n s xa bn tin m khi b m can.

nh gi: vi phng php phn cp ny th xy ra vn v cu trc cy. Rt nhiutrng hp yu cu cy phi c cu trc ng nn phi c mt c ch tm ng chocy ny

c. Multicast nguyn t (Atomic multicast ).

T tng chnh: khi mt tin trnh mun gi bn tin cho mt tp cc tin trnh khc

theo kiu multicast, n s khng gi bn tin ti tt c cc tin trnh ca nhm cha cctin trnh nhn m ch gi n mt nhm nh cc tin trnh cn nhn bn tin .

Vn t ra: phi m bo gi c bn tin ti tt c cc tin trnh trong nhm hockhng c gi ti bt k tin trnh no nu mt tin trnh trong nhm b li sp .

Mt s thut ng:

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Group view (khung nhn nhm): tng chnh ca atomic multicast l mt tin trnhthc hin multicast bn tin m th ch thc hin lin kt ti mt danh sch cc tin trnhcn nhn bn tin m ch khng phi ton b nhm. Danh sch cc tin trnh nytng ng vi mt khung nhn nhm (group view)- mt tp nh cc tin trnh ca mtnhm ln.

View change (thay i khung nhn): khi ang thc hin multicast ti mt group viewG m c mt tin trnh xin gia nhp nhm hay xin ra khi nhm th s thay i vc nys c gi ti tt c cc thnh vin cn li trong nhm. Do , cc tin trnh cn litrong G s nhn c hai bn tin:

m: bn tin cn nhn

vc: bn tin thng bo c thay i trong G.

Nu tt c cc tin trnh trong G u cha nhn c vc th thao tc multicast bn tinm c thc hin.

Nu mt trong s cc tin trnh trong G nhn c vc th pho m bo rng khngmt tin trnh no khc trong G c nhn m na

ng b o (Virtual sychronous).

T tng chnh: m bo bn tin ch c multicast ti tt c cc tin trnh khng cli. Nu tin trnh gi b sp trong qu trnh multicast th qu trnh ny b hy ngayd bn tin c gi ti mt vi tin trnh khc trong nhm ri.

Hnh 50 Nguyn l ng b o

1: P1 tham gia vo nhm c sn ba thnh vin: P2,P3, P4.

2: P2 thc hin multicast bn tin ti tt c cc tin trnh cn li.

3: P1 thc hin multicast bn tin ti tt c cc tin trnh cn li.

4: P3 multicast ti tin trnh P2 , P4 thnh cng nhng P1 cha nh n c th P3 bsp . Lc ny ng b o s hy tt c cc bn tin c gi trc cho P2, P4,

thit lp trng thi trc khi sp ca P3 l cha gi bn tin d.

5: nhm lc ny ch cn P1, P2, P4 v P4 thc hin multicast bn tin,

6: P3 c khi phc v xin gia nhp li nhm.

7: P3 gia nhp nhm thnh cng.

7.3 Cam kt phn tn.

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M hnh thit lp cam kt phi l m hnh phn cp v coordinator lnh trch nhimthit lp cam kt phn tn. cam kt mt pha n gin, coordinator thng bo vi ttc cc thnh vin cn li hoc l thc hin hoc l khng thc hin mt thao tc no. Nu thnh vin no khng thc hin c cng khng th bo li chocoordinator bit. Do ngi ta a m hnh mi l cam kt hai pha v cam kt ba

pha

7.3.1 Cam kt hai pha.

Xt mt giao dch phn tn vi cc thnh vin l mt tp cc tin trnh chy mt mykhc vi gi thit khng c li xy ra.

Cam kt hai pha gm hai: Pha bu c (voting phase )v pha quyt nh (Decisionphase).

Vi pha bu c: bao gm hai bc thc hin:

- Coordinator gi mt bn tin thng bo yu cu bu c VOTE_REQUEST ti tt ccc thnh vin trong nhm.

- Sau khi nhn c bn tin VOTE_REQUEST ca coordinator, nu c th thc hinc th thnh vin s gi li cho coordinator thng bo chp nhn bu cVOTE_COMMIT, nu khng, s gi li cho coordinator thng bo t chiVOTE_ABORT.

Pha quyt nh: gm hai bc thc hin:

- Coordinator tp hp tt c cc bu c ca cc thnh vin. Nu tt c u ng chpnhn giao dch th coordinator s gi mt bn tin GLOBAL_COMMIT ti tt c ccthnh vin. Tuy nhin, ch cn mt thnh vin gi thng bo t chi thcoordinatorquyt nh hy giao dch trn v s gi mt bn tin GLOBAL_ABORTcho tt c cc thnh vin trong nhm.

- Cc thnh vin sau khi gi thng bo chp nhn ti coordinator s i phn hi tcoordinator. Nu n nhn v thng bo GLOBAL_COMMIT th giao dch s cchp thun, cn nu nhn c GLOBAL_ABORT th giao dch s b hy.

Cam kt hai pha a ra mt s cc trng thi hu hn nh hnh 7.17 a, b (394).

Cc trng thi ca mt coordinator l: INIT, WAIT, ABORT, COMMIT. Cn cctrng thi ca mt thnh vin bt k l : INIT, READY, ABORT, COMMIT.

Hnh 51 (a) My trng thi hu hn cho coordinator trong cam kt 2 pha (b). Mytrng thi hu hn cho thnh vin

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Nhc im ca cam kt hai pha: Nhc im chnh ca cam kt hai pha l tn nhiuthi gian ch i. C coordinator v cc thnh vin cn li u phi ch mt bn tinno c gi n cho mnh.

Nhc im th hai l nu coordinator b li th hot ng ca c h thng s b nhhng.

7.3.2 Cam kt 3 pha

khc phc nhc im ca cam kt hai pha trong trng hp coordinator b li,ngi ta a ra m hnh cam kt ba pha. Cc trng thi kh ging hai pha nhng thmmt trng thi PRECOMMIT.

Hnh 52 (a) My trng thi hu hn cho coordinator trong cam kt 2 pha (b). Mytrng thi hu hn cho thnh vin

7.4 Phc hi.

Phc hi l cc phng php a trng thi b li sang trng thi lnh (fault free). Chai cch tip cn cho phc hi li: phc hi li (back forward) v phc hi tin(forward recovery).

Phc hi li: khi thc hin phc hi li s thc hin phc hi trng thi lnh ca hthng trc khi c li v cho h thng chy li t im . c th thc hin ciu ny phi s dng cc im checkpoint. Ti cc im ny s sao lu trng thihin hnh ca h thng khi khi phc s cho chy im checkpoint gn nht. Vic

thc hin theo phng php ny l rt tn km.

Phc hi tin: chuyn h thng t trng thi li sang trng thi mi vi cc thng tin tip tc thc hin

Chng 8: An ton - An ninh.

(Security)

8.1 t vn .

8.1.1 Cc mi e da, chnh sch v c ch an ton , an ninh.

a. Cc mi e da.

H thng my tnh lun b e da bi cc nguy c mt an ton. Mt trong nhng cngvic bo v h thng l lm sao gip h thng trnh khi cc nguy c . C 4 loicc mi e da an ton:

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Interception (chn bt): ch thnh phn khng c php cng c th truy cp n ccdch v hay cc d liu, "nghe trm" thng tin ang c truyn i.

Interruption (t on): l mi e da m lm cho dch v hay d liu b mt mt, bhng, khng th dng c na...

Modification (thay i): l hin tng thay i d liu hay can thip vo cc dch vlm cho chng khng cn gi c cc c tnh ban u.

Fabrication (gi mo): l hin tng thm vo d liu ban u cc d liu hay hotng c bit m khng th nhn bit c n cp d liu ca h thng.

b. Cc c ch an ton, an ninh.

C 4 c ch an ton, an ninh c a ra:

Mt m (Cryptography): l vic thc hin chuyn i d liu theo mt quy tc no thnh dng mi m k tn cng khng nhn bit c.

Xc thc (Authentication): l cc thao tc nhn dng ngi dng, nhn dng clienthay server...

y quyn (Authorization).: chnh l vic phn nh quyn hn cho mi thnh phn ng nhp thnh cng vo h thng. Quyn hn ny l cc quyn s dng dch v, truycp d liu...

Kim ton (Auditing): l cc phng php xc inh c client truy cp n d

liu no v bng cch no.

8.1.2 Cc vn khi thit k.

a. iu khin (focus of control).

C ba cch tip cn:

Chng cc thao tc bt hp l: vic ny thc hin bng cch bo m ton vn chnhcc d liu m khng quan tm n vic phn tch s hp l ca thao tc.

Hnh 53 Chng cc thao tc bt hp l

Chng cc triu gi thao tc khng c y quyn.: khng bo m ton vn d lium tp trung vo cc thao tc. Thao tc no l bt hp l s b hy b ngay.

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Hnh 54. Chng cc triu gi thao tc khng c y quyn

Chng ngi s dng khng c y quyn: cch tip cn ny li tp trung vo qunl ngi dng. Xc nh ngi dng v cc vai tr ca h trong h thng c khngquan tm n m bo d liu hay qun l cc thao tc ca ngi dng.

Hnh 55. Chng ngi s dng khng c y quyn

b. Phn tng cc c ch an ton (Layer of security mechanism)

Mt vn quan trng trong vic thit k mt h thng an ton l quyt nh xem cch an ton an ninh c t tng no. Vic xc nh v tr t ph thuc rtnhiu vo yu cu ca client v cc dch v an ton, an ninh ca tng tng.

Trong mt h phn tn, c ch an ton, an ninh c t tng middleware.

c. Phn tn cc c ch an ton (Distribution of security mechanism)

Xt khi nim TCB (Trusted Computing Base): l tp hp tt c cc c ch an ton, anninh trong h phn tn, cc c ch ny phi tun theo mt rng buc an ton no .

8.1.3 Mt m (Cryptography)

Mt c ch an ton, an ninh c bn trong h phn tn l m mt. T tng c bnl: bn gi m ha bn tin cn truyn, truyn bn tin m ha i, bn nhn s gii m

bn tin nhn c thnh bn tin ban u.

Gi:

Bn tin ban u l P.

Kha m ha l Ek.

Bn tin c m ha theo kha Ek l C: C=Ek(P).

Kha gii m l Dk.

Bn tin c gii m theo kha gii m: P=Dk(C).

C hai loi h thng mt m: mt m i xng (symmetric cryptosystem) v mt mbt i xng (asymmetric cryptosystem).

a. Mt m i xng: dng kha b mt..

Vi mt m i xng: kha m ha v kha gii m l ging nhau. Ta c:

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P=Dk(Ek( P ) ) . C bn nhn v bn gi u phi c kha trn, kha phi c gi bmt.

Nguyn l chung ca gii thut DES (Data Encryption Standard):

Hnh 56 nguyn l chung ca DES

Thc hin trn cc khi d liu 64 bit. Mi khi ny c m ha qua 16 vng lp,mi vng c mt kha m ha 48 bit ring. 16 kha ny c sinh ra t 56 bit khachnh.

u vo ca vng lp m ha th i l d liu c m ha ca vng lp th (i-1).64 bit d liu qua mi vng lp c chia thnh hai phn bng nhau: Li-1 v Ri-1,cng bng 32 bit . Phn d liu bn phi Ri-1 c ly lm phn bn tri ca d liucho vng sau: Ri-1= Li. Hm f vi u vo l Ri-1 v kha Ki sinh ra khi 32 bit c

XOR viLi-1 sinh ra Ri.

Hnh 57 .Mt vng m ha

Phng php sinh kha ca gii thut DES:

Hnh 58. Sinh kha theo gii thut DES

Mi kha 48 bit cho mi vng lp c sinh ra t kha chnh 56 bit nh sau: hon v kha chnh, chia i thnh hai phn 28 bit. Ti mi vng, mi mt na s quay trimt hoc hai bit, sau ly ra 24 bit kt hp vi 24 bit ca na cn li to ra kha.

b. Mt m bt i xng: dng kha cng khai.

Mt m bt i xng: kha m ha v kha gii m l khc nhau. Ta c:

P=DkD(EkD ( P )). Trong h thng ny, mt kha s c gi b mt cn mt kha sc cng khai.

Xt gii thut RAS (c t theo tn ca cc nh pht minh ra n:Rivest, Shamir,Adleman) :

Cch sinh kha ca gi thut RAS: thc hin theo 4 bc:

- Chn 2 s chnh ln: p,q

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- Tnh n = p.q v z = (p-1).(q-1)

- Chn mt s d lin quan n z

- Tnh ton e sao cho e.d =1 mod z.

Nh th d c th dng gii m cn e dng m ha. Ta c th cng khai mt tronghai s ny, ty thut ton.

Nguyn l chung ca gii thut RAS:

Coi bn tin c truyn i l mt dy cc s nh phn. Chia bn tin m thnh cckhi c kch thc c nh mi sao cho 0

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Hnh 61. Reflection Attack

b. Xc thc da trn trung tm phn phi kha.

Nu h thng gm N host, mi host phi chia s mt kha mt vi N-1 host khc thh thng cn qun l N.(N-1)/2 kha, v mi host phi qun l N-1 kha. Nh vy nu

N ln s rt kh khn trong vic qun l. Do , khc phc hin tng trn ta sdng trung tm phn phi kha KDC (Key Distribution Center).

T tng chnh: bn gi s gi bn tin ti trung tm phn phi kha thng bo mnhmun giao tip vi bn nhn. KDC s gi cho c bn gi v bn nhn mt bn tin ccha kha b mt KA,B . Bn tin gi cho bn nhn s c m ha bng KA,KDC .Bn tin gi cho bn gi s c m ha bng KB,KDC .

Hnh 62 Nguyn l ca KDCCch tip cn th hai l KDC s gi c hai bn tin cha kha b mt KA,KDC (KA,B )v KB,KDC (KA,B ) cho bn gi v bn gi c nhim v gi cho bn nhn kha c KDC m ha KB,KDC (KA,B ) .

Hnh 63 Dng ticket

c. Xc thc da trn kha cng khai.

Hnh 64. Xc thc da trn kha cng khai.

Bn gi m ha yu cu bng kha cng khai K+B ca bn nhn. Bn nhn ny l niduy nht c th gii m bn tin bng K-B. Bn nhn s m ha yu cu ca bn gicng vi yu cu ca chnh mnh v kha KA,B va to ra bng kha cng khai K+Aca bn gi nhm xc thc bn gi. Cui cng, bn gi s gi li cho bn nhn yucu RB ca bn nhn gi i xc thc.

8.2.2 Tnh ton vn v tnh mt ca thng ip.

a. Ch k s.

Ch k s m bo tnh ton vn ca thng ip.

C nhiu cch thit lp ch k s cho thng ip:

Cch 1: dng h mt m kha cng khai l RSA.

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Hnh 65 Ch k s cho mt bn tin dng kha cng khai

Bn gi s m ha bn tin bng kha ring K-A ca mnh, sau s m ha tip nidung bn tin v phin bn ch k bng kha cng khai K+B ca bn nhn. Bn tinc m ha ny s c truyn i cng bn tin m. Bn nhn sau khi nhn c bntin s gii m gi tin, ly phin bn ch k ca m v so snh vi m xc thc rng

bn tin ny c gi t bn gi v cng kim tra xem c thay i trn ngtruyn hay khng.

Cch 2: dng hm bm.

Hm bm H dng tnh ton mt bn tin c di c nh l mt chui bit h t mtbn tin c di ty m. Nu gi tr m thay bng gi tr m' th H(m') cng c gi trkhc gi tr h = H(m), do ta c th d dng xc nh c nhng thay i trn bn

tin m trn ng truyn.

Hnh 66. Ch k s cho mt bn tin dng message digest

Bn gi s tnh ton cc bn tin c di c nh t bn tin m v m ha bng kharing ca mnh. Bn tin c m ha ny s c truyn i cng bn tin m. Khi nhn,

bn nhn gii m bn tin v thc hin so snh vi bn tin m c truyn i xcnh c rng bn tin ny gi t bn gi v c k bng ch k s.

b. Kha phin

Trong mt knh trao i an ton, sau pha xc thc s t in hnh truyn thng. Mi knhtruyn thng c xc nh bi mt kha phin tng ng. Khi phin truyn ktthc th kha phin tng ng cng b hy b.

8.2.3 Truyn thng nhm an ton

a. Truyn thng nhm b mt

M hnh n gin l tt c cc thnh vin trong nhm s cng c mt kha b mt

m ha v gii m cc bn tin. iu kin tin quyt cho m hnh ny l phi m borng tt c cc thnh vin trong nhm pha gi bia mt kha .

M hnh th hai l dng mt kha b mt cho tng cp thnh vin trong nhm. Khimt trong hai thnh vin kt thc phin truyn th thnh vin cn li vn s dng kha giao tip vi thnh vin khc trong nhm. Vi m hnh ny phi duy tr ti N(N-1)/2 kha.

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M hnh th ba l dng kha cng khai. Mi mt thnh vin trong nhm s phi duytr mt cp kha cng khai v kha ring, trong kha cng khai c dng bi ttc thnh vin trong nhm.

b. Server nhn bn an ton

Vic nhn bn cc server thng dng trong vic chu li cho h phn tn nhng ikhi cng c dng m bo tnh tin cy cho h thng.

8.3 Kim sot truy nhp (Access Control).

8.3.1 Cc kha cnh tng qut trong kim sot truy cp.

a. Ma trn kim sot truy cp (Access Control Matrix).

Trong ma trn iu khin truy cp, mt hng biu din cho mt ch th (subject), mtct biu din cho mt i tng (object). Gi ma trn kim sot truy nhp l M.

M[s,o]: a ra danh sch cc php ton m ch th s c th yu cu trn i tng o.Khi mt ch th s gi mt phng thc m ca i tng o th monitor s kim tratrong danh sch M[s,o], nu m khng c trong danh sch ny th li triu gi b hy

b.

Thng thng h thng phi lm vic vi rt nhiu user nn c hng nghn ch th cnqun l. Do xy dng mt ma trn thc nh trn l khng hp l. Gii php ra ls dng danh sch kim sot truy cp.

b. Danh sch kim sot truy cp (Access Control List).

Mi mt i tng s duy tr mt danh sch cc truy cp hp l ca cc ch th muntruy cp n gi l ACL nh trnh c s tn ti ca cc entry rng nh ma trnkim sot truy nhp.

Hnh 67 s dng ACL

c. Min bo v (Protection Domains).

Vi vic s dng ACL, tuy khc phc c nhc im ca ma trn kim sot truy

nhp nhng vn c kch thc ln nn a ra cch s dng min bo v. Min bov l mt tp cc cp (i tng, truy cp hp l), mi cp ny s cho ta mt i tngv cc thao tc hp l trn n. Mi mt yu cu u thuc mt min bo v no .Khi mt yu cu gi n, monitor s tm trong min bo v tng ng yu cu ny.

t hiu qu cao hn, ngi ta dng kt hp min bo v vi vic phn nhm cci tng.

8.3.2 Tng la (Firewall).

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Firewall dng ngn chn cc lung khng c php. Firewall c hai loi chnh l:

Packet - filtering gateway: loi ny hot ng nh mt router cho php hoc khngcho php gi tin chuyn qua mng da trn a ch ngun v a ch ch phnheader ca gi tin. Loi ny thng dng ngn chn cc gi tin t ngoi i votrong mng.

Appllication - level gateway: loi firewall ny khng ch kim tra header ca gi tingi n hay gi i m cn kim tra ni dung ca gi tin . Mt v d c bit cho loiny l proxygateway.

8.4 Qun tr an ton - an ninh (Security management ).

8.4.1 Qun tr kha.

a. Thit lp kha.Vic to ra kha b mt gia bn truyn v bn nhn c thc hin nh sau:

Bn A v bn B u to ra hai s ln l n v g - hai s ny c th c cng khai. BnA s to ra mt s ln khc l x, bn B to ra s ln y v gi b mt chng. Bn A sgi cho bn B: n, g v (gx mod n). Bn B s thc hin tnh (gx mod n)y= gxy mod n.do s xc nh c kha b mt x ca bn A. ng thi, bn B cng gi cho bn A(gy mod n). Bn A thc hin tnh ton (gy mod n)x= gxy mod n nh cng xc nhc kha b mt y ca bn B.

Hnh 68 Nguyn l ca Diffie - Hellman key exchange

b. Phn pht kha.

Trong h m mt i xng, kha b mt to ra phi c truyn i trn knh mt ring.

Hnh 69 Phn pht kha theo knh ring.

Trong h mt m dng kha cng khai, kha cng khai phi m bo cng mt cpvi mt kha b mt. Kha cng khai c truyn i nh mt bn r trn ng truynv phi c h tr xc thc. Kha b mt c truyn i trn mt knh ring v cng

phi c xc thc.

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Thng thng, kha cng khai thng cthay bng mt chng ch kha cng khai(public - key certificate). Chng ch ny bao gm mt kha cng khai v mt xu nhdanh xc nh c kha mt lin kt vi n.

b. Thi gian tn ti ca chng ch.

Khi cn hy b mt chng ch ta c th thc hin theo nhiu phng php:

Cch 1: s dng danh sch cc chng ch b hy b CRL (certification revocation list).Khi cllient kim tra mt chng ch th n cng kim tra trong danh sch CRL kimtra xem chng ch ny b hy hay khng. Nh th mi client phi c cp nhtdanh sch ny thng xuyn.

Cch 2: mi chng ch t ng ht hiu lc sau mt thi gian xc nh no . Nhngnu mun hy chng ch trc thi gian th vn phi dng n danh sch CRL nhtrn.

Cch 3: gim thi gian tn ti c hiu lc ca mt chng ch xung gn bng 0. Khi client phi thng xuyn kim tra chng ch xc inh thi gian c hiu lc cakha cng khai.

8.4.2 Qun tr nhm an ton.

Xt nhm G, kha mt CKG c chia s vi tt c cc thnh vin ca nhm mha thng ip ca nhm. Nhm cn c thm 1 cp kha cng khai/ring (KG+, KG-) giao tip vi cc thnh vin ca nhm khc.

Tin trnh P mun tham gia vo nhm G s gi yu cu tham gia JR. RP (Reply pad)

v kha b mt KP,G c m ha s dng kha cng khai KG+ ca nhm. JR cgn bi P v n c gi i cng vi chng ch cha kha cng khai ca P.

Khi mt thnh vin nhm Q nhn mt yu cu t P, n s xc thc P, xc nh temthi gian T m bo rng P vn cn gi tr ti thi im gi. Sau ly ra khacng khai ca P kim tra tnh hp l ca JR.

Nu P c chp nhn vo nhm, Q tr li thng ip GA nhn dng P v cha N(nonce). RP c s dng m ha kha giao tip ca nhm CKG. P s dng khaKG- m ha cng vi CKG. Sau thng ip GA c gn cho Q s dng khaKP,G.

8.4.3 Qun tr y quyn (Authorization management )

S dng capability xc nh quyn truy cp ti nguyn ca tin trnh chim gi n.Mt capability l mt t nh danh 128 bit, c u trc bn trong c m t nh sau:

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48 bit u tin c khi to khi i tng c to ra bi server ca i tng. 48 btny c gi l server port.

24 bit tip theo c s dng xc nh i tng ti server nh sn.

8 bit tip theo xc nh quyn truy cp ca holder ca capability

Trng check (48bit cui) c dng to ra mt capability tht (khng th gi moc).

Khi mt i tng c khi to, server ca i tng chn ly mt trng checkngu nhin v lu tr n trong c capability v trong c table ring ca server

S y quyn(delegation)

S y thc quyn truy nhp l mt k thut quan trng thc thi s bo v trong hthng my tnh v c bit hn l trong h phn tn. tng c bn rt n gin: bng

vic chuyn quyn truy nhp t tin trnh ny sang tin trnh khc, n s tr nn ddng hn phn tn cng vic gia cc tin trnh m khng lm nh hng ti vicbo v ti nguyn.

C vi cch thc thi s y quyn, mt cch l s dng proxy.