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1.Gii thiu1.1. nh ngha h phn tn

Lin quan n khi nim h phn tn, c kh nhiu nh ngha c a ra. Trong s , ba nh ngha v hphn tn di y l hay c nhc ti nht:

nh ngha 1: H phn tn l mt h my tnh c lp giao tip vi ngi dng nh mt h thng thng nht vton vn.

nh ngha 2: H phn tn l mt tp cc my tnh t tr c kt ni vi nhau qua mt mng my tnh v ci t phn mm h phn tn.

nh ngha 3: H phn tn l mt h thng c chc nng v d liu c phn tn trn cc trm (my tnh) khnhau, chng c kt ni vi nhau thng qua mt mng my tnh.Trong ba nh ngha trn th nh ngha mt c xut pht t quan im ngi dng cn nh ngha hai v n

ngha ba c xut pht t quan im k thut.D xut pht t quan quan im no i na, cc nh ngha trn y u ngm ch ra rng hai thnh phn qua

trng nht ca h phn tn chnh l h thng my tnh v phn mm phn tn. Nh vy, chng ta c cng thtng trng sau y :

DS ComputerNetwork + DistributedSoftware (1.1)Vic hnh dung ra khi nim h phn tn m ch da vo mt nh ngha l thuyt nh trn qu tht l rt kh

khn. Sau y l mt s v d in hnh v h phn tn minh ha cho nh ngha trn:Tp hp cc Web server: c s d liu phn tn ca cc siu vn bn v cc ti liu a phng tin.H thng file phn tn trn mt mng LANH thg phn gii tn min DNS

1.2. Phn loi HPT1.2.1. H thng phn tn tnh ton (Distributed Computing System)

L h thng gii quyt cc nhim v tnh ton hiu nng cao, cc h thng c khi lng tnh ton ln. T mnhim v ln ta li phn chia thnh cc nhim v nh hn, thnh cc h thng tnh ton song song

Cluster computer system: H thng tnh ton phn cm. H thng c ghp thnh cc cm my tnh ginnhau, mi cm c kt ni vi nhau thng qua mt mng tc cao. N tr nn ph bin khi t l hiu nng/gica cc my tnh c nhn v my trm c ci thin. c s dng cho tnh ton song song, ni m mt chngtrnh c chy song song trn nhiu my.

Grid computer system: H thng tnh ton li. Cc my tnh trong h thng khng cn phi ging nhau v

platform. N gip chng ta khai thc hiu qu hn cc ti nguyn nhn ri. Ti nguyn t nhiu t chc khnhau c gn vi nhau cho php s hp tc gia mt nhm ngi, hoc gia cc phn vin. S hp tc ny cnc gi l t chc o (virtual org).

Phn bin 2 loi ny: Trong h thng tnh ton cm, phn cng pha di l mt tp hp cc my trm ginnhau, c kt ni vi nhau bi mt mng LAN tc cao, v cng chy 1 OS ging nhau. Trong h thng tnton li, n bao gm 1 HPT c xy dng nh l 1 tp hp ca cc h thg my tnh, mi h thg my tnh c ththuc v cc lnh vc hnh chnh khc nhau, v chng c th c phn cng, phn mm cng nh cng ngh mnc trin khai khc nhau.1.2.2.H thng thng tin phn tn (Distributed Information System)

L cc h thng tch hp cc ng dng khc nhau thnh mt h thng thng tin doanh nghip.Transaction processing system: Cc h thng x l giao tc, quan im ca h thng ny l hoc tt c u

x l, hoc khng c yu cu no c x l. - 4 tnh cht ca giao dch ACID : + Atomic: vi th gii bn ngoigiao dch ny l khng th phn tc c. + Consistent: giao dch khng vi phm cc tnh bt bin ca h thng. Isolated: Cc giao dch xy ra ng thi khng nh hng n nhau. + Durable: 1 khi giao dch commit, s thayi l vnh vin. Nested transaction : c xy dng t nhiu subtransaction. N rt quan trng trong HPT, bi ncung cp 1 cch t nhien phn tn cc giao dch qua nhiu my tnh.

Enterprise Application Integration: Cc h thng tch hp ng dng doanh nghip, chng t th tch hp c lvi c s d liu, v cc phng tin c th truyn thng vi nhau. c th truyn thng gia chng, chnta s dng 1 s m hnh sau: + RPC. + RMI: ging nh RPC, ngoi tr vic n hot ng trn cc i tng thay vhot ng trn cc ng dng. + Nhc im ca 2 hnh thc trn l c ngi gi v ngi nhn u cn phi onlinv chy ti thi in truyn thng. Vi MOM, cc ng dng ch cn gi thng ip ti mt im lin lc logicthng l mt i tng. Nh th,1 ng dng c th ch ra n quan tm n loi thng ip no. Sau , phn trung

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gian m nhim vic truyn thng s chuyn thng ip ti ng dng ph hp. H thng ny cn c gi l hpht hnh/ng k.

V mt vt l, cc h thng ny (v ) thng n nh, thm ch bn thn chng cn c ngun cung cp c nh.1.2.3.H thng phn tn rng khp (Distributed pervasive system)

y l cc h thng trong bao gm cc thnh phn l cc thit b di ng, nhng.. Chng s dng ngun inpin, acqui.

Ba yu cu ca mt ng dng phn tn rng khp: - Embrance contextual changes: thit b phi bit c mtrng ca n c th thay i bt c lc no. Encourage adhoc composition: Cc thit b trong hpt rng khp c

th c s dng theo nhiu cch khc nhau bi nhiu kiu ngi s dng khc nhau. Do , vic cu hnh tp cng dng chy trn thit b phi d dng. recongnize sharing as the default:Mc d tr ngi ca h thng ny l vn v m bo ngun nng lng, nhng n li c tnh di ng cao v

kh nng pht trin rng khp.Chng ta cng c th lit k mt s v d ph bin ca h thng ny:Intelligent home systems: Cc h thng nh thng minh.E-health care systems: Cc h thng chm sc sc khe m rng.Sensor networks: Cc mng sensor.

1.3.Cc c trng ca h phn tn1.3.1.Chia s ti nguyn (resources sharing)

Cc ti nguyn c th chia s trong h phn tn l:Ti nguyn vt lTi nguyn logici vi h phn tn, bi ton chia s ti nguyn phc tp hn v bn thn cc my tnh t tr cng c ti nguy

ring ca n. Tuy nhin, ta s khng cp n vn chia s cc ti nguyn dng ring ny m ch yu i vochia s ti nguyn dng chung.

C th hn, bi ton chia s ti nguyn trong h phn tn c hai i tng chnh:Tp cc ti nguyn dng chung : Cc ti nguyn ny l phn tn, chng l hu hn v c kh nng b sung

( v d nh hiu nng ca CPU) . Tuy nhin, chng ta khng th b sung cc ti nguyn ny mt cch ty , tytin c.

Tp cc ngi s dng (user) : Tp ny c c im l phn tn, hu hn v c tc tng trng rt nhanh. T , chng ta d dng nhn thy, lng ti nguyn trong h phn tn th hu hn m s lng ngi s

dng li cng nhiu. iu dn ti vn chia s ti nguyn ngy cng tr nn cng thng, n c th gy ra xungt, tc nghn trong h phn tn. V h phn tn s phi m bo lm th no vic chia s ti nguyn tr nhiu qu nht. Nh vy, mt bi ton chia s ti nguyn cn m bo gii quyt cc yu cu sau:o Mc mt : Trnh cc hin tng xu ( tc nghn...) xy rao Mc hai: m bo vic chia s ti nguyn mt cch hiu qu.1.0.1. Tnh m (openess)Thng thng, mt h thng no thng cung cp nhiu dch v khc nhau cho cc i tng ngi dng

khc nhau. i vi h phn tn, n c gi l c tnh m nu nh khi ta b sung mt dch v mi, th dch v nyc kh nng chung sng bnh thng vi cc dch v trc . V bi ton chia s ti nguyn vn c gii quymt cch hp l.

Mt s c trng: - Interoperability: Cc thnh phn khc nhau trn cc h thng khc nhau c th cng tn t

v lm vic vi nhau. Portability: cc ng dng c trin khai trn hpt A cng c th c thc thi m ko cchnh sa trn 1 hpt B khc c cng giao din nh hpt A. Extensible: d dng thm cc thnh phn mi, thay thcc thnh phn c m ko h nh hng n phn cn li ca hpt.

1.0.2. Tnh trong sut(transparencyH phn tn d c hon ho, tt p n bao nhiu th bn cht ca n vn l ri rc, v ngi thit k phi lm

th no che giu, lm gim nh hng, khim khuyt ca h phn tn i vi ngi s dng.Nh vy, tnh trong sut c hiu l s che giu s phn tch, ri rc ca cc thnh phn trong h phn tn

vi ngi s dng. Qua , ngi s dng s coi h thng nh l mt h thng thng nht. Tnh trong sut l mtnh cht rt mnh, v cng rt kh t c. Thng thng, c mt vi dng tnh trong sut chnh sau:

Access Transparency: Ti nguyn ton cc v cc b cng c truy cp theo cch ging ht nhau.

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Location Transparency: Ngi s dng s khng nhn bit c v tr vt l thc t ca ti nguyn m h ondng

Migration Transparency : Ti nguyn c th c di chuyn t ni ny sang ni khc m ngi s dng khngnhn ra s thay i

Replication Transparency: Ngi s dng khng nhn ra s tn ti ca nhiu bn sao ca ti nguyn trong hthng

Failure Transparency: Ngi s dng khng nhn ra h thng m h ang s dng ang c li mt thnh phno .

Concurrency Transparency: Ngi s dng khng h bit h ang chia s ti nguyn dng chung vi rt nhingi s dng khc.1.0.3. Tnh co dn (Scalability)Tnh co dn l mt tnh cht quan trng trong h phn tn. Mt h thng c gi l c th co dn (scalable) nu

n c th kim sot c vic gia tng ca ti nguyn, cng nh ngi s dng m khng lm nh hng ti hinng, cng nh lm tng phc tp ca h thng 1. Hay ni cch khc, iu ny lin quan n vic xy dng mh phn tn sao cho trong bt c trng hp thay i no, th phn mm h phn tn ca chng ta phi c kh nngp ng c.

Thng thng, vic co dn thng lin quan n s gia tng kch thc ca h phn tn theo ba kha cnh chnhsau:

Size : S lng ngi s dng v s lng ti nguyn, ng vi s gia tng ny, h thng c th i mt vi nguc b qu ti ( do n phi x l nhiu user request hn). Tng t nh vy, do h thng phi qun l mt s lngti nguyn ln hn, n cng c th b qu ti.

Geography: H phn tn cng c th pht trin theo khong cch a l, l khong cch vt l thc t ginhng ngi s dng, nhng ti nguyn vi nhau. Vn lin quan n s m rng v a l, chnh l vn vtruyn thng ( khong cch cng xa th tr truyn thng cng ln, v nguy c xy ra li cng cng cao)

Administration: H phn tn c pht trin v m rng theo nhiu lnh vc hnh chnh khc nhau. Vn linquan n vic m rng ny, l s xung t cc chnh sch gia cc t chc trong qu trnh m bo vic sdng, qun l v bo v ti nguyn mt cch ng n.

1. Cc nguyn l thit k cc h phn tn1.1. Kin trc1.1.1. Kin trc phn mm (Software architectures)y l mt m hnh t chc logic ca cc phn mm h phn tnKin trc phn tng tng ca kin trc phn tng l rt n gin: Cc thnh phn c t chc theo tng tng gi ln nhau sao

cho mt thnh phn tng Li c cho php gi ti cc thnh phn tng Li-1 bn di n, nhng khng c chingc li (Error: Reference source not found). Vi m hnh ny, lung iu khin thng c chuyn t tng nsang tng khc : Trong khi cc yu cu (request) i t tng cao xung tng thp th cc tr li (result) li i theochiu ngc li.

Mt v d ca kin trc ny, l v d v Mng my tnh. N l mt trng hp ring ca h phn tn, vi ccphn mm phn tn l phn mm ci t cc giao thc.

Kin trc da trn hng i tng.Vi kin trc ny, mi i tng tng ng vi mt thnh phn, v cc thnh phn ny c kt ni vi nhau

thng qua c ch RPC. Trong kin trc hng i tng, kin trc phn mm ca n vn tha mn cc tnh chca m hnh Client/Server. Hin nay, kin trc phn tng v kin trc da trn i tng vn ang l m hnh kintrc quan trng nht cho cc h thng phn mm ln.

Kin trc da trn d liu tng ca kin trc ny l cc tin trnh s lin lc vi nhau thng qua mt th mc dng chung (common

repository). i vi cc h phn tn, kin trc ny cng quan trng nh hai kin trc m ta cp trc y. V dmt tp cc ng dng mng c pht trin da trn mt h thng file phn tn chia s (shared distributed filsystem), , tt c cc qu trnh truyn thng din ra thng qua cc file. Tng t vy, cc h phn tn da tr

1

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web cng l mt v d: Cc process lin lc vi nhau thng qua vic s dng cc dch v d liu da trn web(shared Web-based data services)

Kin trc da trn s kinTrong kin trc da trn s kin (Error: Reference source not found) Cc tin trnh lin lc vi nhau thng qu

qu trnh lan truyn s kin. Trong h phn tn, vic lan truyn s kin thng lin quan n khi nimpublish/subscribe system . tng ng sau khi nim ny l, cc tin trnh s pht ra cc s kin, v sau phntrung gian s m bo rng ch c cc tin trnh ng k nhn cc s kin ny mi nhn c chng. u im chnhca kin trc ny, l cc thnh phn trong h thng c tch ri nhau, n khng cn quan tm n s tn t

ca cc thnh phn khc.Khng gian d liu chia s (Shared data spaces)Kin trc hng s kin c th c kt hp vi kin trc hng d liu to thnh khng gian d liu chia s

tng c bn ca khng gian chia s d liu, l cc tin trnh c tch bit vi nhau v thi gian, chnkhng cn phi cng active khi qu trnh truyn thng din ra. Hn na, rt nhiu khng gian d liu chia s sdng giao din tng t SQL cho th mc c chia s vi ngha rng d liu c th c truy cp bng cch sdng mt m t hn l mt tham chiu c th.

1.1.2. Kin trc h thng (System architectures)Dng mo ti vic b tr, ci t cc thnh phn trong h phn tn trn cc my trm khc nhau nh th no.

phn di y, em xin tm hiu v mt s m hnh kin trc h thng c th.Kin trc tp trungV d in hnh ca kin trc tp trung l m hnh Client/Server. Trong m hnh Client/Server, cc tin trnh

trong h phn tn c chia ra thnh hai nhm (c th gi ln nhau). Mt server l mt tin trnh ci t mt dcv c th no , v d, dch v h thng file (file system service) hoc dnh v c s d liu (database service)Mt client l mt tin trnh yu cu dch v t server bng cch gi request v i tr li t server. Tng tc giclient server c m t nh trn cn c gi l hnh vi hi-p (request-reply behavior)

Hnh 2.1: Tng tc trong m hnh Client-ServerTruyn thng gia client v server c th c ci t theo mt giao thc v hng n gin (connectionles

protocol) hoc giao thc c hng ng tin cy (reliable connection-orriented protocol).Nhc im : Khi client gi request n server th n phi ch ( v thi gian ch ny coi nh thi gian cht)

Chng ta c th khc phc iu ny bng cch sau khi server nhn c request ca client, th n ch cn gi ack lcho client, trong thi gian server x l, client s lm cc cng vic khc..

Kin trc phi tp trung: Vi v d l mng ngang hng p2pLin quan n vn ny l khi nim phn tn theo chiu ngang (horizontal distribution). Vi hnh thc ph

tn ny, mt client hay server c th b phn chia mt cch vt l thnh cc phn bng nhau, mi phn hot ngtrn mt phn m n c chia s t mt tp d liu hon chnh, qua , cn bng ti gia cc phn. cng chnl t tng ca kin trc phi tp trung.

Mt v d in hnh ca kin trc phi tp trung, l v d v mng ngang hng (peer to peer network)Kin trc lai: L s kt hp ca c hai kin trc trn.

Lin quan n kin trc tp trung, chng ta c khi nim kin trc client-server nhiu phn (multitiers clientserver architectures) . N l kt qu ca vic chng ta chia ng dng ra thnh cc phn nh giao din, cc thnphn x l v thnh phn d liu. Cc phn khc nhau ny c lin quan trc tip n vic t chc logic ca ndng. Trong nhiu trng hp, vic x l phn tn c th c hiu l t chc ng dng client-server nh mkin trc nhiu phn. Chng ta gi l cch phn tn theo chiu dc (vertical distribution). c im ca phtn theo chiu dc, l chng ta c cc thnh phn khc nhau mt cch logic trn cc my khc nhau

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1.2. Tin trnh1.2.1. Mt s khi nim c bnTin trnh : Mt tin trnh thng c nh ngha l mt chng trnh ang trong qu trnh thc thi, tc l, m

chng trnh ang c thc thi bi mt trong nhng b vi x l o (virtual processor) ca h iu hnh. H i

hnh cng phi m bo mt tin trnh s khng th (d v tnh hay c ) lm nh hng n hot ng ca mtin trnh khc, hay ni cch khc, vic cc tin trnh c th cng chia s mt CPU v cc ti nguyn phn cngkhc s c h iu hnh lm cho trong sut.

Lung: Mt lung thng c xem l mt n v x l nh nht m c th c lp lnh, thc thi bi h ihnh. Tng t nh process, lung cng thc thi nhng on code ring ca n c lp vi cc thread khc, tunhin, tnh trong sut v s thc thi ca cc lung khng c h iu hnh m bo nh trong trng hp cprocess. Do , cc lung thng ch cha mt s lng nh nht cc thng tin cho php mt CPU c th cchia s gia mt vi lung. Thng thng, ng cnh lung (thread context) ch bao gm thng tin v ng cnh CPU(CPU context) cng vi mt vi thng tin v qun l lung.

Li gi h thng : L li gi m h iu hnh cung cp c giao din i vi ngi s dng.o Li gi h thng b phong ta (block system call): thc hin, chng ta ch cn gi sys call, sau i k

qu. Cch thc ny th d lp trnh hn, nhng li km hiu qu v c thi gian cht.o Li gi h thng khng b phong ta (nunblocking system call): Mt mt tin trnh gi syscall, mt khc, nli tip tc cng vic ca mnh. Phng php thc hin ny em li li hiu qu cao, nhng vic lp trnh, ci t lphc tp hn.

1.2.2. Cc h thng a lung vi m hnh Client/ServerMultithreaded Clients

thit lp mt mc cao ca phn tn trong sut , h phn tn hot ng trong mt mng ln cn phi che du sln truyn cc thng ip ni b cc qu trnh . Vng tr trong mng din rng c th d dng xy ra c th t hntrm ms n mt vi s . Thng thng n tr trong kt ni l thit lp kt ni v ngay lp tc thc thi vic khc .

C mt s li ch ca vic s dng multithreaded Web browser :

Ti ni dung HTML, load nh bng cc lung khc nhauMt vi kt ni c th c m ng thi.Khi s dng multithreaeded client, connection c th c thit lp ti cc bn copy khc nhau ca web server,

cho php d liu truyn song songMultithreaded Servers

Mc d nhng li ch quan trng ca vic s dng a lung trn ng dng pha khch hng nhng ng dng chnhca a lung trong h phn tn li nm bn pha my ch . Thc t cho thy rng a lung khng ch lm n ginm trn my ch m cn lm cho vic pht trin my ch khai thc x l song song t c hiu nng cao , thmch ngay trn cc h thng n x l . Tuy nhin , by gi cc h thng a x l c s dng rng ri. chng ta c 2 thit k : mt my ch tp tin a lung v 1 my ch n lung . Mt thit k th 3 l chy my ch nh 1my trng thi hu hn . Khi 1 yu cu n , ch 1 lung kim tra n v nu tha mn th thc hin nu khng th 1thng ip s c gi t a .

1.2.3. Di tr m tng ca vic di tr m, l vic di tr cc tin trnh ( ch khng phi l source code) t v tr ny sang v

tr khc. Thc cht y l process migration.u im: Gim ti ti nguyn, tng hiu qu s dng ti nguyn, qua , to ra c tnh mm do trong th

thi h phn tn. Tht vy, hiu nng ca ton b h thng s c ci thin nu cc tin trnh c di chuyn t nphi chu ti nng sang ni ang c ti nh

Cc m hnh di tr mTa coi mt tin trnh gm c ba thnh phn:

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1.3. Truyn thngTruyn thng lm mt yu t ti quan trng trong h phn tn. S l v ch khi chng ta tm hiu v h phn tn

m li khng quan tm n cch thc cc tin trnh trong cc my khc nhau trao i thng tin. Trong phn ny, emxin tm hiu v cc quy tc m cc tin trnh phi tun theo trong qu trnh truyn thng, hay cn gi l cc giathc, v cch thc t chc cc giao thc ny thnh tng.

Bn cnh , em cng tm hiu v ba m hnh truyn thng ph bin, l RPC (Remote Procedure Call)MOM (Message-Oriented Middleware) v data streaming.1.3.1. Kin trc phn tngM hnh giao thc phn tng (Layered Protocol)

Hnh 2.2: M hnh OSI by tngM hnh giao thc phn trung gian (Middleware Protocol)

Hnh 2.3: Middleware ProtocolC bn loi Middleware ph dng:1.3.2. Li gi th tc t xa (RPC)Thc hin ch yu theo m hnh Client/Server. Khi mt tin trnh trn my A ( client) gi mt th tc trn my

B (server), tin trnh gi trn A s b treo, v qu trnh thc thi th tc c gi din ra trn my B. Thng tin ctruyn t pha gi (A) sang pha nhn (B) trong cc tham s, v c tr v di dng kt qu ca th tc. Phnphp ny c gi l li gi th tc t xa ( RPC Remote Procedure call)

Trong khi tng ca phng php l dn gin nh vy, th trong thc t, vn cn tn ti nhiu vn lin quan n. Th nht, do pha gi v pha b gi nm hai my tnh khc nhau, nn chng s c thc thi trn nhngkhng gian a ch khc nhau, v iu ny gy ra s phc tp khng nh. Th hai, tham s v kt qu tr v cnphi c truyn di, iu ny cng gy ra s phc tp nu nh hai my l khng tng t nhau. Cui cng, tronqu trnh thc hin, c th mt trong hai my xy ra li, v li ny c th gy ra mt s vn khc na.

Tuy nhin, cc kh khn ny u c khc phc phn no, v RPC vn ang l mt trong nhng k thuph bin c s dng bi nhiu h phn tn.

Ngi ta phn RPC ra lm hai loi : Synchronous RPC v Asynchronos RPC.Hnh 2.4: Synchronous RPC (a) v Asynchronous RPC (b)

Vi truyn thng ng b, sau khi client a ra request, n s phi ch phn hi t pha server. Trong thi gian, n khng lm g c, v cng chnh l mt nhc im ca vic truyn ng b. iu ny s dn ti vic chi v ch ca tin trnh gi trn client, nht l khi li gi khng c kt qu tr v, trong khi n c th thc hin ms cng vic hu ch khc. Mt s v d v vic tin trnh gi khng cn phi i kt qu tr v, l vic thmmt entry vo c s d liu, khi ng mt dch v t xa, x l hng lot ( batch processing) Chnh v vy mkhi nim truyn thng khng ng b c a ra. Chi tit v n c em trnh by ngay di y.

Vi truyn thng khng ng b, sau khi server nhn c request, n s gi mt tn hiu ACK v cho client bo nhn. Khi client nhn c tn hiu ack ny ,n c th thc hin cc cng vic khc trong thi gian server x lrequest kia.

1.3.3. Truyn thng ip (MOM) phn bn trn, em tm hiu v RPC, tuy nhin, cng c trng hp RPC t ra khng ph hp, n i hi c

bn gi v bn nhn cng phi sn sng (active) ti thi im din ra truyn thng. Tuy nhin, i khi chng tkhng th bit c liu bn nhn yu cu c ang sn sng (active) hay khng. Chng ta c th gii quyt vn ny thng qua h thng messages-hng i, hoc Messages -Oriented Middleware (MOM). H thng Messagehng i cung cp h tr rng ri cho cc giao tip khng ng b lin tc. Bn cht ca cc h thng ny l chncung cp dung lng lu tr cho, m khng i hi mt trong hai ngi gi hoc nhn c hot ng trong qutrnh truyn messages.

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1.3.4. Truyn thng hng dng (SOM)Vi cc hnh thc truyn thng bn trn, chng ta ch quan tm n vic trao i thng tin, m khng h qua

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 victhi gian truyn l nhanh hay chm. Hay ni khc i, trong cc m hnh truyn thng bn trn, thi gian khng hnh hng ti tnh ng n ca vic truyn tin.

Vi hnh thc truyn thng hng dng, thi gian ng mt vai tr c k quan trng. V d, nu mt on mthanh c pht li vi tn s khc vi tn s ly mu ca on m thanh ban u, n c th cho ra kt qu khvi on m thanh gc. Tng t nh vy, nu chng ta mun xem mt b phim trc tuyn trn mng, nu thgian truyn ca d liu l qu chm, b phim c th b gin on. Nh vy, hnh thc truyn thng ny c cuncp nhm to iu kin thun li cho vic trao i cc thng tin ph thuc vo thi gian, tc l, thi gian s nhhng n tnh ng n ca thng tin c trao i.

1.3.5. Truyn thng a im (MultiCast)Truyn thng a im chnh l kh nng gi d liu t mt im ti nhiu im nhn. Vi m hnh ny, chng

ta c hai trng hp:T mt im truyn n nhiu im: T mt im, chng ta s truyn ng thi n mt nhm cc im th

mn mt tiu ch no .

T mt im truyn n tt c cc im: broadcast ( qung b ) : t mt im, chng ta s truyn n tt c cim cn li c kh nng tip nhn iu ny l rt c ch, v n c thc hin kh nhiu trong bi cnh h phntn. V d, nu chng ta c mt tin trnh, nu ta mun a ra mt thng nht, mt chu trnh no m phi thamkho tt c cc tin trnh cn li. Trong trng hp ny chng ta s s dng broadcast.

1.4. nh danh1.4.1. Phn bit cc khi nimThc th trong h phn tn l mt khi nim kh chung chung. N c th hiu l bt c mt th g, v d nh c

ti nguyn nh host, my in, a t, file, n cng c th l cc tin trnh, l ngi s dng, l mailbox.Tn (name) : L mt xu cc bit, hoc cc k t c dng tham chiu n mt thc th trong h phn tn

Tt c cc thc th trong h phn tn mun c tham chiu n th u cn phi c tn.

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nh danh (Identifier): N l mt kiu tn c bit v tha mn cc tnh chto Mt nh danh tham chiu ti nhiu nht mt thc tho Mi thc th c tham chiu bi nhiu nht mt nh danh.o Mt nh danh lun lun tham chiu ti cng mt thc th.a ch (Address) : L mt tn c bit tham chiu n mt im truy cp (Access point) ca thc th. a ch l

tn access point ca thc th. Do mi thc th c th c nhiu access point nn n cng c th c nhiu a ch.Access Point: l 1 kiu thc th c bit trong hpt, tn ca 1 AP c gi l 1 a ch. a ch ca AP ca 1 thc

th cng c gi 1 cch n gin l a ch ca thc th

1.4.2. Cc kiu nh dng trong h phn tn.3 kiu nh danh chnh trong HPT :2.4.2.1nh danh phng (flat naming) : Cc tn c t ngu nhin, phi cu trc v khng cha bt k thn

tin g v access poin ca thc th tng ng.1 s gii php xc nh thc th khi ch bit nh danh ca n:

* cc gii php n gin : + Broadcast : thg ip cha nh danh ca thc th c broadcast ti cc my trogmg. Cc my khi nhn c thng ip s kim tra xem n c cha thc th khng. Ch nhng my tnh cim truy nhp ca thc th mi tr li li thng ip = 1 thng ip khc cha a ch ca im truy nhp Nhc : thiu hiu qu khi mng ln, lng ph bng thng. + Multicast: thng thng khi tin trnh bit thc th 1 nhm c th no, n ch cn multicast n nhm ma fko cn phi broadcast ti tt c cc my. + ForwardinPointer :khi 1 thc th di chuyn t A sang B, n li ti A 1 tham chiu ti a ch mi B ca n. Nh th, clien

c th tm kim c a ch ca thc th. Nhc: chui c th qu di -> t, tt c cc da im trung gian phi lutr 1 phn ca ng i, khi ng i b t.* Home based approaches: 1 cch tip cn c th h tr cc thc th trong hpt, l a vo 1 home location

n s lu tr thong tin a ch hin thi ca thc th. 1 s bin php k thut c th p dng bo v trog trnhp c li xy ra. Trog thc t, a ch nh thng c chn l ni m thc th c khi to.

* Distributed Hash table:* Hierarchical Approaches:

2.4.2.2 nh danh c cu trc (structured naming) : nh danh phng thng ch ph hp vi my tnh, vthng thng, chng thng gy kh khn cho con ngi. Do cc h thng nh danh cng thng h tr ctn c cu trc, c to thnh t cc tn c th c c (human-readable names). Vi nh danh c cu trc, ctn c t chc thnh mt khng gian tn, l mt th c hng. Vic phn gii tn (name resolution) l vic

duyt th v hng tm ra cc thnh phn ca tn. V d in hnh v phn gii tn chnh l DNS.* Khng gian tn: Tn thng c t chc thnh KGT. KGT cho nh danh c ctruc thg c biu din l 1 th chng, c gn nhn, vi 2 loi node chnh: - Node l: biu din 1 thc th c t tn, n ko c cnh i ra. Nodth mc: c 1 s cnh i ra, mi cnh c gn nhn vi 1 tn. Mi node trong KGT c xem nh l 1 thc th khtrog HPT. Mi node ng dn lu tr 1 bng trong , cc cnh c m r l 1 cp (nhn ca cnh, nh danh cnode). Bng gi l bng ng dn. Ngoi ra cn 1 node c bit gi l node gc, l node ch c cnh i ra.

* Phn gii tn: Thng thng, khi chng ta a ra 1 path name, chng ta phi c kh nng tm kim cc thng tinlu trong node c tham chiu bi tn . Qu trnh tm kim nh trn dc gi l PGT. Qu trnh PGT: cho 1 tn cdng N: qu trnh phn gii tn bt u ti N, ti , nhn c tn l label1 s c tm tronbng ng dn, v tr v nh danh ca node c tham chiu bi label1 .. qu trnh phn gii c tip tc cho tlabeln , v cui cng l tr v ni dung ca node .

* Dch v nh danh: + Dch v nh danh l mt dch v cung cp cc truy cp ti mt khng gian tn, n chphp chng ta c th thc thi mt s thao tc trn khng gian tn y. Cc dch v ny bao gm: - Thm hay xa 1 ng dn, hay 1 l. Chnh sa ni dung ca 1 node. Tm kim 1 node theo tn cho trc. + Cc dch v nhdanh c ci t bi cc name server, v vic phn gii tn s c thc hin bi cc resolver. Mt resolver c thc ci t bi bn thn client, bi mt name server, hay n cng c th c ci t nh l mt dch v ring rThng thng, dch v nh danh c phn thnh hai loi chnh l dnh v nh danh khng phn tn v dch vdnh danh phn tn, sau y l mt s nt chnh ca hai loi dnh v ny: - Dch v nh danh ko phn tn : c name server, n l t xa (remote) hay a phng (local). ch v nh danh phn tn: nhiu name server, KGc nhn bn v /hoc phn on, c gng phn tn vic qun l KGT v vic PGT. + Tuy nhng kha cnh l thuyca dch v nh danh l kh r rng, nhng vic ci t n i hi chng ta phi c mt s ch . V im bt

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ca qu trnh phn gii tn thng phi c nh, nn resolver cn phi lu tr mt danh sch cc name server, vi khi, vic qun l danh sch ny l khng h d dng

2.4.2.3.nh danh da trn thuc tnh: nh danh phng v nh danh c cu trc thng cung cp mt ctham chiu va duy nht, nhng cng va ph thuc vo v tr n mt thc th. Thm na, cc tn c cu trthng c thit k theo cch thn thin vi con ngi c th nh danh cc thc th, sao ch vic truy cp ntr nn thun tin. Tuy nhin, do c ngy cng nhiu thng tin c a ln, mt vn c t ra l phi tmkim hiu qu cc tn . nh danh c cu trc c a ra vi mc ch ny, n cho php ngi s dng ch cncung cp mt m t v mt th no m anh ta mun tm kim. Mt trong nhng cch m t l m t mt th

th di dng (thuc tnh, gi tr), hay ni cch khc, mi mt thc th c biu din bng mt tp hai tham s l thc tnh v gi tr ca thuc tnh (attributed and value).* Dch v th mc: H thng nh danh da trn thuc tnh cn c gi l dch v th mc (directory services)

trong khi h thng h tr nh danh c cu trc c gi l h thng nh danh (naming systems). Vi dch v thmc, cc thc th c mt tp cc thuc tnh i km m c th c dng tm kim. V d, vi h thng mail, cthng ip c th c nh du vi cc thuc tnh ngi gi, nhn tm kim. Vn t ra l phi thit kmt tp ph hp cc thuc tnh. Trong bi cnh h phn tn, chng ta thy c mt v d, l RDF (resourcdescription framework).

* Ci t theo th cp* Ci t phn tn

1.5. ng b ha. cc phn trc y, em tm hiu v cc tin trnh, cng nh vic truyn thng gia cc tin trnh. Tu

nhin, vn cn mt vn quan trng na, l, lm th no cc tin trnh c th hp tc cng nh ng b vnhau. Trong phn ny, em s tp trung tm hiu v vic, lm th no m cc tin trnh c th ng b ha c vnhau. Nh vy, vn ng b ha m chng ta nhc ti thc cht l vic ng b ha gia cc tin trnh giquyt bi ton chia s ti nguyn.

Trc khi i su vo chi tit, em xin nu ra mt s v d v ng b ha, th nht, m bo cc tin trnkhng cng ng thi truy nhp vo ti nguyn chia s, nh my in, m thay vo , chng ta s cp quyn truy cti nguyn ny c quyn (ti mt thi im no ) theo mt cch hp l nht. Mt v d khc, l, i khi ctin trnh c th cn phi tun theo mt th t no , v d nh thng ip m1 s c gi t tin trnh P trchoc sau thng ip m2 ca tin trnh Q.

1.5.1. BH gia cc tin trnh ng b ha gia cc tin trnh, chng ta c hai cch chnh, l da trn thi gian vt l v da trn th

gian logic, phn tip theo y, em xin tm hiu c th hn v vn ny.1.5.1.1. BH theo thi gian vt lNu cc my tnh nm trn phm vi rng khp, vn ng b ha s kh hn nhiu, v khi s xut hin kh

nim thi gian tr (time skew khi h thng c nhiu my tnh, mi my tnh li c mt ng h ring, cc ng hny c th cho cc gi tr khc nhau khi chng ta cng c ti mt thi im. S khc bit gia cc gi tr thi gianny c gi l time skew hay clock skew). iu dn n vic phi ng b thi gian ca cc my tnh, hay ncch khc, chng ta cn phi c mt thi gian chun quc t ly lm c s. T xut hin khi nim UTC(Universal Coordinated Time).

Thi gian chun UTC : Vin thi gian chun quc gia Hoa K (National Institute of Standard Time) thit lmt trm radio sng ngn c nhim v nh k pht thi gian chun cho cc trm c my thu. Khi xut hin kh nng

Trong h c mt my c b thu: Khi chng ta s x l ng b (UTC Receiver). Khi thu, chng ta cn phnh v my tnh cn thu tnh b vo khong cch truyn, v thc t, c mt h thng h tr vic ny, l hthng GPS (Global Positioning System)

Trong h thng khng c my thu no c tch hp UTC, mi my s c thi gian ring ca n. Khi , chngta s c gii thut h tr:o NTP ( Network Time Protocol) : gii thut crystalo RBS (Reference Broadcast Synchronization) : y l gii thut dng cho mng khng dy

Gii thut NTPGi s trong h phn tn c mt my c WWV (gi l Time server ) v chng ta s tin hnh ng b cc my

khc vi my ny.Trong khong thi gian /2p mi my s gi mt thng ip n my ch hi thi gian hin ti.

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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). my ch trng thi b ng (ch hi)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 ti C ca bn gi..C th gii

quyt bng cch thay i nhp ngt li nhanh hn hoc chm hn cho n lc khp nhau. (kiu nh t nhin bay vqu kh nn sinh li?)

Hai l s chnh lch t lc C(uct) c gi cho n lc nhn c c th gy li.Gii quyt bng cch ghi nhn

khon thi gian gia lc gi v nhnGii thut BerkeleyNu nh trong gii thut Cristian, server thi gian ng vai tr b ng, ch tr li yu cu ca client khi c h

th trong gii thut Berkeley, n li ng vai tr ch ng: server thi gian ( y c gi l time deamon) chng hi thi gian hin ti trn cc my khc, tnh ton chnh lch so vi thi gian hin ti trn server. Sau n tnh trung bnh chnh lch v tgian iu chnh ng h trn my mnh, cng nh gi n cc my clienyu cu chnh nhanh or chm ng h cho ph hp vi my server.

y, khng cn thit phi c bt k my nhn UTC no: thi gian trn my server c thit lp bng cc thaotc theo chu k. Mt im cn lu khc l cc my trong h thng ch c thi gian ging nhau, ch khng nhtthit u phi c thi gian ging nh thi gian trong thc t.

1. Time deamon hi cc my khc v gi tr ng h ca chng

2. Cc my ny tr li3. Time deamon ch cho cc my bit cch chnh li ng h cho ph hpGii thut RBSGii thut ny khngyu cu trn h thng c node cha thi gian chun (khi so snh vi thi gian chun thc t)

Do , thay v hng ti ng b ha vi thi gian UTC, n ch ng b trong ni b h thng, nh vi gii thutBerkeley. Mt khc, trong cc gii thut trc , chng ta c gng ng b ha c pha gi v pha nhn, nhngRBS ch pha nhn ng b ha.

C th hn, trong RBS, pha gi qung b mt thng ip tham chiu, thng ip ny cho php pha nhn c thchnh ng h ca n. Mt c im quan trng, l trong mng sensor, thi gian lan truyn tn hiu ti ccnode khc gn nh l hng s, Thi gian lan truyn trong trng hp ny c tnh bt u khi thng ip ri khigiao din mng (network interface) ca pha gi. Chnh v th, hai nguyn nhn chnh gy ra chnh lnh thi gian b loi b, l, khong thi gian to ra thng ip, v khong thi gian tiu tn c th truy cp vo mng(time spent to access the network)

1.5.1.2. ng b ha da trn thi gian logic.Vn ng b ha, nh em cp n trn, c lin quan mt cch t nhin n thi gian thc. Tuy nhin

trong thc t,c nhiu tnh hung chng ta khng cn phi ng b ha theo thi gian vt l, ta ch cn s dng mmc thi gian logic no l c. ng vi vic ng b ha theo thi gian Logic, Trong mt bi vit ca mn(1978), Lamport ch ra rng, chng ta khng nht thit phi ng b ha mt cch tuyt i (hay ng b hatheo thi gian thc). Lamport cng a ra mt gii thut ng b ha theo thi gian logic, gii thut ny cs dng mt khi nim mi, l khi nim v tem thi gian. Khi nim tem thi gian:

nh ngha quan h xy ra trc happens beforeK hiu : a bMt quan h c gi l xy ra trc khi n tha mn hai tnh hung:

i. Nu a v b l cc s kin trong cng mt tin trnh v a xy ra trc b th quan h a b l ng.ii. Nu a v b l hai s kin thuc hai tin trnh khc nhau, trong a l mt s kin gi thng ip i b l s

kin nht thng ip th quan h a b l ng.Quan h a b c tnh cht bc cu, tc l

Trn c s quan h xy ra trc , chng ta nh ngha khi nim tem thi gianVi mi s kin x, tem thi gian ca x, k hiu l C(x) c nh ngha tha mn cc iu kin sau:

i. Nu a v b l hai s kin xy ra trong cng mt tin trnh v a b th C(a) < C(b)ii. Nu a l s kin gi mt thng ip ca mt tin trnh no

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b l s kin nhn thng ip ca tin trnh khc thC(a) < C(b)Ch : Quan h ny mi ch l mt chiu, tc l nu a b th C(a) < C(b) nhng iu ngc li cha chc

ng, tc l nu C(a) < C(b) th cha chc chng ta c a bKhi nim tem thi gian vectorTrong khi nim v tem thi gian, nu s kin a xy ra trc s kin b th C(a) < C(b). Tuy nhin, chng t

khng th ni trc iu g v quan h gia a v b nu ch bit C(a) < C(b), hay ni cch khc, nu C(a) < C(b)chng ta s khng th ni rng a xy ra trc b. Nh vy, vn y l khi nim v tem thi gian khng c tnh

cht nhn qu (causality). Do , khi nim tem thi gian vector (vector clock) c a ra.Vi khi nim ny, mt vector thi gian VC(a) c gn cho mt s kin a c tnh cht sao cho nu VC(a) VC(b) th s kin a s xy ra trc s kin b.

1.5.2. Mt s gii thut h tr qun l tin trnh1.5.2.1. Gii thut loi tr nhau.Trong mt khong thi gian no , s c mt tin trnh c ton quyn s dng ti nguyn ca h thng.C hai cch tip cn chnhToken based solution: Cc gii php da trn token.Gi s chng ta c mt bus network nh Error: Reference source not founda, trong mng ny, cc tin trn

khng tun theo mt th t no c. Tuy nhin, chng ta c th dng phn mm xy dng mt logical ring thError: Reference source not foundb. Cc v tr trn vng c th c t chc theo th t s hc ca cc a ch

mng, hoc tng t vy. Tc l, chng ta phi m bo mt tin trnh c th bit rng, s kin no ng ngasau n.Khi vng c khi to, tin trnh 0 c a cho mt th bi (token). Th bi s quay vng quanh cc ti

trnh, t tin trnh k n k+1 ( chia module vi kch thc vng). Khi mt tin trnh nhn c th bi, v nu ncn phi s dng ti nguyn dng chung, n s thc hin cc cng vic cn thit vi ti nguyn , sau giphng ti nguyn. Sau khi mi vic kt thc, n li chuyn th bi cho tin trnh ngay sau n, v qu trnh ltip tc

Vn ln nht trong thut ton truyn th bi l th bi c th b mt, khi chung ta phi dinh li th bi bv vic d tm li th bi l rt kh.Permission based solution:sd giy php:

Khi mt tin trnh mun s dng ti nguyn dng chung, n to ra mt thng ip cha tn ca ti nguyn

process number ca n, v thi gian hin ti (thi gian logic). Sau , n s gi thng ip ti tt c cc tin trnh(bao gm c bn thn n). y, chng ta gi s khng c mt mt trong qu trnh truyn tin. Sau , tin trnh s i cho n khi tt c cc tin trnh khc tr li. Nu tt c cc tin trnh khc u cp giy php, tin trnh nys c php s dng ti nguyn dng chung.

1.5.2.2. Cc gii thut bu chnRt nhiu h phn tn i hi phi c mt tin trnh hot ng nh mt ngi iu phi. Thng thng, vi

process no m nhn vic iu phi l khng quan trong, tuy nhin, chng ta phi lun m bo c mt tin trnlm nhim v ny. phn ny, em s tm hiu v mt s gii thut bu chn ph bin.

Thng thng, nu tt c cc tin trnh l ging nhau, tc l khng c cc c trng c bit no phn bigia chng, chng ta s gi s mi tin trnh c mt id duy nht, v d nh a ch mng. Thng thng, cc githut bu chn c xu hng c gng tm ra process c s process (process number) cao nht v gn cho n vai triu phi. Cc gii thut iu phi s khc nhau cch thc tm kim ca chng.

Cc gii thut bu chn ny gi s rng cc process u bit n process number ca cc process khc. iu mchng khng bit, l process no hin ang hot ng (up) v process no khng hot ng (down). Di y lmt s gii thut bu chn tiu biu, mc ch ca chng l m bo rng, khi mt qu trnh bu chn c thhin, n s kt thc vi vic tt c cc process cng ng la chn mt process lm ngi iu phi mi.

Bully algorithmeKhi bt k mt tin trnh no bit c process khng cn p ng li yu cu, n s khi to qu trnh bu chn.

Mt process bt k P, thc hin vic bu chn nh sau:P gi thng ip Election ti tt c cc tin trnh c s th t cau hnNu khng ai p li, P s thng c, v tr thnh ngi iu phi miNu c mt tin trnh cao hn tr li, n s tip tc cng vic, v P hon thnh nhim vRing algorithme

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Trong gii thut ny, ta gii s cc tin trnh c sp xp theo mt th t vt l hay logic no , tc l, nbit tin trnh no ng ngay sau n.

u tin, khi mt tin trnh nhn thy tin trnh iu phi khng cn hot ng na, n to ra thng ipELECTION cha process number ca n v gi ti tin trnh ngay sau. Nu tin trnh sau n b down, sender s bqua tin trnh ny v chuyn ti thnh vin tip theo, hoc tip theo na nu tin trnh kia cng hng Ti mibc, sender s thm s process number ca n vo danh sch.

Sau , thng ip s quay tr li tin trnh m khi to thng ip (khi mt tin trnh nhn c thng ipm c s process id ca n ht 1 vng) . Ti thi im , thng ip chuyn t ELECTION sang

COORDINATION v quay vng mt ln na thng bo cho tt c cc tin trnh khc bit ngi iu phi mi( l tin trnh c Pid cao nht trong list).Khi thng ip c quay vng xong 1 ln na, n s b xa i, v cc tin trnh quay li cng vic nh trc

.Bu chn trong mi trng khng dy

Chng ta hy xt trong mi trng mng khng dy, c th bu ra tin trnh iu phi, bt k node no trongmng c th bt u vic bu c bng cch to thng ip ELECTION cho node ngay bn cnh n. Khi node nynhn c tn hiu ELECTION ln u tin, n s gn node gi l node cha ca n, v tip tc gi ELECTION chott c cc node khc cnh n.. Khi mt node nhn c ELECTION t mt node khng phi cha n, n ch gi tinchp nhn m khng lm g

Khi node R nhn node Q l cha, n s forward thng ip ELECTION cho cc node cnh n (tr Q) v i s

chp nhn t cc node kia gi v c php i vo. Sau , bn thn n gi thng ip chp nhn ELECTIONt Q. Nh th, nu cc node hng xm chn cho mnh cha ri s ngay lp tc phn hi li R, v nu tt c ccnode c cha, R s l mt node l, v n s c th ph hi li Q ngay lp tc. Nh vy, n s cung cp cc thngtin thm, nh thi gian sng ca pin, hoc dung lng mt s ti nguyn khc.

Thng tin ny s cho php Q so snh kh nng ca R vi cc node khc, v la chn node tt nht. Ngoi ra, Qgi thng ip ELECTION bi cha ca n,P cng gi nh vy. Khi Q gi thng ip chp nhn thng ipELECTION c gi bi P, n s chuyn node ph hp nht cho P. Bng cch n, node ngun s bit cch lachn node tt nht tr thnh iu phi.

Sau , n broadcast thng tin ny cho tt c cc node khc.

Bu chn trong cc h chng quy m lnCc thut ton va cp s dng trong cc h phn tn nh v ch chn mt node duy nht lm node iu phi.Trng hp c nhiu node c chn ( nh trong trng hp superpeers trong mng p2p) -> thut ton khc.Cc iu kin cn tha mn:Cc node bnh thng c th truy cp t tr ti superpeers ( normal nodes should have low-latency access to

superpeers)Superpeers c phn b trn ton mng.Phi phn chia trc mi superpeer s phc v bao nhiu node trong mng.Mi superpeer khng nn phc v qu mt s node nht nh.Rt may nhng iu kin ny c tha mn trong hu ht cc mng P2P,gm c mng c cu trc v khng c

cu trc.Trong mng c cu trc, mi node c gn mt id m bit. tng c bn l s dnh k bit u nh dansuperpeers. Nu mng cn N superpeers ta s phi dnh log2(N) bit u nh danh superpeers

1.6. Nht qun v nhn bn1.6.1. V sao li cn phi s dng bn saoC hai l do chnh khin chng ta s dng bn sao, l tng tin cy v tng hiu nng cng nh tnh sn

sng ca h thng.Tng tin cy: iu ny l kh r rng, gi s nu ta khng nhn bn, h thng ch c mt bn sao gc. Tron

trng hp bn sao ny b hng th ton b h thng s phi ngng hot ng.Tng tnh sn sng v hiu nng ca h thng: Lin quan n vn ny, chng ta c phng n qun l t

trung v phng n qun l phn tn. c th hn, chng ta hy cng xt qua v d h thng qun l du lch(Error: Reference source not found), vi chi nhnh gc t ti A.

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o i vi phng n qun l tp trung,trong trng hp v d ca chng ta, l h thng ny ch c mt khd liu gc duy nht ti A chng ta s khng cn phi quan tm n vn v tnh nht qun, tuy nhin, phng s dng ny s lm tng lu lng mng v mi giao dch u phi thng qua im tp trung (trong trng hny l A) x l. R rng, tnh sn sng v tnh p ng ca h thng b hn ch, bi khi c nhng yu cu ti C rt xa so vi A, th h thng ti im C khng c kh nng tr li yu cu ti , m n s gi cho A x l, v lkt qu v.o i vi phng n qun l d liu phn tn, chng ta s c hng li t vic tnh sn sng ca h thn

c tng ln.V mt l thuyt, do d liu ti A v C l hon ton ging nhau, nn nu c mt yu cu pht sinh t

C, n s c p ng ngay m khng cn phi gi qua A na. Tuy nhin, trong thc t, phng php qun l dliu phn tn cng bc l mt nhc im ln, l vn v tnh nht qun li d b vi phm hn nu chng takhng lm tt cng tc cp nht d liu.

Tm li, chng ta cn phi ch u im, nhc im ca tng cch s dng cho ph hp.1.6.2. Cc m hnh nht qunCc m hnh nht qun ly d liu lm trung tmThng thng, tnh nht qun s c cp trong bi cnh cc thao tc c v ghi trn d liu chia s. D liu

ny tn ti di dng b nh dng chung, hay c s d liu chia s, hay h thng file phn tn. Chng c gchung l kho d liu (data store).

Mc tiu ca m hnh ny l cung cp mt ci nhn tng qut ca tin trnh i vi kho d liu.Mt m hnh nht qun theo cch ly d liu lm trung tm thc cht l mt cam kt gia cc tin trnh vi kh

d liu (v nu tt c cc tin trnh u thc hin tt th kho d liu s tun theo tnh nht qun)Mt kho d liu c th b phn tn trn nhiu my v tin trnh c th truy cp n kho d liu t cc bn sa

cc b. Thao tc ghi trn mt bn sao cc b s c lan truyn ti cc bn sao khc ca kho d liu. y chnta phn loi, mt thao tc c gi l ghi nu n lm thay i d liu ca kho, tri li, n c gi l thao tc c.

Mt s v d v m hnh nht qun ly d liu lm trung tm:Continouos consistency model

c th nhn bn d liu mt cch hiu qu, chng ta cn phi ni lng s nht qun. Yu v Vadat a ra bayu t c lp nh ngha s nht qun

S chnh lch trong gi tr s hc gia cc bn sao: c hiu l s lng cp nht c thc hin trn mt bncopy nhn vn cha c thc hin trn bn ghi khc.

S chnh lch v trng thi (c mi) ga cc bn sao : Lin quan n ln cui cng m bn ghi c cp nht.S chnh lnh c tun theo th t cp nht ca cc thao tc : Cho php c s khc bit v th t cp nht trongcc bn ghi, chng no n cn trong gii hn cho php.

Sequential consistency modelMt kho d liu c xem l nht qun tun t nu n tha mn iu kin sau:Kt qu ca mt vi s thc thi l tng ng nhau nu nh (iu kin sau c tha mn :D) cc thao tc (

v ghi) ca tt c cc tin trnh ln kho d liu c thc thi theo mt vi th t tun t v cc thao tc ca mitin trnh c th u theo ng th t c a ra bi chng trnh ca n

nh ngha ny c ngha rng, nu cc tin trnh chy ng thi trn cc my khc nhau, th bt k s xen k hpl no ca cc thao tc c v ghi u l hnh vi chp nhn c.

Causal consistency modelM hnh nht qun nhn qu th hin mt s ni lng ca m hnh nht qun tun t. Trong m hnh ny, chng

ta phn bit gia cc s kin c lin h nhn qu vi nhau, v nhng s kin khng c mi lin h ny.Nht qun nhn qu l m hnh yu hn nht qun tun t, m hnh ny ch quan tm n cc thao tc ghi c quan

h nhn qu vi nhau. Cc thao tc ghi c quan h nhn qu tim nng phi c nhn thy bi tt c cc tin trnhtheo cng mt trt t, cn cc thao tc ghi ng thi c th c nhn bit theo th t khc nhau trn nhng mykhc nhau.

Hai thao tc ghi c coi l c quan h nhn qa vi nhau nu vic thc hin mt thao tc c kh nng gy nhhng n d liu c ghi bi thao tc kia.

Cc m hnh nht qun ly ngi dng lm trung tmM hnh nht qun m chng ta cp trn tp trung vo mc ch cung cp mt ci nhn nht qun tr

ton h thng i vi kho d liu vi mt s tha nhn quan trng rng cc tin trnh ng thi c th cng mt l

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cp nht kho d liu. Tuy nhin, vn cn mt m hnh nht qun khc kh ph bin, l m hnh nht qun lyngi dng lm trung tm.

i vi kiu m hnh ny, chng ta s khng quan tm n vic chia s kho d liu gia cc tin trnh m quantm n vic tr li cu hi nhng ngi s dng di ng c cung cp kh nng m bo tnh nht qun nhth no?

M hnh ny thng dng cho cc c s d liu phn tn vi ngi dng di ng. Nht qun cui cng

Yu cu duy nht ca m hinh ny l tt c cc bn sao cui cng ri s phi ging nhau. Tt c cc cp nht phi

m bo l cui cng cng s c truyn ti tt c cc bn sao khc. Yu cu ny s c thc hin tt nu miclient lun chu kh cp nht cho cc bn sao. Nhng vn s xy ra nu qu trnh din ra lin tc v s lmnh hng n hiu nng ca h thng

=> gii php l ch p dng vi cc h phn tn ch c v t cp nht . Cc bn cp nht s c cp nht tuynhin tin trnh ca ngi dng s truy cp c vo bn cp nht ny sau 1 thi gian nht nh , m bo ko cvn li khi 1 tin trnh ang ghi trong lc 1 tin trnh khc c .

Tuy nhin m hinh ny s gp mt cht kh khn nu cc client l di ng.Khi ngi dng cp nht ti 1 a im ( ghi d liu vo database chng hn )( Trong 1 khong thi gian ngn ) Ri n 1 a im khc , truy cp vo d liu . Ngi s tng tc

vi 1 c s d liu khc , v s thy rng cng vic mnh lm hm nay bin mt . Nht qun ch c

M hnh nht qun c u m bo rng mt client s lun nhn thy nhng d liu mi hn v khng bao giphi nhn thy nhng d liu c hn nhng g m mnh c trc . iu c ngha l khi mt client thchin 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 nhtcng phi c ghi ging vi bn sao u tin.

Nht qun ch ghiM hnh nht qun ghi u m bo rng mt thao tc ghi ln mt mc d liu xc nh s c hon tt trc kh

c bt k tao tc ghi no khc ln mc d liu bi cng mt client. Ni cch khc th cc thao tc ghi ln mtmc d liu s c sp xp mt cch c trt t.

Nht qun c KQ ghiTrong m hnh nht qun ny, ngi dng c m bo rng s lun c nhn thy nhng kt qu ghi mi nhtTc ng ca mt thao tc ghi ca mt tin trnh ln mc d liu x s lun c nhn thy bi mt thao tc

ln lt trn x ca cng tin trnh .Tin trnh nhn nhim v ghi th s m trch lun vic c nh vy th qu trnh ghi bt buc s phi hon ttrc khi vic c c thc hin => ngi dng s nhn thy kt qu ghi mi nht .

Nht qun ghi theo sau cM hnh nht qun ghi u m bo rng mt thao tc ghi ln mt mc d liu xc nh s c hon tt trc kh

c bt k tao tc ghi no khc ln mc d liu bi cng mt client. Ni cch khc th cc thao tc ghi ln mtmc d liu s c sp xp mt cch c trt t.

1.6.3. Qun l bn sao (Replica management)Chng ta cn phi ch n vn lan truyn cp nht (update propagation) . Cn phi m bo lm th no

sau thi gian ngn nht, s thay i d liu c cp nht trn tt c cc bn sao cn li.C ba hnh thc lan truyn cp nht chnh:

Lan truyn thng bo (notification) cp nht: Thng bo v vic mt hiu lc ca 1 giao thc. P2 ny tt khi t lcc thao tc c so vi thao tc ghi nh.Lan truyn ton b d liu c cp nht : Vi hnh thc ny, bn pha gi s phi vt v hn, nhng pha ti

nhn cp nht s khng phi thc hin nhiu vic.Ch lan truyn thao tc c cp nht: tn t bng thng nhng i hi nng lc x l cao v trong nhiu TH th

cc thao tc l rt phc tp.ng vi ba hnh thc lan truyn cp nht trn, chng ta c hai cch tip cn gii php lan truyn cp nh

thng dng trong vic cache d liu.o Push-based approach (y) hay server-based protocols: Cc trm m trn c d liu thay i s ch ng

y d liu lan truyn cp nht n trm khc m khng cn i yu cu cp nht gi n.

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o Pull-based (ko) hay client-based protocols: bn thn cc trm khng cp nht ch ng kho cc trm cpnht cc trm khc v pha mnh.

1.6.4. Cc giao thc nht qun.Cc giao thc nht qun m t cch thc ci t ca m hnh nht qun. Tun theo m hnh nht qun tun t

(sequential consistency model) v cc bin th ca n, chng ta phn bit primary-based protocols v replicatedwrite protocols

Primary-based protocol: Tt c cc thao tc cp nht c chuyn n mt bn ghi gc,bn ghi ny sau , nlt n, s m bo vic cp nht c thc hin v c chuyn tip ng th t.

Giao thc ghi t xa: tt c cc thao tc ghi c thc hin ch trn 1 server t xa. Giao thc ny thng kt hp vcc h thng client-sv truyn thng. u : s dng giao thc ghi k theo khi, cc thao tc ghi c th c gi ncc bn sao d phng theo cng 1 th t. Nhc :vn hiu nng. Tt c cc thao tc ghi trog giao thc u chimkh nhiu thi gian.

Giao thc ghi cc b: - mt bn sao ca mc d liu c duy tr. Khi c yu cu thao tc ghi, mc DL c nhn bt SV xa chuyn dn sv cc b. vic ny c gi l tip cn theo di tr hon ton. Nhc: thi gian thc nh vc 1 mc DL c th ln hn thi gian tin trnh s dng DL .

Replicated-write protocols: Thao tc cp nht c chuyn tip ti mt vi bn sao ti cng mt thi imTrong trng hp ny, vic m bo cc thao tc c thc hin ng th t s thng kh khn hn.

Giao tc nhn bn ch ng: cc thao tc c dc thc hin cc b. Giao thc ghi c th c truyn s dng giaotip Point2P hay multicast. u: tt c bn sao u nhn c cc thao tc cng lc v theo cng 1 trt t, v n cn

ko cn h du 1 bn chnh hay phi gi tt c cc thao tc ti SV.Nhc: i truyn theo multicast g, phi c 1 bsp xp dy tp trug m c 2 u kh tip cn linh hot.Giao thc Quorumbased: cc thao tc ghi c thc hin trn 1 tp nh nht cc bn sao. Khi thc hin 1 thao t

c, ng dng cng phi lin h vi 1 tp cc bn sao tm ra phin bn mi nht ca DL. Tt c cc mc DL kt hp vi 1 s phin bn. Mi ln 1 mc b sa i th s phin bn cng c tng ln.

Cache-coherence protocols: Cache l 1 dng c bit ca nhn bn, n c iu khin bi client thay v bi svCo

1.7. Tnh chu li1.7.1. Mt s khi nim c bn1.7.1.1. Phn bit cc thut ng c th hiu r hn v tnh chu li, trc tin chng ta phn bit mt s khi nim c bn v li nh sau:

Hng hc, khng chy (h thng khng cung cp cho ngi dng

y cc tnh nng vn c ca nError(n) Li sai st (mt phn trng thi ca h thng c th dn ti failure)Fault(n) Nguyn nhn gy ra Error

Tnh chu li c th hin ch h thng vn c th hot ng tt, cung cp dch v cho ngi dng ngay ckhi c mt s li nht nh.

1.7.1.2. Cc loi liLi nht thi (transient faults) : L li xy ra mt ln ri bin mt, nu chng ta c lp li th thao tc li cn

khng xy ra, v d trong qu trnh truyn tin, khi chng ta ang truyn th c mt my bay bay qua, gy nhiu.Li lp (intermittent): L li c th lp i lp li nhiu ln, v d nh connector b lng, c th dn ti kt n

khng c thit lp, vic ny c th lp i lp li nhiu ln.Li bn vng (permanent error) : l li m khi xy ra, n tip tc duy tr v tn ti n khi chng ta khc phhon ton Bn cht ca vic a ra cc loi li l gip chng ta phn bit cc nguyn nhn dn ti li. (cc fault m

qua dn ti error)1.7.1.3. Cc m hnh liLi sp (hay l sp crash) Server b treo, trc SV hot ng tt n khi b li. Khi SV gp li ny

n ko th lm g c na. VD in hnh l OS ca PC, OS ngng -> restart.Li b st (omission) Server khng p ng c c yu cu gi n k c vic gi d liu i v nhn d li

v. Chia 2 loi:

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Li khi nhn thng ip ti: SV k nhn c yu cu ngay c t client gn n nht mc d kt ni c thit lpch ko nhn c thng ip ch ko nh hng n trng thi ca SV.

Li khi gi thng ip: SV vn nhn c cc yu cu, nhng v l do no ko th gi kt qu ti my yu cuL do thng gp l do b nh m gi y -> SV cn chun b tnh hun client gi li yu cu .

Li thi gian (timing failure) xy ra khi SV phn ng li qu chm, sau c thi gian cho php. 1 h thnlun c cc rang buc v thi gian. Nu gi nhanh qu -> b nh m bn nhn k cha hoc ngc li th nhng hiu nng chung ca h thng.

Li p ng (reponse) Server khng p ng ng cc yu cu ca ngi dng. C 2 loi:

Li v mt gi tr: l li khi sv tr li yu cu ca client vi gtr ko chnh xc.Li v chuyn trng thi: l li khi sv hot ng lch hng khi lung iu khin. C ngha sv tr li cc ycu c gi tr v k lin quan ti yu cu ng dng

Li ty tin (arbitrary) Server c cc p ng ty tin v mt thi gian, quy trnh. C 2 kh nng:Nht: 1 sv to ra 1 kt qu sai m k th pht hin ra cHai: sv b li c lin kt vi cc sv khc to 1 kt qu sai.

1.7.2. Mt s gii php xy dng cc h thng c tnh chu li1.7.2.1. Che giu li bng s d tha lm gim tc ng ca vic xy ra li i vi ngi dng. Mc d h thng ang c li nhng n vn tip t

cung cp cc dch v v ngi s dng khng cm nhn c li trong h thng ang xy ra. C ba cch chgiu li bng s d tha:

D tha thng tin: Vi cch ny, chng ta s a thm vo mt s bit. Cc bit ny cho php chng ta c thphc hi li cc bit b li. V d, m Hamming c th c thm vo d liu truyn i c th phc hi d liutrong trng hp n b nhiu trn ng truyn.

D tha thi gian: Vi cch ny, khi mt hnh ng c thc thi, v nu cn thit, chng ta c th thc hinli hnh ng . V d, nu mt giao dch b t chi, chng ta c th d dng thc hin li n. D tha v thi giac bit hu ch khi chng ta gp li nht thi hay li lp ( faults are transient or intermittent).

D tha vt l: Vi cch ny, cc thit b, hoc cc tin trunhif s c thm vo m bo ton h thng cth chu li trong trng hp mt thnh phn no vn hnh sai. D tha vt l c th c thc hin c trn khcnh phn cng cng nh kha cnh phn mm.o Kha cnh phn mm: Cc process c th c thm vo h thng nu nh c mt trong s n b li, h

thng vn c th vn hnh ng chc nng ca n. Hay ni cch khc, bng cch nhn bn cc tin trnh, kh nng

chu li ca h thng c th c nng cao.o Kha cnh phn cng: chng ta c th thm vo cc mch in t trong trng hp mt thit b b li, h

thng vn vn hnh v a ra kt qu ng. Mt hnh thc thit k kh ph bin, l TMR (Triple ModulRedundancy)

1.7.2.2. Nhn bn tin trnhNu ta nhn bn cc tin trnh v tp hp cc thnh phn ging nhau vo tng nhm th nu mt tin trnh hng cc tin trnh khc vn s tip tc p ng. V iu ny s lm tng kh nng chu li ca h thng.

V vic tp hp cc tin trnh ging nhau vo tng nhm, chng ta c mt s ch sau:Cc nhm phi ng: phi c kh nng thm hay bt thnh vin mt cch thun li.V s lng tin trnh : trong mt nhm c bao nhiu tin trnh l ph hp nht.Vn v qun l cc nhm.

ng vi vic phn nhm, chng ta c hai cch phn nhm nh sauNhm phng (flat group) : L nhm m trong , tt c cc tin trnh c vai tr nh nhau.Nhm phn cp (hierarchy group) : L nhm m trong , tn ti tin trnh ng vai tr iu phi (coordinate Chng ta phi gii quyt cc c ch tha thun, cacm kt gia cc tin trnh trong nhm. V d, nu tin trnhiu phi b hng, chng ta phi c c ch chn mt tin trnh iu phi mi ph hp.

ng vi vn nhm phn cp, ngi ta a ra khi nim distributed commit. N thng c p dng cho ch giao tc. C hai gii thut ni ting lin quan n distributed commit:

2PC (2 phase commit)o Vote Phaseo Decision phase: da trn tt c cc thng ip t cc thnh vin cn li. Nu ch cn c mt thnh vin t

chi, chng ta s hy lun.

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o ly v d l mt giao dch phn tn vi s tham gia ca mt s lng cc process chy trn cc my khcnhau. Qu trnh gm 4 bc chnh sau:

1.Tin trnh iu phi gi thng ip VOTE_REQUEST ti tt c cc tin trnh khc.2.Khi cc tin trnh tham gia nhn c thng ip VOTE_REQUEST, n tr li hoc l VOTE_COMMIT hoc

l VOTE_ABORT3.Tin trnh iu phi tp hp tt c cc thng ip bu c t cc thnh vin. Nu tt c cc thnh vin u gi

VOTE_COMMIT th tin trnh iu phi s a ra thngip GLOBAL_COMMIT n tt c cc thnh vin. Nuch c mt thnh vin gi VOTE_ABORT th thng ip GLOBAL_REQUEST s c gi ti tt c cc thnh

vin.4.Nu cc tin trnh tham gia cng nhn c GLOBAL_COMMIT, n s cam kt giao dch mt cch aphng (locally commits the transaction). Nu nhn c thng ip GLOBAL_ABORT th giao dch s b t chNhc im : Khi tt c cc tin trnh tham gia cng nhn c v x l thng ip VOTE_REQUEST t tin

trnh iu phi, trong thi gian tin trnh iu phi li gp li. Khi , cc tin trnh tham gia s khng c cchno quyt nh cch thc hin hnh ng cui cng. Chnh v vy m n b gi l giao thc cam kt b phong ta.

3PC (3 phase commit) khc phc nhc im ca 2PC trong trng hp coordinator b li, ng ta a ra m hnh 3PC. Cc trng thi kh

ging 2PC nhng c thm trng thi PRECOMMIT.1.7.2.3. To kn truyn thg tin cyTheo Client/Server

Theo Unicast1.7.3. Phc hi li.C hai cch tip cn l phc hi tin v phc hi li, sau y, em xin trnh by c th v tng cch tip cn.Phc hi li(Backward recovery) : a h thng v trng thi lnh mnh gn nht trc thi im gp li.

thc hin iu ny, chng ta cn phi c thng tin trng thi hot ng ca h thng ti mt thi im no . Tcl chng ta phi nh k lu cc thng tin trng thi ca h thng hoc ch ra cc im nh k lu thng tin(check point).

Phc hi tin (Forward recovery) : c th phc hi tin, chng ta cn phi c nhng d bo ng, qua mc th nhy n cc im tng lai. Phng php ny thng t c dng h phng php u.

1.8. An ton anh ninh1.8.1. Mt s khi nimCc mi e da an ton an ninh security threats:

Chn bt: ch thnh phn ko c php cng c th truy nhp n cc dch v hay cc DL, nghe trm thng tin truyn

t on: l mi e da lm cho dch v hay DL b mt mt, b hng, k th dng c na.Thay i: l hin tg thay i DL hay can thip vo cc dch v lm cho chng k cn gi c cc c tnh bn u.Gi mo: l hin tg thm vo DL ban u cc DL hot ng c bit m k th nhn bit c n cp Dl ca h

thng.Chnh sch an ton an ninh security policy : N chnh l tp cc yu cu t ra m bo an ton an ninh cho

h thng. Ni cch khc, n m t tp tt c cc quy nh, cc hnh ng c php hoc b cm m bo aton an ninh cho h phn tn.

C ch an ton an ninh security mechanism : Tp hp cc gii php thc hin chnh sch an ton an ninhC bn loi c ch an ton an ninh chnh.o Mt m data encription : Nhm bo v thng tin v mt ni dung, n gip cho thng tin khng b mt m

hay khng b nhng ngi khng c quyn truy nhp vo bit c.o Xc thc - authentication : nhm kim tra cc nh danh xc nh hoc ng k ca cc thc th trong h

phn tn. V d, mt user mun truy cp vo h thng, chng ta phi xem user c trong h thng cha. iny l v cng quan trng, c bit l trog cc giao dch.o y quyn Authorization : nhm kim tra cc quyn c php thc hin ca thc th trong h thng. T

l sau khi xc nh c danh tnh ca user, phi xem user s dng h thng vi ng quyn c cp haykhng, nu sai th phi xem xt li

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o Kim ton audity : nhm theo vt cc hnh ng ca cc thc th trong h thng, c bit l cc user. Nhvy, sau khi bit c mt kt qu no , v xc thc c tnh chnh xc ca n, chng ta s kim tra theo dxem trong qu trnh hot ng ca h thng c iu g bt thng khng, nh vy chng ta s d dng bit c agy ra s kin .

1.8.2. Tm hiu cc c ch security2.8.2.1.Mt mMt m kha b mt ( hay mt m i xng) tc l kha m ha v gii m l nh nhau. Gii thut c trn

cho trng hp ny l Data Encryption Standard (DES)

Mt m kha cng khai (hay mt m phi i xng) : kha m v kha gii m l khc nhau. Phng php tibiu cho trng hp ny l RSA, l tn vit tt ca ba tc gi ngh ra mt m kha cng khai.ng vi khi nim mt m kha b mt, cn c mt s khi nim lin quan na, l ch k s v tin in tCh k s : nu chng ta va mun bo v ni dung, li va mun xc thc (ngun gc ca ngi gi) chng t

c th dng ch k in t. Phng php ny c s dng rng ri trong cc giao dch, cc hp ng kinh t, cthanh ton in t.

Tin in t (electronic money) : l mt n v tin vi cc thng tin cn thit nh ni pht hnh, mnh gi y ny sinh mt vn , nu c nhiu ngi c kh nng thay i cc thng tin ny, th r rng tin in t s bmt gi tr ca n, do , chng ta va phi bo v ni dung ln ngun gc ca n. Trong trng hp ny, chng tc th s dng ch k s.

2.8.2.2 Xc thc:

Xc thc trn kha b mtBn gi mun giao tip vi bn nhn s gi yc A ti bn nhn, Nhn tr li bng yc RB. Gi m ha yc RB = khab mt KA,B v gi v bn nhn. Nhn xc thc c gi nh nhn bit c yc RB mnh gi trong gi tin va nhnLc ny gi mun xc thc bn nhn s tip tc gi yc RA ti nhn. Nhn li m ha RA bng kha b mt KA,Bv gi v nhn -> nhn xc minh c bn gi, sau thc hin trao i.Xthc da trn trug tm phn phi kha

H thng gm N host, mi host chia s 1 kha vi N-1 host khc. Khi N ln, ra s dng thm trung tm phn phkha KDC (Key Dis Cent)

Bn gi s gi bn tin ti trung tm phn phi kha thng bo mnh mun giao tip vi bn nhn. KDC s gi chc bn gi v bn nhn 1 bn tin c cha kha b mt KA,B. Bn tin gi cho b nhn s c m ha bng KA,KDCbn gi s l KB,KDC.

Cch khc l KDC gi c 2 bn tin cha kha b mt KA,KDC(KA,B) v KB,KDC(KA,B) cho bn gi v bn gs gi cho bn nhn kha c KDC m ha KB,KDC(KA,B).Xc thc trn kha cng khai

Bn gi m ha yu cu = kha cng khai K+B ca bn nhn. Nhn l ni duy nht c th gii m bn tin = KB. Nhn s m ha yu cu ca bn gi cng vi yu cu ca chnh mnh v kha KA,B va to ra bng kha cngkhai K+A cu bn gi nhm xc thc bn gi. Cui cng, gi s gi li cho nhn yu cu RB ca nhn gi i xc thc.

2.8.2.3. y quynS dng capability xc nh quyn truy nhp ti nguyn ca tin trnh chim gi n. 1 capa l 1 t nh dan

128 bit, cu trc bn trong : 48 bit u c khi to khi i tng c to ra bi sv ca i tng (gi l sv port) 24bit tip c s dng xc nh i townjg ti SV nh sn, 8 bit tip xc nh quyn truy nhp folder ca capa, 4

bit cui (trng check) dng to 1 capa tht (ko th gi mo).S y quyn: bng vic chuyn quyn truy nhp t tin trnh ny sang tin trnh khc, n s tr nn d dng hn phn tn cng vic gia cc tin trnh m k lm nhr hng ti bo v ti nguyn.

2.8.2.4. Kim sot Ma trn kim sot truy nhp

Gi ma trn kim soat lsaf M (M[s,o]): a ra danh sch cc php ton m cchur th s c th yu cu trn i tngo. Khi 1 ch th s gi 1 phng thc m ca i tng o th monitor s kim tra trog danh sch M[s,o] nu m k ctrong danh sch ny th li gi b hy. V h thng c nhiu user nn m trn l ko hp l.Danh sch kim sot truy nhp

Mi 1 i tng s duy tr 1 danh sch cc truy cp hp l ca cc ch th mun truy cp n gi l ACL nh trnh dc s tn ta ca cc entry rng nh ma trn kim sot truy nhp.

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Nhc: vn c kch thc qu ln.Min bo v

L 1 tp cc cp (i tng, truy cp hp l), mi cp ny s cho ta mt i tng v cc thao tc hp l trn nMi 1 yu cu u thuc 1 min bo v no . Khi 1 yu cu gi n, monitor s tm trong min bo v tng ngyu cu ny. t hit qu cao hn, ngi ta dng kt hp min bo v vi vic nhm cc i tng.