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Cu 1. Lm r mt cc dng trong sut ca h phn tn. Cc c trng c bn ca h phn tn (6 c trng) a) Kt ni ngi s dng vi ti nguyn. b) Tnh trong sut (transparency). c) Tnh m (openess). d) Tnh co dn (scalability).

e) Tnh chu li (Fault tolerance). f) Tnh an ton an ninh (security).

Tnh trong sut (transparency) Mt h phn tn c gi l trong sut nu n c kh nng che du tnh ri rc v nhng

nhc im c th ca n i vi ngi s dng cui v ngi lp trnh ng dng. C 8 dng trong sut : 1. Trong sut truy cp: che du cch biu din d liu v cch thc truy cp ti nguyn. (ko cn bit dl c ly ntn.) 2. Trong sut v tr: che du v tr thc ca ti nguyn. (ko cn bit ly t u) 3. Trong sut di tr: che du kh nng di tr (di chuyn t ni ny sang ni khc) ca ti nguyn m ngi sd ko h bit. 4. Trong sut nh v li: che du kh nng ti nguyn c th di chuyn t ni ny n ni khc ngay c khi ang c s dng. 5. Trong sut v nhn bn: che du cc bn sao c nhn ra. 6. Trong sut v tng tranh.(Cc my ko cnh tranh v ti nguyn). 7. Trong sut v li. (c li v h thng nhng ngi dng ko bit)

Cu 2: Tnh co dn ca h phn tn. Cc bin php nng cao kh nng co dn ca h phn tn.

a. Tnh co gin (Scalability) Mt h phn tn c gi l c tnh co gin nu n thch nghi vi s thay i quy m ca 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 l dn n s thay i v v tr a l ca ngi 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 (hiu nng ca h thng l hiu qu nng lc hot ng ca i tng).

b. C ba gii php ph dng gii quyt vn co gin ca h phn tn: Cc vn v quy m l mt ch quan trng trong vic pht trin h phn tn. Chng

bao gm s dng cc bn nhn rng d liu, cc k thut lin quan n b nh m v trin

khai nhiu my ch thc hin cc nhim v, cho php mt vi nhim v tng t c thc

hin ng thi.

- Rt ngn thi gian i: c s dng trong mt trng hp m rng v mt a l. tngca n l hn ch ti a thi gia ch i i vi nhng tr li t server xa. - 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 min con. Vi phng php ny s gii quyt c vn khi thm ngi dng hay ti nguyn vo h thng.

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

Cu 3. Phn bit kin trc nhiu phn v kin trc a tng. Mt kin trc m hnh ca H phn tn l mi quan h cc v tr ca cc thnh phn v mi quan h gia cc thnh phn ny. Mt h phn tn c hp thnh bng cc qu trnh x l thng tin trn mt h thng mng. M hnh kin trc a tng.

Lin quan n kin trc tp trung, chng ta c khi nim kin trc client-serve nhiu phn. N l kt qu ca vic chng ta chia ng dng ra thnh cc phn nh giao din, cc thnh phn x l v thnh phn d liu. Cc phn khc nhau ny c lin quan trc tip n vic t chc logic ca ng dng. Trong nhiu trng hp, vic x l phn tn c th c hiu l t chc ng dng client-server nh mt kin trc nhiu phn. Chng ta gi l cch phn tn theo chiu dc. c im ca phn tn theo chiu dc, l chng ta t cc thnh phn khc nhau mt cch logic trn cc my khc nhau. Kin trc phn tn theo chiu dc l mt s m rng ca kin trc client-server, phn tn cc.

Ging nhau: u l m hnh clien-server (cc tin trnh trong h phn tn chia lm 2 nhm c th gi ln nhau. Mt serve l mt tin trnh ci t mt dch v c th no , v d, dch v h thng file file system serve mt client l mt tin trnh yu cu dch v t serve bng cch gi request v i tr li).

ng i ca Resquest v Response theo 2 chiu khc nhau. Resquest i t tng cao xung tng thp, Response theo chiu ngc li. Resquest i t Client ti Server v Response theo chiu ngc li. (Khng c chuyn o ngc c ng i ny. Server ko yu cu Client, tng thp ko yu cu tng cao).

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Khc nhau:

Mt server tr li c th cho kt qu:Mt dy cc request-response (m hnh a tng)

+ Kin trc phn tng: Phn tng dui gc nhn logic. H thng chia ra thnh cc tng, lin kt gia cc tng l lin kt logic.

+ Kin trc nhiu phn: H thng phn thnh cc phn khc nhau (Client, Server) da trn gc nhn vt l. Cc phn khc nhau s nm cc my tnh khc nhau, lin kt gia cc phn l lin kt vt l. Cu 4 Trnh by nhng hiu bit v thin client v fat client trong cc h phn tn. T nhng hiu bit hy cho bit trng hp vn dng c th. Cch tip cn t chc cc my client v server l phn phi cc chng trnh trong cc lp ng dng ca phn tng ng dng vi cc my khc nhau, dng n gin my client c s dng trong hai dng l thin client v fat client.

Nu s dng thin - client, server s cung cp cc qu trnh x l v qun l d liu; cung

cp cho khch hng nhng giao din ha n gin. Vi kin trc ny ta s d dng qun l hn, ng tin cy hn, cc my client khng cn phi qu ln v cu hnh cao nhng li lm mt cc hot ng nhn thc ca client.

Mt khc, nu s dng my fat - client tt c cc ng dng x l d liu v mt s d liu ti pha client s lm gim khi lng cng vic ti my ch, c kh nng m rng hn nhns li kh qun l bi qun tr h thng v km an ton. Vn dng: Fat- client:vi cc game cn nng lc x l cao th khng server x l nhiu, server ch x l cc thng tin quan trng , client s x l nhiu hn, m hnh d,e (hnh trn)

Thin - client: Hc trc tuyn , s dng m hnh a,b v cn qun l d liu tp trung. c s thng nht trong vic test, cp nht nng lc ca Server hon ton m bo qun l tp trung dl.

Cu 5. Trnh by nhng hiu bit v c im phn cng ca h phn tn. Phn bit h thng multiprocessor v multicomputer. Quan nim v gii php phn cng cho h phn tn C 2 loi my tnh: - Cc loi my tnh c chia s b nh (Shared Memory): cc loi my a vi x l (multiproccessor).

- Cc my tnh khng chia s b nh (Private Memory): cc h thng a my tnh (multicomputers). Trong mi loi li chia thnh 2 loi theo kiu mng kt ni: - Kt ni da trn bus (bus-based): ch c mt ng kt ni chung. - Kt ni da trn chuyn mch (switch-based): c nhiu ng kt ni t my ny sang my khc.

Mt im khc nhau gia chng l multiprocessors ch c mt khng gian a ch duy nht c s dng cho tt c cc b vi x l. Hay ni cch khc, tt c cc my tnh u s dng chung mt b nh. V d: nu mt b vi x l no ghi thng tin, gi tr 44 vo a ch

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1000, th tt c cc b vi x l khc sau nu c gi trj ti a ch 100 u nhn c gi tr 44.

Khc vi h thng nu trn, trong mt h thng Multicomputers, mi my tinh s dng b nh ca mnh. Sau khi mt b vi x l ghi gi tr 44 vo a ch 100 th b vi x l khc nu c gi tr ti a ch 1000 s nhn c gi tr m n lu vo trc (khng nht thit l 44). V d in hnh v h thng ny l: mt vi my tnh c ni vi nhau qua mng.

Lin kt kiu BUS c hiu l lin kt theo mt mng duy nht, panel, bus, cable hay cc mi trng khc kt ni cc my vi nhau. V d d hiu nht l mng tivi, cng ty cung cp dch v s ko dc theo cc cable chnh, cn ngi s dng s kt ni t cable chnh vo ti vi ca mnh. Lin kt theo kiu SWITCHED khng tn ti mt ng kt ni duy nht m thay vo t my tnh ny n my tnh kia c kt ni vi nhau theo knh ring v c kt ni theo cc cng ngh khc nhau. Cc thng bo c chuyn i theo cc knh vi s ra quyt nh r rng l s kt ni vi knh ra c th ca tng my tnh.

KIN TRC A B X L - MULTIPROCESSORS

H thng Multiprocessors c im c trng l: Tt c cc b vi x l u c kh nng truy cp vo b nh chung. H thng Multiprocessors lin kt theo kiu BUS c to thnh

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bi mt vi b vi x l c kt ni vi nhau bi panel chung v thng qua n l b nh dng chung. Cu hnh n gin nht l panel vi bus hay mainboard, t cc b vi x l v b nh dng chung. Do chng dung chung b nh duy nht, khi b vi x l A ghi 1 t vo b nh, cn b nh B khong microsecond sau c t b nh A. B nh mang c trng thng nht. Vn ch nu lc ny c to thnh bi 4 hay 5 processor th bus s tr nn qu ti v hiu sut lm vic s gim trng thy. Gii php c a ra l t vo gia processor v bus mt b nh m. b nh m ny s lu thng tin c truy cp thng xuyn nht. Tt c cc yu cu u c x l thng qua b nh m ny. Nu cc thng tin yu cu nm trn b nh m th b vi x l s s dng thng tin trn b nh m ny m khng cn thng qua bus. Nu kch thc ca b nh m ln th lu lng bus tnh theo b vi x l s cng nh, iu ny cho php kt ni vo h thng nhiu b vi x l hn. Tuy nhin, a b nh m vo th li to ra mt vn khc. Cng xem v d hai b vi x l A v B cng c 1 t t b nh m ca ring mnh, sau b vi x l A ghi ln t trong khi b vi x l B ln sau li mun s dng t th n s c gi tr c ca mnh t b nh m m khng phi l gi tr mi c ghi bi b vi x l A. Nh vy l b nh tr ln khng thng nht v lp trnh cho h thng tr ln kh khn hn. Tuy nhin s dng b nh m trong h phn tn vn c s dng rng ri v y chng ta li vng phi vn khng thng nht thng tin trong b nh. Vn ca h thng Multiprocessors nm vic hn ch kh nng m rng, thm ch trong trng hp s dng b nh m. i vi h thng vi hn 256 vi x l, khi kt ni cc b phn vi x l ny vi b nh cn phi s dng phng php khc. Mt trong nhng phng php l phn b nh ra lm cc modul nh v kt ni chng vi cc b vi x l thng qua li kt ni (crossbar switch) ging nh hnh v:

(a): Chuyn mch thanh cho (crossbar) (b): Chuyn mch Omega hnh v (a) th mi b vi x l u c th kt ni vi module b nh bt k. Cc im ni gia b nh v vi x l l mt im kt ni. u im ca s ny l mi b vi x l c th truy cp vo bt c module b nh no. Mt u im na l ng thi 2 b vi x l c th truy cp vo mt module b nh, mc d nu trong cng mt thi im hai b vi x l truy cp vo mt b phn ca b nh th mt b vi x l phi i n lt. Tuy nhin, nhc im ca s trn l nu tn ti n b vi x l v n module b nh th chng ta phi c n2 im kt ni. Nu s lng n l qu ln th phng php trn l khng hiu qu. Pht hin ra iu ny ngi ta xut mt m hnh mi, i hi t im kt ni hn l m hnh mng Omega nh hnh (b). Trong hnh (b) ta thy, tn ti 4 im chuyn i 2x2, mi im c 2 u vo v 2 u ra. Chng ta c th thy rng vi s ny th mi b vi x l cng c th truy cp n mt khi b nh bt k. Nhc im ca s trn chng ta thy rng mi b vi x l c th truy cp

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n cc khi b nh th n cn phi i qua vi im kt ni v nh vy r rang lm gim thi gian truy cp. Nh vy cc switch ny phi x l rt nhanh. Ngi ta c gng gim bt chi ph vo cc switch k trn bng cch chuyn qua s hnh cy. i vi trng hp ny th mi b vi x l c th nhanh chng truy cp n phn b nh ca mnh. Truy cp vo cc phn b nh khc nu so snh s chm hn. tng ny c thc hin bi my NUMA (NonUniform Memory Access). Mc d my NUMA c thi gian truy cp trung bnh nhanh hn so vi my Omega Network. Tuy nhin n vn cn tn ti yu im ca n, l khi phn phi cc chng trnh v ti nguyn i hi phi phn b sao cho phn ln cc truy cp l trn phn b nh ring ca tng b vi x l. KIN TRC A MY TNH - MULTICOMPUTERS H thun nht/ H khng thun nht: H thng thun nht: Mng my tnh cng s dng mt cng ngh, cc b vi x l l nh nhau, truy cp n cng mt b nh ging nhau. Thng dng trong h thng c tnh ton song song.

H thng khng thun nht: Nhng my tnh khc nhau kt ni vi nhau. H thun nht Multicomputers Khc vi h multiprocessers, xy dng h multicomputers l tng i d hn. Mi b vi x l lin kt trc tip vi b nh ca ring mnh. Vn cn li l lin kt gia cc b vi x l vi nhau. Xem xt h ng u multicomputers. nhng h ny chng ta lm quen vi tn gi System Area Networks SAN. Cc my tnh c lp t ring r v chng lin kt vi nhau thng qua mng kt ni vi tc cao. Ging nh trng hp trc, chng ta phi la chn hoc h thng theo kiu BUS hoc kiu SWITCH. Trong cc h thng theo kiu BUS, cc my tnh kt ni vi nhau thng qua mng phn chia nhiu truy cp, v d Fast Ethernet. Tc truyn tin thng thng l 100 Mb/s. Cng ging nh trng hp multiprocessors, cu trc kiu BUS ny c nhng hn ch lin quan n vic m rng h thng. Cng nhiu my tnh hay cng nhiu nt th hiu qu lm vic ca h thng cng gim bt. Trong cc h thng theo kiu SWITCH thng tin c truyn t b vi x l ny sang b vi x l khc. c mt s xut gii quyt theo mt s topology sau:

(a): Kiu li; (b): Kiu siu khi.

Trn hnh v, trn cc nh l cc processor, cn cc cnh l lin kt gia cc processor. i vi cc h thng theo kiu SWITCH tn ti nguyn tc: Cc b vi x l hot ng song song. Mt my tnh khng l hon ton c th c to ra bi cc b vi x l dng cho cc my trm thng thng, chng ch khc ch phng php kt ni chng vi mng chung. Cc mng ny c thit k sao cho thi gian ch l t nht v hiu qu lm vic l cao nht. ng thi cng phi tnh ton n vic bo v h thng khi h hng. Vi s tn ti hang ngn b vi x l th hng tun phi c vi b b hng, cn khc phc. Phi m bo lm sao cho h thng lun trong trng thi lm vic.

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H khng thun nht Multicomputers Thng thng th cc h phn tn c xy dng theo nguyn tc khng ng u

multiconputers. iu ny c ngha l cc my tnh l mt b phn ca h v chng hon ton c th khc nhau, v d khc nhau v loi vi x l, v kch thc b nh, v hiu sut lm vic ca knh xut nhp d liu. Cc cch kt ni nhng my tnh nh th vo mng chung cng hon ton c th ging nhau.

Khi pht trin, m rng nhng h thng k trn cn phi quan tm n vn ton din, p ng c phn ln ca h thng, rng khi thnh lp nhng chng trinh i vi h thng khng ng u trn cn phi s dng nhng chng trnh ring bit sao cho ngi lp trnh khng phi lo lng v vn phn cng. Phn mm phi m bo cho chng trnh c th hot ng i vi mi c im khc nhau ca phn cng (m bo tnh trong sut). Cu 6. Hy nu c im ging nhau v khc nhau ca truyn thng RPC v truyn thng ROI.

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Ging nhau ca truyn thng RPC v ROI:

Truyn thng RPC v ROI u l cc middleware(mt ci tin trong h phn tn l gp

tng trnh din v tng phin thnh 1 tng mi l middleware.Tng ny nm gia h iu hanh

v ng dng mi u ca h thng phn tn nm trong mng my tnh) c s dng vi mc

ch tng tnh mm do, trong sut v hiu qu cho h thng, to giao din v cung cp dch v

tt cho tng vn chuyn pha di.

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Khc nhau ca truyn thng RPC v ROI:

RPC: c s dng rng ri nht u im l n gin, hng ti s trong sut v truy

cp. Tuy nhin nhng tham chiu thng qua vic gi th tc t xa l hn ch.

ROI: Bn cht l mt RPC nhng iu c bit l n hng ti i tng phn tn. im khc bit chnh vi RPC l roi cung cp cc th tc rmi cho php tham chiu i tng trong ton h thng thng qua cc tham s thch hp. ROI m bo tnh trong sut cao hn RPC Cu 7. Trnh by nhng hiu bit v truyn thng RPC. Yu t no m bo cc li triu gi th tc t xa khc nhau th thc hin cc cng vic khc nhau. Cc m hnh dch v Middleware.

a. Gi th tc t xa - RPC (Remote Procedure Call) b. Triu gi i tng t xa - ROI (Remote Object Invocation) c. Truyn thng hng thng ip - MOC (Message Oriented Communication) d. Truyn thng hng dng SOC (Stream Oriented Communication) 1. RPC gi th tc t xa a. Tng quan v RPC:

Li gi th tc xa l mt c ch cho php mt chng trnh c th gi thc thi mt th tc (hay hm) trn mt my tnh khc. Trong chng trnh lc ny, tn ti hai loi th tc: th tc cc b v th tc xa.

Th tc cc b l th tc c nh ngha, ci t v thc thi ti my ca chng trnh. Th tc xa l th tc c nh ngha, ci t v thc thi trn mt my tnh khc.

C php gia li gi th tc cc b v xa th ging nhau. Tuy nhin, khi mt th tc xa c gi n, mt thnh phn ca chng trnh gi l Stub s chuyn hng kch hot mt th tc tng ng nm trn mt my tnh khc vi my ca chng trnh gi. i vi ngi lp trnh, vic gi th tc xa v th tc cc b th ging nhau v mt c php. y chnh l c ch cho php n gin ha vic xy dng cc ng dng Client-Server. Trong h thng RPC, Server chnh l my tnh cung cp cc th tc xa cho php cc chng trnh trn cc my tnh khc gi thc hin. Client chnh l cc chng trnh c th gi cc th tc xa trong qu trnh tnh ton ca mnh.

Mt Client c th gi th tc xa ca nhiu hn mt my tnh. Nh vy s thc thi ca chng trnh Client lc ny khng cn gi gn trn mt my tnh ca Client m n tri rng trn nhiu my tnh khc nhau. y chnh l m hnh ca ng dng phn tn (Distributed Application).

Kin trc ca chng trnh Client-Server ci t theo c ch li gi th tc xa

Mt ng dng Client-Server theo c ch RPC c xy dng gm c su phn nh s di

y:

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Kin trc chng trnh kiu RPC

Phn Client l mt qu trnh ngi dng, ni khi to mt li gi th tc t xa. Mi li gi th tc xa trn phn Client s kch hot mt th tc cc b tng ng nm trong phn Stub ca Client.

Phn ClientStub cung cp mt b cc hm cc b m phn Client c th gi. Mi mt hm ca ClientStub i din cho mt hm xa c ci t v thc thi trn Server.

Mi khi mt hm no ca ClientStub c gi bi Client, ClientStub s ng gi mt thng ip m t v th tc xa tng ng m Client mun thc thi cng vi cc tham s nu c. Sau n s nh h thng RPCRuntime cc b gi thng ip ny n phn Server Stub ca Server.

Phn RPCRuntime qun l vic truyn thng ip thng qua mng gia my Client v my Server. N m nhn vic truyn li, bo nhn, chn ng gi tin v m ha thng tin.

RPCRuntime trn my Client nhn thng ip yu cu t ClientStub, gi n cho RPCRuntime trn my Server bng lnh send(). Sau gi lnh wait() ch kt qu tr v t Server.

Khi nhn c thng tip t RPCRuntime ca Client gi sang, RPCRuntime bn pha server chuyn thng ip ln phn ServerStub.

ServerStub m thng ip ra xem, xc nh hm xa m Client mun thc hin cng vi cc tham s ca n. ServerStub gi mt th tc tng ng nm trn phn Server.

Khi nhn c yu cu ca ServerStub, Server cho thc thi th tc c yu cu v gi kt qu thc thi c cho ServerStub.

ServerStub ng gi kt qu thc trong mt gi tin tr li, chuyn cho phn RPCRuntime cc b n gi sang RPCRuntime ca Client .

RPCRuntime bn pha Client chuyn gi tin tr li nhn c cho phn ClientStub. ClientStub m thng ip cha kt qu thc thi v cho Client ti v tr pht ra li gi th tc xa.

Trong cc thnh phn trn, RPCRuntime c cung cp bi h thng. ClientStub v ServerStub c th to ra th cng (phi lp trnh) hay c th to ra bng cc cng c cung cp bi h thng.

C ch RPC c h tr bi hu ht cc h iu hnh mng cng nh cc ngn ng lp trnh.

b. Xt chi tit cc thao tc RPC - ng gi cc tham s: vic ng gi cc tham s chun b truyn i do client stub thc hin. Client stub s sp xp cc tham s theo th t v a vo hng i. Cc tham s c truyn i gip cho server hiu c cng vic mnh cn thc hin tng ng l g xc nh li gi n th tc thch hp. - Truyn tham s: Vic truyn tham s t client ti. C hai cch truyn tham s l: truyn tham bin v truyn tham tr. + Truyn tham tr: cc tham s c truyn i l cc gi tr c th. Cc th tc c gi n 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 vic biu din d liu v cc kiu d liu.

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+ Truyn tham bin: cc tham s c truyn i l con tr hay bin cha a ch ca ni 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 th tc th s c thng bo cho client v cc ln gi sau s dng gi tr mi . c. Nhc im: - Pha gi v b gi nm 2 my khc nhau nn K.gian a ch khc nhau, phc tp. - Tham s v k.qu tr v cng phi c truyn i. Nu tham s khng thng nht th rt phc tp. * Yu t no m bo cc li triu gi th tc t xa khc nhau th thc hin cc cng vic khc nhau.: ng gi cc tham s Cu 8. Nu nhng hiu bit v truyn thng hng thng ip v trng hp vn dng.

Truyn thng hng thng ip - MOC (Message Oriented Communication). Trong mt s trng hp RPC t ra khng ph hp, v n i hi c bn gi v bn nhn

cng phi sn sng ti thi im din ra truyn thng. Tuy nhin, i khi chng ta khng th bit c liu bn nhn yu cu c ang sn sng hay khng. Chng ta c th gii quyt vn ny thng qua Messages-Oriented Middleware (MOM).

a. Cc loi truyn thng hng thng ip c bn Truyn thng nht thi (Message-oriented Transient communication): bn tin gi i

ch c lu li trong phin trao i . Khi phin trao i hon thnh hoc khi kt ni b hy b th cc bn tin cng b hy b trn cc server. Do , v mt l do no m mt server trung gian khng th chuyn tip bn tin i c th bn tin ny s b hy b.

Hnh 5: Mt s dng truyn thng hng thng ip

Truyn thng kin tr (Message-oriented Persistent communication): Th in t l mt v d minh ha r nt cho khi nim truyn thng kin tr. Khi mt trm mun gi bn tin i trn mng, 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 cc b. Mail server ny tm thi lu tr bn tin vo b nh m ca mnh, xc nh a ch trm ch, ri gi ti server cc b ca trm ch tng ng (c th i qua nhiu mail server trung gian khc). Ti mail server cui cng, bn tin lc ny s c lu li trc khi pht cho trm ch tng ng.

Vi truyn thng nht thi bn tin c lu trong h thng truyn thng ch khi ng dng ca bn gi v bn nhn u ang hot ng. Middleware khng th chuyn giao bn tin do

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vic truyn thng b gin on hoc nu bn nhn ang offline th bn tin s b hu b. Cn vi truyn thng kin tr, bn tin c gi i s c lu tr trong tng trung gian truyn thng (communication middleware). Do , ng dng gi s khng cn tip tc trng thi hot ng sau khi gi bn tin, v ng ng dng bn nhn cng khng cn phi hot ng khi bn tin c gi. Hai loi truyn thng c bn c s dng

Truyn thng ng b (Synchronous communication): khi trm gi gi i mt bn tin th 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 s tip tc thc hin cng vic ca mnh. iu ny cng c ngha l bn tin c lu li trn b nh m ca trm gi hoc ca server cc b.

b. Mt s loi truyn thng hn 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 th in 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 my

mnh, trm gi s tip tc thc hin cng vic ca mnh. Cng lc, bn tin s c truyn ti trm nhn. Khi bn tin n c trm nhn m trm nhn li khng lm vic, 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 mt bn tin, trm gi s ch bn tin bo nhn thnh cng ca trm nhn gi v mi thc hin tip cng vic ca mnh. Cu 9 Nu nhng hiu bit v truyn thng hng dng. Ti sao trong truyn thng hng dng th ng b ha cc dng li c ngha quan trng.

Truyn thng hng dng (stream - oriented communication) Mt s khi nim: Medium (media): ch cc phne tin dng truyn thng tin nh cc thit b lu tr, ng truyn, cc phng tin hin th... Data stream: l mt chui cc n v d liu. Dng n (simple stream) l dna chi gm mt chui n v d liu. Dng phc (complex stream): bao gm nhiu chui n v d liu khc nhau. Mi chui ny c si l mt dns con (sub stream).

Vi data stream th thi gian l yu t quyt 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 cho mi n v d liu trong data stream. -Truvn ng thi (isochronous transmission mode): quy nh tr ln nht v nh nht cho mi n v d liu trona data stream. Cch truyn ny ng mt vai tr quan trna trons vic trnh din audio v video ng b cc dng:

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-ng b n gin : thc hin ng b gia dng tr v dng lin tc . V d irons vic trnh din slide c km m thanh. Dng hnh nh slide l dna tr cn dng m thanh l dna lin tc Phi ng b hai dng ny thu c kt qu trnh din nh mun. -ng b phc tp: l vic ne b gia cc dna d liu lin tc. V d trong vic xem phim trc tuyn, c dng m thanh v dng hnh nh u l cc dng lin tc cn phi c ng b

Ti sao trong truyn thng hng dng th ng b ha cc dng cn li c ngha quan trng

Vi cc hnh thc truyn thng RPC, RMI, MOM, ta ch quan tm n vic trao i thng tin, m khng h quan tm c th rng vic trao i thng tin s din ra ti mt thi im c th no, v cng khng h quan tm n vic thi gian truyn l nhanh hay chm. Ni cch khc, trong cc m hnh truyn thng RPC, RMI, MOM, thi gian khng h nh hng ti tnh ng n ca vic truyn tin.

Vi hnh thc truyn thng hng dng, thi gian ng vai tr cc k quan trng. V d, nu mt on m thanh c pht li vi tc khc vi tc ca on m thanh ban u, n c th cho ra kt qu khc vi on m thanh gc. Tng t nh vy, nu chng ta mun xem mt b phim trc tuyn trn mng, nu thi gian truyn ca d liu l qu chm, b phim c th b gin on. Nh vy, hnh thc truyn thng ny c cung cp nhm to iu kin thun li cho vic trao i cc thng tin ph thuc vo thi gian, khi m thi gian s nh hng n tnh ng n ca thng tin c trao i. Cu 10. Trnh by nhng hiu bit v tin trnh trong cc h phn tn. 1. Khi nim v tin trnh

Tin trnh (process) l 1 chng trnh ang trong qu trnh thc thi ngha l mt chng trnh hin ang c thc thi bi mt trong cc b x l o ca h iu hnh. i vi tin trnh th vn qun l v lp lch cho cc tin trnh nhng vn quan trng cn gii quyt. Nhiu tin trnh c th ng thi chia s cng mt CPU v cc ti nguyn phn cng khc. 2. c im Mt tin trnh gm: a. M ngun chng trnh (code) (khng thay i) b. D liu (data) c. B m CT (Program Counter) d. Ngn xp (Stack) e. Gi tr cc thanh ghi (Register values) 3. Cc trng thi ca tin trnh - Trng thi ca tin trnh ti mt thi im xc nh bi hot ng ca tin trnh ti thi im . - Trong qu trnh sng, tin trnh c th thay i trng thi do cc nguyn nhn: Phi dng hot ng do ht thi gian i mt thao tc I/O hon tt Phi ch mt s kin xy ra -Ti mt thi im, tin trnh c th c mt trong cc trng thi: new: Tin trnh ang c to running: Tin trnh ang chim hu CPU & thc hin cc lnh. waiting: Tin trnh ang ch cung c cp ti nguyn hoc ch mt s kin no xut hin chuyn sang trng thi sn sng. ready: Tin trnh trng thi sn sng, c phn phi ti nguyn cn thit, ang ch n lt c thc hin theo c ch lp lch ca h iu hnh.

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terminated: Tin trnh kt thc. N khng bin mt cho n khi mt tin trnh khc c c trng thi thot ca n.

. Hot ng(qu trnh chuyn trng thi) Ti mt thi im, ch c mt tin trnh c th nhn trng thi running. Trong khi , nhiu tin trnh c th trng thi waiting hay ready. Tin trnh mi to c a vo h thng, c cung cp ti nguyn trng thi ready(ch c phn phi CPU thc hin) Khi tin trnh ang thc hin(running), n c th chuyn sang trng thi: Kt thc(terminal) nu thc hin xong Ch(waiting) tin trnh yu cu mt ti nguyn nhng cha c p ng v ti nguyn cha sn sng cp pht ti thi im ; hoc tin trnh phi ch mt s kin hay thao tc nhp/xut Sn sng(ready) khi xy ra ngt chuyn CPU cho tin trnh c mc u tin cao hn B iu phi cp pht cho tin trnh mt khong thi gian s dng CPU hoc ht thi gian chim hu CPU B iu phi chn mt tin trnh khc c trng thi ready cho x l. Ti nguyn m tin trnh yu cu tr nn sn sng cp pht; hay s kin hoc thao tc I/O tin trnh ang i(c trng thi waiting) hon tt, tin trnh chuyn sang ready Cu 11 Lung trong cc h phn tn. ngha ca vic s dng cc h thng a lung. Mi tin trnh c mt khng gian a ch v ch c mt dng x l tng tc v s dng CPU hiu qu hn cn nhiu dng x l cng chia s mt khng gian a ch, v cc dng x l ny hot ng song song tng t nh cc tin trnh phn bit (ngoi tr vic chia s khng gian a ch) Mi dng x l c gi l mt lung(thread) hay mt tiu trnh

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*)Li gi h thng (System call): l tp lnh m rng do h iu hnh cung cp xc nh giao 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 s dng 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 v tin trnh ny tip tc thc hin song song vi li gi h thng. * Mt thuc tnh quan trng ca lung l chng cho php kha cc li gi h thng m khng cn phi kha ton b tin trnh m c lung ang chy. * K thut a lung (multi-threading) cho php khai thc tnh song song khi thc thi mt chng trnh trn mt h thng nhiu b x l. Khi , mi lung c gn cho mt CPU khc nhau trong khi d liu dng chung c lu tr trong b nh chnh dng chung. Phng php ny thng c dng trong UNIX.

Cu 12. Trnh by nhng hiu bit v m hnh a lung pha client v m hnh a lung pha server. Cho v d s dng trn thc t. M hnh a lung pha Server

Tuy nhin c th thc hin bng cch khc, nhng yu cu c gi n mt v tr c bit no trn my ch, sau khi kim tra cc yu cu my ch chn ra mt lung nhn ri no thc hin yu ny. Nh vy c th thc hin nhiu yu cu m vn m bo hot ng ca h thng.

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Tin trnh ny s thc hin vic c cc tp tin trn h thng ni b, lc ny c th lm cho lung b dng cho n khi d liu c c t a. Nu lung b dng, lung khc s c chn thc thi My ch tp tin a lung ngn xp: Khi mt yu cu n, ch mt lung kim tra n v nu tha mn th thc hin nu khng th mt thng ip s c gi t a. Tuy nhin thay v ngn chn n ghi li trng thi ca yu cu hin ti vo mt bng v tip tc ly thng ip tip theo. Nu thng ip tip theo l cng vic mi th thc hin cn nu l li tr li t a cho cng vic trc th n s ly cc thng tin lin quan t bng, li tr li c x l v sau gi li cho pha my khch. Theo cch ny my ch s s dng n cc li gi khng chn gi v nhn cc yu cu M hnh a lung pha Client

Cu 13. Trnh by cc m hnh di tr m. ngha.

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V d: mt server gi v mt on code cho browser ca client, client xly on code im yu ca di tr m l bo mt: khng c c ch kim tra ngha di tr m : - Di tr m trong h thng phn tn din ra di hnh thc tin trnh di chuyn trong

ton b tin trnh c chuyn t mt my sang mt my khc - Di chuyn tin trnh ang chy l cng vic tn km v phc tp - Hiu sut tng th h thng c th c ci thin nu cc tin trnh c chuyn t ti

mt cch nng n sang ti mt cch nh nhng Cu 14 Trnh by cc hiu bit v tn, a ch v im truy cp.

Cu 15. ngha ca phn gii tn. Trnh by phng php phn gii tn tun t Trong h thng tn c cu trc, khng gian tn c biu din bng biu c hng vi cc node v nhn. Qu trnh tm kim tn trong khng gian tn c gi l phn gii tn (name resolution). Kt qu qu trnh phn gii tn tr v tn gi mt node.

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Phng php phn gii tn tun t (interactive name sesolution): Vic phn gii tn thc hin bng cch truyn v nhn qua li gia client v cc name server cc mc khc nhau. 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 theo l do client m nhim.

Cu 16. ngha ca phn gii tn. Trnh by phng php phn gii tn quy. Trong h thng tn c cu trc, khng gian tn c biu din bng biu c hng vi cc node v nhn. Qu trnh tm kim tn trong khng gian tn c gi l phn gii tn (name resolution). Kt qu qu trnh phn gii tn tr v tn gi mt node. Phng php phn gii tn quy (recursive name resolution):

Mi name server s gi kt qu n name server tip theo m n tm thy. V c nh vy cho n khi hon thnh phn gii ton b ng dn.

Cu 17. Trnh by nhng hiu bit v phn gii tn tun t v phn gii tn quy. Trng hp no th s dng kiu phn gii tn no. Hiu bit v phn gii tn tun t v phn gii tn quy nh cu 15,16. Trng hp p dng (khng chc nm mong cc bn ng gp kin) Phn gii tn min quy c 2 li ch c bn l: + Nu s dng caching lu kt qu phn gii tn cc lp cao nh Global hoc Administrational (cc nt thng rt t thay i) th hiu qu phn gii s cao hn phng php phn gii tun t.

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+ Chi ph truyn thng c th gim xung. T li ch th 1 c th suy ra vic phn gii tn min quy thng c p dng trong trng hp name server mc managerial (s dng caching lu kt qu phn gii tn cc lp cao nh Global hoc Administrational) - Phn gii tn min quy thng i hi cc name server khi lng cng vic rt ln, cho nn cc name server lp Global ch h tr phn gii tn min tun t. (Suy ra phn gii tn min tun t thng c p dng trong trng hp name server mc Global) ( Ng Hng b sung- theo nh vd thy ging: * c im ca phn gin tn tun t: Cng vic client tng; cng vic server gim v Client thng xuyn hi li server --> nu K/c gia Client v server xa mt nhiu tg. * c im ca phn gin tn quy: Cng vic client gim; cng vic server tng. * NX: Nu s Client t v K/cch gia Client & Server xa nn chn Phn gii tn QUY Ngc li chn phn gii tn TUN T ) Cu 18,19,20,2. Ti sao i vi cc thc th di ng th phi c cch t chc tn v phn gii tn c bit. Trnh by phng php.

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Chuyn con tr: Mt trong nhng gii php tm thc th trong trng hp thay i v tr thc th l s dng phng php chuyn con tr. Nguyn l ca n tng i n gin. Nu thc th chuyn t A sang B th con tr lu a ch mi ca c lu tr A. V nh vy Client c th tm c thc th cn tm bng cch duyt qua mt lot cc a ch m n tng chuyn trong qu kh. Mt trong nhng vn ca phng php ny l: Nu thc th lun lun thay i th phng php ny khng hiu qu, bi n phi duyt qua qu nhiu a ch khc. V d nh cc h thng lu tr file trn internet. vn th hai l cc thc th trung gian phi lu tr con tr rt lu khi thc th vn cn tn ti. Vn th 3, cng l vn nghim trng nht: Khi thng tin v a ch b mt trn mt host trung gian bt k th n khng th c tm thy na. Phng php da trn im gc: Tm thc th bng phng php truyn thng tin nhm v chuyn con tr c mt s vn ln v s dng cc phng php ny nh v cc thc th trong cc h phn tn ln t ra khng ph hp. Mt trong nhng phng php c u chung hn l phng php da vo im gc (Home-Based Approaches), s dng mt im gc (home location) t y s theo di a ch hin ti ca thc th. Nh vy vic bo v im gc ny l v cng quan trng v cn p dng mt s phng php c bit, bi nu im ny down - tt c cc thc th do n qun l s khng th tm thy. Mi lin h vi host u thng qua im gc ny. Khi thc th mun thay i v tr th phi ng k ti im gc. Nh vy qu trnh lm vic vi thc th c th m t qua 4 bc sau: - Gi thng tin n im gc. - im gc gi a chi thc th - Chuyn thng tin n thc th - Chuyn cc thng tin tip theo n thc th.

Cu 21. ng b ha. Khi nim, l do. KN: ng b ha trong h phn tn l m bo cho cc MT trong h phn tn lm vic ng th t (ch khng phi ng b v d liu).

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Trong h phn tn, vic tnh thi gian ca mi my tnh l khc nhau, v vy cn phi c mt khi nim gi l thi gian vt l thng nht v thi gian gia cc my tnh trn ton cu. Bn cnh , khi cc tin trnh cng yu cu mt n v d liu trong cng mt thi gian th s khng trnh khi tng tranh bt kh khng. Nhng iu trn chnh l cc vn chnh cn c gii quyt trong vic ng b ho. Trong h phn tn,mi my tnh l mt ng h nn vic ng b cc ng h ny l rt cn thit v rt kh khn. Cu 22 ngha ca ng b ha ng h vt l. Trnh by cc gii thut ng b ha ng h vt l.

Trong mt h thng phn tn, mi thnh phn phn tn c mt h thng m gi ring v n hon ton khng ging nhau, thm ch c v tc .

Mt gii php a ra l dng mt h o chun sao cho ti cc thnh phn khc nhau th thi gian o c l nh nhau.

S dng ng h vt l l mt gii php, l phng php s dng cch chng ta vn lm o thi gian, da vo gc quay ca tri t quanh mt tri v gc quay ring ca tri t quanh trc ca n. Mc d vy vn c s sai lch do tri t quay chm li, d l rt rt nh. Ba trm triu nm v trc thi gian 1 nm l 400 ngy, trong khi by gi l hn 365 ngy ngy mt cht.

Theo phng php ny chng ta c 2 h o, TAI v UTC. TAI International Atomic

Time c n v chia khng i da vo s dao ng ca nguyn t, trong khi UTC li da vo vng quay ca tri t quanh mt tri, v m bo khp vi TAI, UTC phi c nhng on chia khng u.

tng c bn ca phng php ny l tt c cc thnh phn s cng quan st mt ng h chung v iu chnh ng h ca mnh theo ng h chung ny. C th iu chnh nhanh hn hoc chm i, ty tnh hung. C 3 gii thut ph bin, o la:

1. Gii thut Cristian. S dng mt time server, cc thnh phn trong h phn tn s request ti hi gi. Tuy nhin tnh ng thi gian nhn c, pha client phi thc hin mt s tnh ton tr i qung thi gian x l trn server (I) v thi gian truyn ((T1-T0-I)/2)

2. Gii thut Berkeley. Mt thnh phn l time deamon s thc hin ng b gi cho cc thnh phn khc. Theo nh k n s gi thi gian ca n cho cc thnh phn, cc thnh phn ny so snh vi gi ca mnh v gi li sai khc cho time deamon, sau time deamon s gi li cc thnh phn khong thi gian cn iu chnh ca n, tnh c qung thi gian x l trn time deamon v thi gian truyn gi thng ip.

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3. Gii thut trung bnh. Nu nh 2 phng php trn l phi x l tp chung th phng php ny c th trnh c iu . Sau mi khong thi gian nht nh, tt c cc thnh phn trong h phn tn thc hin broadcast gi ca mnh, v sau tnh li gi theo gi tr trung bnh m n nhn c t cc thnh phn khc. Gii thut ny thc hin chia thi gian thnh nhng khong ng b c nh. Khong thi gian I s bt u t thi im (To + i.R) v chy n khi To+(i+1)R vi To l thi im 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 thi gian ca mnh. Sau khi broadcast n s bt u thu thp thi gian m cc my khc gi n trong khong thi gian S. Sau b i gi tr ln nht v nh nht ri tnh trung bnh ca cc gi tr thi gian cn li. Cu 23 ng b ha ng h logic. ngha, trng hp vn dng. Trong thc t vic s dng ng h vt l khng m bo chnh xc v phi thc hin phc tp. Trong nhiu trng hp, gia cc tin trnh khng nht thit phi ph hp theo thi gian thc t m ch cn khp vi nhau v thi gian. Do ngi ta a ra khi nim ng h logic. C 2 gii thut ph bin, o la:

1. Tem thi gian Lamport (Lamport timestamps) Lamport nh ngha nh sau: Nu a xy ra trc b th a->b Nu a l s kin gi thng ip, b l s kin nhn thng ip th a->b Mi quan h ny c tnh cht bc cu, nu a->b v b->c th a->c. thc hin c iu ny, chng ta nh ngha mt nhn thi gian C cho mi s kin trong h thng. Sao cho nu a->b th C(a)

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Mi s kin c gn vi mt vector tem thi gian VT sao cho nu Vtt(a) < VT(b) th s kin a l nguyn nhn ca s kin b.

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

- Vi[i] l s cc s kin xy ra ti Pi - 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 m bo 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 hn pha nhn s bo cho bit l c bao nhiu s kin cc tin trnh khc xy ra trc khi Pi gi thng ip m.Ni cch khc vector tem thi gian 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]

Cu 24 Trnh by cc gii thut bu trn trong ng b ha ng h logic. Nhiu thut ton phn tn i hi 1 tin trnh ng vai tr nh iu phi vin

(coordinator), ngi khi xng (initiator), hoc khng th thc hin 1 vai tr c bit. Trong phn ny ta s xem xt cc thut ton bu chn iu phi vin. Thut ng iu phi vin c dng nh 1 tn tng qut cho tin trnh c bit.

Nu tt c cc tin trnh u ging ht nhau, khng c cc c im phn bit, th khng c cch no chn ra mt tin trnh c bit. V th chng ta s gi s rng mi tin trnh c mt con s duy nht, v d nh a ch mng ca n ( n gin ta cng cho rng mi tin trnh trn 1 my). Ni chung, cc thut ton bu chn s c gng xc nh tin trnh vi s tin trnh (process number) l cao nht v ch nh n l iu phi vin. Cc thut ton khc nhau th s khc nhau trong cch xc nh ny.

Khi tin trnh iu phi gp li th s phi c qu trnh bu chn chn ra mt tin trnh khc lm iu phi thay cho n. C hai gii thut bu chn hay c s dng l: 1. Gii thut p o (Bully Algorithm) Gi thit Mi mt tin trnh u c mt ID duy nht. Tt c cc tin trnh khc u c th bit c 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 chn nu nh n khi phc li sau qu trnh xy ra li hoc tin trnh iu phi b trc trc. Cc bc ca gii thut:

- P gi thng ip ELEC n tt c cc tin trnh c ID cao hn - Nu khng c tin trnh no phn hi th P s tr thnh tin trnh iu phi - Nu c mt tin trnh c ID cao hn phn hi th n s m nhim vai tr iu phi.

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hnh: V d theo gii thut p o

2. Gii thut vng (Ring Algorithm) Gi thit Cc tin trnh c mt ID duy nht v c sp xp trn 1 vng trn Logic. Mi mt tin

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 trnh

gi theo 1 hng nht nh. Thm d lin tip trn vng cho n khi tm c 1 nt cn 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 ng

v gi thng ip iu phi cho tin trnh .

Cu 25: ng b ha ng h logic. Nu cc gii php cp kha, kha file v thu hi kha trong cc h thng file phn tn. 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 l rt cn thit v rt kh khn. ng b ha ng h logic (Logical Clock) Trong nhiu trng hp, gia cc tin trnh khng nht thit phi ph hp theo thi gian thc t m ch cn khp vi nhau v thi gian. Do ngi ta a ra khi nim ng h logic. a. 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" ( B : A xy ra trc B th tt c cc tin trnh trong h phn tn tha thun s kin A xy ra trc ri n s kin B.Khi c A B l ng.A v B l hai s kin ca cng mt tin trnh. Nu A xy ra trc B th A 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. C. C th A B , BQuan h xy ra trc c tnh bc cu: A 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 v tha mn cc iu kin sau: B trong cng mt tin trnh th C(A)Nu 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)

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Vi mi s kin phn bit (khng c lin quan) th C(A)C(B) C. Vector thi gian (Vector Timestamps) Gii thut vector timestamp a ra mt vetor timestamp VT(a) gn cho s kin a c thuc 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 trnh khc nhau th N khc nhau)

- Vi[i] l s cc s kin xy ra ti Pi - 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 m bo 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 hn pha nhn s bo cho bit l c bao nhiu s kin cc tin trnh khc xy ra trc 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] * Cc gii php cp kha, kha file v thu hi kha trong cc h thng file phn tn 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. Bn A s to ra mt s ln khc l x, bn B to ra s ln y v gi b mt chng. Bn A s gi 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 nh c kha b mt y ca bn B. b. Phn pht kha.

Trong h m mt i xng, kha b mt to ra phi c truyn i trn knh mt ring. Trong h mt m dng kha cng khai, kha cng khai phi m bo cng mt cp vi mt kha b mt. Kha cng khai c truyn i nh mt bn r trn ng truyn v phi c h tr xc thc. Kha b mt c truyn i trn mt knh ring v cng phi c xc thc. 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 nh danh xc nh c kha mt lin kt vi n. Cu 26: Trnh by nhng hiu bit v nht qun v nhn bn trong cc h thng phn tn.

Nht qun v nhn bn (Consisstency & replication) 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 v mt 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 khng chnh xc. - Dng bn sao tng hiu nng ca h thng: c th tng quy m h thng c v s lng ln phm vi a l. - Tuy nhin vic s dng nhn bn cng phi tr gi, l tnh nht qun d liu ca h thng b suy gim. Do s dng bn sao nn c th xy ra trng hp c s thay i trn mt d liu

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m khng cp nht trn cc bn sao ca n. iu ny s gy ra cc sai st trong h thng. Do phi tn nhiu cng sc xy dng cc m hnh m bo tnh nht qun ca d liu. * Cc m hnh nht qun ly d liu lm trung tm. 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 v mt 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 h thng xc nh ng khi nim "gn nht". iu ny l kh kh thi vi h phn tn. 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 cu: - Kt qu ca s thc hin bt k l nh nhau nu thao tc c v ghi do cc tin trnh thc hin trn mc d liu mt cch tun t v cc thao tc ca mi tin trnh xut hin 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 cc thao tc l nh nhau.

b. M hnh nht qun tuyn tnh: L m hnh yu hn m hnh nht qun cht nhng mnh hn m hnh nht qun tun t. 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 s kin 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. 4 /M hnh nht qun FIFO . Nht qun FIFO cn c gi l nht qun PRAM. y l m hnh yu nht v m hnh ny b qua gii hn v trt t ca bt k thao tc ng thi no. 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 c ng 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 nht qun tun t. Khng c thao tc no ln cc bin ng b ha c php thc hin cho n khi tt c 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 hin cho n khi tt c cc thao tc trc ln cc bin ng b ha c thc hin. 6/ M hnh nht qun i ra (Release consistency). S dng thm hai lnh: lnh acquired bo mun vo vng ti hn (critial region) v lnh release bo gii phng vng ti hn. Hai lnh ny cng c hai cch thc thi khc 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. 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 lnh acquired v release khi mun s dng vo vng ti hn. Nhng cc lnh ny thao tc trn tng mc d

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liu ca vng d liu chia s. Tin trnh no mun s dng mc d liu th phi i cho tt c cc tin trnh khc gii phng mc d liu . * Cc m hnh nht qun ly client lm trung tm. 1/ Nht qun cui cng (eventual consistency). Khi mt d liu c nhiu bn sao th yu cu a ra l sau cc thao tc cp nht th tt c cc bn sao cui cng l phi bng nhau. Yu cu ny s c thc hin tt nu mi client 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 lun m bo rng ngay c khi client thay i v v tr vt l th vic s dng cc bn sao cng phi chnh xc. Tc l cc bn sao lun lun l nht qun. 2/ Nht qun c u (monotonic - write consistency). Mt tin trnh thc hin thao tc c trn mt mc d liu th phi m bo bt k thao tc 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 mt bn sao ri tip theo li c trn mt bn sao khc th bn sao th hai kia t nht cng phi c ghi ging vi bn sao u tin.

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). 3/ Nht qun ghi u (monotonic - read consistency). Thao tc ghi trn mc d liu x ca mt tin trnh phi c hon thnh trc bt k mt 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 c trt t. 4/ Nht qun c kt qu ghi (Read - your - write consistency) Trong m hnh nht qun ny, ngi dng c m bo rng s lun c nhn thy nhng kt qu ghi mi nht. "Tc ng ca mt thao tc ghi ca mt tin trnh ln mc d liu x s lun c nhn thy bi mt thao tc c ln lt trn x ca cng tin trnh ". 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 mt ngi 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 nhn thy bi mt thao tc c lin tip ln x ca cng tin trnh ".. Cu 27 Trnh by nhng hiu bit v cc giao thc ko v y trong qun l cc bn sao.

Qun l cc bn sao = lan truyn cp nht. C 3 kh nng lan truyn cp nht: 1. Ch lan truyn thng bo v cp nht ti cc bn sao khc Thng bo v vic mt hiu lc ca mt giao thc. Tc l vic n truyn i mt thng bo

rng cc bn sao khc khng nht qun vi bn sao hin ti. Thc hin tt khi t l cc thao tc c vi thao tc ghi nh (ghi nhiu) 2. Truyn d liu cp nht t bn sao ny ti mt bn sao khc Thc hin tt khi c nhiu thao tc c. Ghi li cc thay i v tp hp cc cp nht li truyn i ( ch truyn i cc thay i ch

khng truyn i c d liu bi thay i, v th tit kim c bng thng) 3. Lan truyn cc thao tc cp nht ti cc bn sao khc (nhn bn ch ng)

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Tn t bng thng nhng i hi nng lc x l cao v trong nhiu trng hp th cc thao tc l rt phc tp.

Trong 3 phng php trn th khng phng php no c th coi l tt nht v l do cn ty thuc vo bng thng sn c cng nh t l cc thao tc c so vi cc thao tc ghi ln cc bn sao Cc giao thc ko v y:

Cp nht y (Push based approach): Trong giao thc ny server l ch ng, mi khi c cp nht, server s y cc d liu cp nht v cho client. Nhc im ca phng php ny l server cn gi danh sch cc client lan truyn khi c cp nht.

Cp nht ko (Pull based approach): Giao thc ny thng c s dng kt hp vi client caches. Client lun lun ch ng hi server c cp nht no hay khng. Khi c cp nht, client s ly d liu cp nht v v b sung vo client caches.

Cu 28 Phn bit bn sao khi to bi client v bn sao khi to bi server. ngha v trng hp vn dng. Bn sao do Server khi to Cc bn sao ny c s dng lm tng hiu nng. Cc bn 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 nhn c cc yu cu t mt v tr xa, h s to ra mt bn sao tm thi vng m yu cu n. Vic a ra cc m ch bn sao nh vy s n gin hn vic phi to tm ra v tr t bn sao.

c im - Gim ti trn my ch - Cc tp tin c ch nh s c a ti cc v tr gn vi my khch hn.

Mi my ch theo di vic truy cp vo cc tp tin (m) v v tr m yu cu n. V d nh, mi my ch c th bit c my khch no l gn vi mt my ch no nht, nu c hai my trm cng chung mt my ch gn nht no th tt c cc yu cu ti my ch ban u s c chuyn ti my ch . Nu nh s yu cu ti my ch ban y m gim xung di mt gi tr no th tp tin s b xa khi my ch . iu s lm gim s lng cc bn sao v lm tng cc yu cu ti cc bn sao khc. Tuy nhin vn phi m bo rng t nht cn c mt bn sao tn ti. Cc bn sao do client khi to Mt loi bn sao quan trng l bn sao c to ra bi my khch, n thng thng c gi l b nh m cache) 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 da vo yu cu ca client. B nh m ca my khch c s dng tng thi gian truy cp d liu. Thng thng, khi my khch mun truy cp mt s d liu no n kt ni ti bn sao gn nht ca d liu v ly d liu n cn v c v lu tr d liu m n mi thay i. Nu cc thao tc u l c d liu th hiu nng c th tng ln nh my trm lu tr d liu trong b nh tm. Cc d liu ny c gi trong b nh tm trong mt thi gian nht nh no . Thng thng b nh tm nm ngay trn my client. Cu 29 Sao chp v caching c s dng trong cc h phn tn nh th no?

Sao chp : Mt trong cc vn khi m rng h thng l gim hiu xut lm vic ca h thng. mt trong nhng tng hn ch iu l ng dng sao chp gia cc phn t

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ca h thng. Sao chp ko ch lm tng kh nng truy cp ca h thng n cn cho php cn bng ti gia cc thnh phn ca h thng. iu ny lm tng hiu xut lm vic ca h thng

Caching : c xem nh mt hnh thc sao chp (replication) c bit, thm vo s khc bit gia chng l khng nhiu. Kt qu ca cachhing cng l to mt sao chp d liu

Cache l mt dng c bit ca nhn bn, n c iu khin bi client thay v c iu khin bi server. C nhiu gii php ho vic cache d liu. Chin lc pht hin coherence: xc nh khi no th s khng nht qun xy ra l t d loi b nhng d liu gy ra s khng nht qun. C hai gii php khc nhau: o Gii php tnh: ti thi im bin dch chng trnh th nhng ch th ph thuc s c thm vo pht hin nhng d liu khng nht qun. o Gii php ng: ti thi im chy chng trnh th c nhng on m kim tra tnh khng nht qun ca d liu cache vi d liu ca server. Chin lc p buc coherence: xc nh xem d liu cache c gi nht qun vi d liu lu trn server nh th no. C 2 cch buc d liu phi cht ch vi nhau o cho client m bo s nht qun ca d liu. o cho server gi i mt thng ip v s khng hp l mi khi d liu b thay i Vic thc hin cc thao tc ghi d liu vo cache c tin hnh nh th no? Cache ch c (Read - only Cache): Cc cp nht c thc hin bi server (bng giao thc y) hoc client (bng giao thc ko mi khi client nhn thy d liu cache c) Cache ghi thng (Write - through cache): client s thay i ni dung ca cache, sau s gi cc cp nht n cho server. Cache ghi li (Write - back cache): client tr hon s lan truyn cp nht, cho php nhiu cp nht c to ra cc b sau gi cc cp nht mi nht cho server. Cu 30 Trnh by cc cch thc lan truyn cp nht (ch thng bo l c cp nht, truyn d liu cp nht t bn sao ny ti bn sao khc, lan truyn cc thao tc cp nht) C 3 kh nng lan truyn cc cp nht 1. Ch lan truyn thng bo v cp nht ti cc bn sao khc. Thng bo v vic mt hiu lc ca mt giao thc. Tc l vic n truyn i mt thng bo rng cc bn sao khc khng nht qun vi bn sao hin ti. Thc hin tt khi t l cc thao tc c so vi thao tc ghi nh (ghi nhiu) 2. Truyn d liu cp nht t bn sao ny ti mt bn sao khc. Thc hin tt khi c nhiu thao tc c Ghi li cc thay i v tp hp cc cp nht li truyn i (ch truyn i cc thay i ch khng truyn c d liu thay i, v th tit kim c bng thng) 3. Lan truyn cc thao tc cp nht ti cc bn sao khc (nhn bn ch ng) Tn t bng thng nhng i hi nng lc x l cao v trong nhiu trng hp th cc thao tc l rt phc tp. Trong 3 phng php tip cn trn th khng phng php no c th coi l tt nht v n cn ty thuc vo bng thng sn c cng nh t l cc thao tc c so vi thao tc ghi ln cc bn sao.

Cu 31. Tnh chu li trong cc h phn tn. ngha, v d mt h thng c tnh chu li - Mt c tnh ring bit ca h phn tn gip phn bit vi h thng my n l khi

nim v li ring phn (partial failure). Mt li ring phn c th xy ra khi mt thnh phn trong h thng b s c, v li ny c th nh hng n hot ng chung ca cc thnh phn khc. Mt mc tiu quan trng trong thit k h phn tn l xy dng nn 1 h thng m n c

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th t ng hi phc li cc li ring phn m khng lm nh hng g nghim trng n ton b hiu nng ca h thng.

Mt h thng c kh nng chu li l mt h thng m bo c nhng yu cu sau: 1. Tnh sn sng 2. Tnh tin cy 3. Tnh an ton

4. Tnh bo tr - Tnh chu li c ngha to ln trong cc h phn tn. Trong thit k h thng ni

chung, ty thuc vo mc quan trng v tnh cht sng cn ca h thng m ngi thit k phi bo m tnh lin tc trong hot ng ca h thng.

V d: h thng iu khin bay, h thng c s d liu ca cc ngn hng, h thng active-directory ca mt doanh nghip Cu 32. Lm r tnh sn sng, tnh tin cy, tnh an ton v kh nng bo tr ca mt h thng c tnh chu li.

Mt h thng c kh nng chu li l mt h thng m bo c nhng yu cu sau: 1. Tnh sn sng: h thng hot ng chnh xc trong bt k thi im no v sn sng

thc hin chc nng ca n. 2. Tnh tin cy: kh nng hot ng trong mt thi gian di m khng b gin on,

khng xy ra li. Tnh tin cy khc vi tnh sn sng: - H thng hot ng lin tc, nhng c mi gi b down 1 giy, tc l tnh sn

sng t 99,99% nhng l h thng khng tin cy c. - H thng khng b sp bao gi, nhng c vo thng 8 hng nm th ngh

lm vic 2 tun c tnh sn sng 96% nhng l h thng c tnh tin cy cao. 3. Tnh an ton: khi xy ra li cng khng dn ti thm ha. Cc h thng cn phi c

an ton cao l cc h thng iu khin. V d cc h thng iu khin tin trnh nh h thng dng iu khin nh my ht nhn hay a con ngi vo v tr yu cu an ton cao. Nu h thng iu khin ch b li trong mt khong thi gian rt ngn, hu qu c th rt thm khc. Nhiu v d trong qu kh chng t rt kh xy dng mt h thng an ton

4. Tnh bo tr: h thng c kh nng phc hi li c sau khi c li. Nu s phc hi ny din ra t ng th c th ni h thng ny cng c tnh sn sng cao.

Cu 33+34+35. Cc loi d tha c s dng tng tnh chu li trong cc h phn tn. Lm r cc hiu bit v che du li bng d tha.

H thng c kh nng chu li, n phi c kh nng che giu nhng li xy ra vi cc tin trnh khc. K thut chnh che giu li l s dng s d tha. C 3 loi c th thc hin c l: d tha thng tin; d tha thi gian; d tha vt l Trong : - D tha thng tin: B sung thm cc bit d tha pht hin li v phc hi li. V d trong vic truyn d liu thng thm vo cc bit kim tra chn l, m Haming, CRC... pht hin li v khi phc li. - D tha thi gian: mt hnh ng c thc hin, sau nu cn thit n s c thc hin li mt ln na. Cc giao dch s dng phng php ny. Nu mt giao dch b b qua, n c th c thc hin li m khng c tn hi g. S d tha thi gian t ra c bit hu ch khi li l tm thi hoc khng lin tc. - S d tha vt l ngha l cc tin trnh hoc thit b d phng c thm vo gip cho h thng hon thin chng li s thiu hoc hot ng sai chc nng ca mt s thit b.

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V d: K thut TMR, mi thit b c sao chp thnh 3 bn. Vi cc voter V l cc mch c 3 u vo, 1 u ra, nu hai hoc 3 u vo l ging nhau th cho kt qu u ra, ngc li, nu c ba u vo u khc nhau th u ra khng xc nh:

Cu 36. Cc thao tc khc phc li pha server v client trong truyn thng RPC

Vi RPC c th xy ra 5 loi li sau:1. Client khng th nh v c server.2. Tin nhn yu cu t client ti server b mt. 3. Server gp li sp ngay sau khi nhn c yu cu t client.4. Tin nhn tr li t server ti client b mt. 5. Client gp li sp sau khi gi tin nhn yu cu. Khc phc li: Li 1: Client khng nh v c server

Nguyn nhn: do server v client dng cc phin bn khc nhau hoc do chnh server b li.

Khc phc: s dng 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 phi ngn ng no cng h tr ngoi l hay iu khin tn hiu. Nu t vit mt ngoi l hay iu khin tn hiu th s ph hy tnh trong sut.

Li 2: Tin nhn yu cu t clien n server b mt H iu hnh 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 s gi

li yu cu . Nu bn client nhn thy c qu nhiu yu cu phi gi li th n s xc nhn rng server

khng hot ng v s xc nh li l "khng nh v c server. Li 3: Server gp li sp ngay sau khi nhn c yu cu t client Chia li ny thnh hai loi:

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

Loi 2: Va nhn c yu cu t client th server b li ngay. Phng php khc phc: client ch cn truyn li yu cu cho server. Nhng vn l client khng th ni cho server bit yu cu no l yu cu c gi li.

Cc thao tc khc phc li pha server:

K thut 1: i n khi n hot ng tr li, c thc hin yu cu nhn c trc khi li . Nh th RPC thc hin t nht mt ln.

K thut 2: sau khi c khi phc server s khng thc hin yu cu nhn c trc 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.

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

Cc thao tc khc phc li pha client: 1. 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. 2. Client lin tc gi li yu cu: c th dn ti trng hp mt yu cu c thc hin nhiu ln. 3. Client ch gi li yu cu no khi khng nhn c bn tin ACK phn hi t server 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 gi li yu cu . 4. Client gi li yu cu nu nhn c thng bo hng t server

Li 4: Tin nhn tr li t server ti client b mt. Phng php khc phc: thit k cc yu 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 th t ny. Do server s khng thc hin lp li cc yu cu. Tuy nhin server vn phi gi tr v bn tin thng bo yu cu no b tht lc. Hoc ta c th s dng mt bit phn header ca yu cu phn bit yu cu no l yu cu c gi li.

Li 5: Client gp li sp sau khi gi tin nhn yu cu. Client gi yu cu ti server ri b li trc khi nhn c tr li t server gi v. Cng

vic m server thc hin nhng khng c ch no i nhn c gi l mt orphan. Nh th s gy lng ph chu k CPU.

C 4 gii php c a ra trong trng hp ny l: 1. Trc khi gi i yu cu, client stub lu li yu cu cng vic. Sau khi phc hi sau

li, vic thc hin cc orphan ang din ra s dng li 2.Chia thi gian hot ng lin tc ca client thnh cc thi k. Khi client khi phc th ch s thi k li tng ln mt n v. Lc ny client s gi thng bo n tt c cc my khc thng bo s thi k mi ca mnh. Khi nhn dc thng bo ny th cc orphan s dng li. 3. Khi nhn c bn tin thng bo thi k mi, server s kim tra xem c ang thc hin yu cu t xa no khng v s c xc nh xem client no gi yu cu ny. Nu khng xc nh c th qu trnh tnh ton ny s b hy b. 4. Quy nh mi RPC ch c mt khong thi gian xc nh T thc hin, sau khi gp li, client s phi i thm mt khong thi gian T trc khi khi ng li nhn cc orphan.