CHNG 1: TNG QUAN V H THNG CN BNG TI SERVER 1.1 Ti sao phi xy dng h thng cn bng ti? Trong thi i bng n ca cng ngh thng tin hin nay, mng my tnh ng vai tr ngy cng quan trng hn trong hot ng ca cc doanh nghip, t chc cng nh cc c quan nh nc. Thm ch mt s n v, chng hn nh cc cng ty hng khng hoc cc ngn hng ln, mng my tnh c th v nh h thn kinh iu khin hot ng ca ton doanh nghip. S ngng hot ng ca mng my tnh hay s hot ng km hiu qu ca mng my tnh trong nhng c quan ny c th lm t lit cc hot ng chnh ca n v, v thit hi kh c th lng trc c. Chng ta u bit cc my ch l tri tim ca ca mng my tnh, nu my ch mng hng, hot ng ca h thng s b ngng tr. iu ng tic l d cc hng sn xut c gng lm mi cch nng cao cht lng ca thit b, nhng nhng hng hc i vi cc thit b mng ni chung v cc my ch ni ring l iu khng th trnh khi. Do vy, vn t ra l cn c mt gii php m bo cho h thng vn hot ng tt ngay c khi c s c xy ra i vi my ch mng. Vic la chn mt server n l c cu hnh cc mnh p ng nhu cu ny s ko theo chi ph u t rt ln v khng gii quyt c cc vn t ra ca cc t chc. Gii php hiu qu c a ra l s dng mt nhm server cng thc hin mt chc nng di s iu khin ca mt cng c phn phi ti - Gii php cn bng ti. C rt nhiu hng a ra gii php cn bng ti nh Cisco, Coyote Point, Sun Microsystems... vi rt nhiu tnh nng phong ph. Tuy nhin, v c bn, nguyn tc cn bng ti vn xut pht t nhng quan im k thut kh tng ng. Mt k thut cn bng ti in hnh l RRDNS (Round Robin DNS). Vi gii php ny, nu mt server trong nhm b li, RRDNS s vn tip tc gi ti cho server cho n khi ngi qun tr mng pht hin ra li v tch server ny ra khi danh sch a ch DNS. iu ny s gy ra s t qung dch v. Sau nhng pht trin, t cc thut ton cn bng ti tnh nh Round Robin, Weighted Round Robin n cc thut ton cn bng ti ng nh Least Connection, Weighted Least Connection, Optimized Weighted Round Robin v Optimized Weighted Least Connection, k thut cn bng ti hin nay nh s kt hp cc thut ton trn ngy cng tr nn hon thin mc d nhc im vn c nh to im li n v vn nt c chai do s dng b iu phi tp trung (centralized dispatcher)

vn cn. Ngoi kh nng p dng vi Web server, k thut ny cn c th p dng vi cc h server ng dng khc. SLB khng ch lm nhim v phn phi ti cho cc server m cn cn cung cp c ch m bo h thng server lun kh dng trc cc client. SLB khng c yu cu c bit g v phn cng, bt c my tnh no hp chun u c th c s dng lm server. Chi ph trin khai nh gim ng k. Kin trc phn mm phn tn ca SLB cho php cung cp hiu nng v tnh kh dng ca k thut ny mc cao nht. 1.1.1.So snh h thng cn bng ti v h thng thng thng

Kch bn A Tnh sn sng cao Tnh m rng ng dng C C X l a nhim

Kch bn B Khng Khng X l nhanh n nhim

u im ca cn bng ti Tnh m rng: thm hoc b bt server mt cch d dng

Tnh sn sng cao do h thng dng nhiu Server V vy h thng c tnh d phng. Tnh qun l: Theo di v qun l tp trung h thng Server, bo dng h thng server m khng cn tt cc dch v C th tch cc ng dng khi server Lm vic c vi nhiu h iu hnh Hiu sut cao Server c nhm li thc hin a nhim v tt hn Tt c Server u hot ng ng cng sut khng c tnh trng mt Server lm vic qu ti trong khi server khc li ang nhn ri. Nhng t chc no cn c gii php cn bng ti Cc doanh nghip Nh cung cp dch v ISP Trung tm x l d liu Chnh ph Phng th nghim Trng i hc, vin nghin cu 1.2.Cc gii php chia ti trn th gii Vic chia ti c th thc hin bng nhiu phng cch, hnh thc khc nhau, vi cc cng ngh khc nhau hoc kt hp chng li: 1.2.1. Chia ti bng phn mm ci trn cc my ch Kt hp nhiu server mt cch cht ch to thnh mt server o (virtual server). Cc h iu hnh cho my ch th h mi ca cc hng Microsoft, IBM, HP... hu ht u cung cp kh nng ny, mt s hng phn mm khc nh Veritas(Symantec) cng cung cp gii php theo hng ny. Cc gii php thuc nhm ny c u im l quen thuc vi nhng nh qun tr chuyn nghip, c th chia s c nhiu ti nguyn trong h thng, theo di c trng thi ca cc my ch trong nhm chia ti hp l. Tuy nhin, do s dng phn mm trn server, tnh phc tp cao nn kh nng m rng ca gii php ny b hn ch, phc tp khi trin khai cng nh khc phc khi xy

ra s c, c ro cn v tnh tng thch, kh c c nhng tnh nng tng tc v bo mt cho ng dng. 1.2.2. Chia ti nh proxy Nhm ny thng tn dng kh nng chia ti sn c trn phn mm proxy nh ISA Proxy ca Microsoft hay Squid phn mm m ngun m ci trn my ph dng. Proxy ny s thc hin nhim v chia ti trn cc server sao cho hp l. Gii php ny v hot ng mc ng dng nn c kh nng caching (l cng ngh lu tr cc b d liu c truy cp vi tn sut cao) v kh nng firewall tng ng dng. V s dng my ph dng nn gii php ny c u im l chi ph thp, kh nng m rng tt v ci t trn mt my c lp, d qun tr. Tuy nhin, cng v ch hot ng mc ng dng nn hiu nng khng cao, v s dng my ph dng nn khng c ti u, d tn ti nhiu li h thng, v ci t trn mt my c lp nn vic theo di trng thi ca cc my ch gp kh khn. Nhc im ln nht ca cc gii php dng ny thng c tnh n nh km, hiu nng thp, d mc li. y l iu khng th chp nhn c i vi cc h thng i hi tnh sn sng cao nh ngn hng, ti chnh. 1.2.3. Chia ti nh thit b chia kt ni Nhm ny thng s dng cc moun cm thm trn cc thit b chuyn dng nh B nh tuyn (Router) hay hay b chuyn mch (Switch) chia ti theo lung, thng hot ng t layer 4 tr xung. V s dng thit b chuyn dng nn c hiu nng cao, tnh n nh cao, kh nng m rng tt hn nhng kh pht trin c tnh nng bo mt phc tp nh gii php proxy, thng thut ton chia ti rt n gin nh DNS round-robin (y l thut ton chia ti ph bin nht v n gin, tuy nhin cng nhc v hiu qu thp. Vi thut ton ny cc yu cu v IP ca mt tn min ng vi nhiu server s c bin dch thnh a ch IP ca cc server theo th t quay vng. Nhm ny c kh nng chia ti ng km, khng theo di c trng thi ca my ch, x l kt ni mc ng dng rt km, d gy li ng dng v gi thnh cao. Cch thc ny cng hon ton khng ph hp i vi cc h thng yu cu tnh chun xc ca cc hot ng giao dch nh ti chnh, ngn hng. Nh vy, gii php c kh nng theo di trng thi ng dng tt th m rng, tng tc, bo mt km(GP dng phn mm). Gii php m rng, tng tc, bo mt tt, th theo di trng thi ng dng km, khng n nh, hiu nng thp(GP s dng proxy), gii php hiu nng cao, n nh, m rng tt th km thng minh, d gy li ng dng, tng tc km(GP chia ti nh thit b chia kt ni). Trong khi , tt c cc

yu cu v hiu nng cao, n nh, m rng tt, tng tc tt v bo mt l rt quan trng i vi cc hot ng ca ngn hng, chng khon v cc nh cung cp dch v. GP sn c ca cc hng ch p ng c mt phn trong cc yu cu trn nh Module CSS ca Cisco, ISA ca Microsoft, hay Netscaler ca Citrix) Nh vy yu cu thc t t ra l phi xy dng c mt h thng hon chnh c kh nng qun tr lu lng, c kh nng kim sot, iu khin v ti u ha lu lng mng chy qua n. SLB l mt gii php c th gii quyt c hu ht cc yu cu t ra trn vi mt s ti u cao nht. 1.3. Cc thnh phn ca SLB 1.3.1. Chc nng ca cc thnh phn trong SLB Mt gii php cn bng ti phi (Server Load Balancer) c nhng chc nng sau y: Can thip vo lung d liu mng ti mt im ch. Chia lung d liu thnh cc yu cu n l v quyt nh my ch no s x l nhng yu cu . Duy tr vic theo di cc my ch ang hot ng, m bo rng cc my ch ny vn ang p ng cc yu cu n. Nu my ch no khng hot ng ng chc nng, my ch bt buc phi a ra khi danh sch xoay vng. Cung cp s a dng bng vic tn dng nhiu hn mt n v trong cc tnh hung fail-over (fail-over l kh nng t ng chuyn qua cc thit b d phng khi gp tnh hung hng hc hoc trc trc. Vic thc thi ny c thc hin m khng c s can thip ca con ngi cng nh khng c bt s cnh bo no). Cung cp s phn phi d trn s hiu bit v ni dung v d nh c URL, can thip vo cookies hoc truyn XML. Server Load Balancers: Load Balancer l mt thit b phn phi ti gia cc my tnh vi nhau v cc my tnh ny s xut hin ch nh mt my tnh duy nht. Phn di y s tho lun chi tit hn v cc thnh phn ca cc thit b SLB. VIPs: Virtual IP (VIP): l mt dng th hin ca ca cn bng ti. Mi VIP s dng mt a ch cng khai IP. Bn cnh , mt cng TCP hay UDP s i km vi mt VIP nh cng TCP 80 c dnh cho lung d liu ca web. Mt VIP s c t nht mt my ch thc s c gn a ch IP v my ch ny s lm nhim v phn phi

lung d liu c chuyn n. Thng thng th s c vi my ch v VIP s dn u lung d liu n cho cc my ch bng cch s dng cc metric hoc cc phng thc c m t trong phn Active - Active Scenario sau y. Cc my ch (Servers): My ch chy mt dch v c chia s ti gia cc dch v khc. My ch thng c m ch ti cc my ch HTTP, mc d cc my ch khc hoc ngay c nhng dch v khc c lin quan. Mt my ch thng c mt a ch IP v mt cng TCP/UDP gn lin vi n v khng c a ch IP cng khai (iu ny cn ph thuc vo topo ca mng). Nhm (Groups): Dng ch mt nhm cc my ch c cn bng ti. Cc thut ng nh farm hoc server farm c cng mt ngha vi thut ng ny. Cp ngi dng truy nhp (User - Access Levels): L mt nhm cc quyn c gn cho mt ngi dng no khi ng nhp vo mt thit b cn bng ti. Khng ch nhng nh cung cp thit b khc nhau cung cp nhng cp truy nhp khc nhau, m hu ht cc dch v cng s c nhng cch thc truy nhp rt khc nhau. Cch trin khai ph bin nht l ca Cisco, cung cp truy nhp da trn ti khon ngi dng (cho php c ti khon superuser). Mt phng thc ph bin khc l cch thc truy cp cp ngi dng c dng trong cc h thng Unix.

Read-only: Cp truy cp ch c (Read-only) khng cho php bt k mt thay i no c thc hin. Mt ngi dng c quyn ch c ch c th xem cc thit t, cc cu hnh, v nhiu th khc na nhng khng th thc hin c bt k mt thay i no c. Mt ti khon nh th c s dng xem cc thng k hiu sut hot ng ca thit b.Truy nhp ch c thng l cp truy cp u tin ca mt ngi dng khi ng nhp vo h thng trc khi thay i sang cc ch vi quyn truy cp cao hn. Superuser: Superuser l cp truy cp cho php ngi dng c y quyn iu khin h thng. Superuser c th thm cc ti khon khc, xa file, cu hnh li h thng vi bt k tham s no. Cc cp khc: Rt nhiu sn phm cung cp thm mt vi cp ngi dng trung gian gia hai cp trn, c nhng quyn gii hn trn h thng.

Gii php d phng (Redundancy) Gii php d phng rt n gin: nu mt thit b gp trc trc, thit b s c thay th bi mt thit b khc m khng hoc gy t nh hng nht n hot ng ca ton b h thng. Thit b c thay th s thc hin nhng chc nng ging nh thit b b thay th. Hu ht cc thit b trn th trng u c kh nng ny.

C mt vi cch thc hin kh nng ny. Cch thng thng nht l s dng hai thit b. Mt giao thc s c s dng bi mt trong hai thit b kim tra tnh trng hot ng ca thit b cn li. Trong mt vi tnh hung, c hai thit b u hot ng, p ng cc lung d liu n. Trong mt vi tnh hung khc, s ch c mt thit b hot ng chnh, thit b cn li s c s dng trong tnh hung hng hc hoc trc trc. Vai tr ca vic d phng Trong gii php d phng, tn ti mt quan h l active - standby. Mt thit b, hay cn gi l thit b ang hot ng thc hin mt vi hoc y cc chc nng chnh, trong khi thit b d phng s i thc hin nhng chc nng ny. Mi quan h ny cng c th c gi l mi quan h master/slave. Trong nhng tnh hung nht nh, c hai thit b s l ch (master) trong mt vi chc nng v lm phc v (slave) trong mt vi chc nng khc nhm phn tn ti. Cng trong mt vi tnh hung khc, c hai thit b u l ch (master) ca tt c cc chc nng c chia s gia hai thit b. Quan h ny cn c gi l quan h active active. Kch bn Active - Standby (hot ng - ch) Kch bn d phng hot ng - ch l cch d nht thc hin. Mt thit b s nhn ton b lung d liu n, trong khi thit b cn li s ch trong cc tnh hung trc trc.

Hnh 2.1: Kch bn Active Standby

Nu thit b ang hot ng gp trc trc, mt thit b kia s xc nh trc trc v nhn x l ton b lung d liu n.

Hnh 2.2: Hot ng ca kch bn Active - Standby Kch bn Active - Active C mt vi bin th ca kch bn ny. Trong tt c cc trng hp, c hai thit b u chp nhn x l lung d liu n. Trong tnh hung mt trong hai thit b gp trc trc th thit b cn li s nhn thc thi lun c nhng chc nng ca thit b gp trc trc. Trong mt bin th khc, VIPs c phn phi gia hai thit b cn bng ti (Load Balancer - LB) chia s lung d liu n. VIP 1 n LB A, VIP 2 n LB B. Xem hnh 2 - 3.

Hnh 2.3: Kch bn Active-Active

Trong mt bin th khc, c hai VIPs s u tr li trn c hai LB vi mt giao thc ph v s hn ch rng c hai LB s khng c cng mt a ch IP.

Hnh 2.4: Hot ng ca kch bn Active-Active

Trong tt c cc kch bn active - active, nu mt LB gp trc trc, cc VIP cn li s tip tc tr li trn LB cn li. Nhng thit b cn li s thc thi ht tt c cc chc nng. Hnh 2-5.

1.3.2. Hot ng ca h thng cn bng ti server phn trn tm hiu v cc khi nim v cc thnh phn c bn ca h thng cn bng ti server. Phn tip theo ny s trnh by SLB hot ng nh th no nhn trn kha cnh mng. M hnh SLB n gin c m t nh hnh di y.

Hnh 2.5: H thng SLB n gin SLB m rng hiu nng ca cc server ng dng, chng hn nh Web server, nh phn phi cc yu cu ca client cho cc server trong nhm (cluster). Cc server (hay cn gi l host) u nhn gi IP n, nhng gi ch c x l bi mt server nht nh. Cc host trong nhm s ng thi p ng cc yu cu khc nhau ca cc client, cho d mt client c th a ra nhiu yu cu. V d, mt trnh duyt Web cn rt nhiu hnh nh trn mt trang Web c lu tr ti nhiu host khc nhau trong mt nhm server. Vi k thut cn bng ti, qu trnh x l v thi gian p ng client s nhanh hn nhiu. Mi host trong nhm c th nh ra mc ti m n s x l hoc ti c th phn phi mt cch ng u gia cc host. Nh s dng vic phn phi ti ny, mi server s la chn v x l mt phn ti ca host. Ti do cc client gi n c phn phi sao cho mi server nhn c s lng cc yu cu theo ng phn ti nh ca n. S cn bng ti ny c th iu chnh ng khi cc host tham gia vo hoc ri khi nhm. i vi cc ng dng nh Web server, c rt nhiu client v thi gian m cc yu cu ca client tn ti tng i ngn, kh nng ca k thut ny nhm phn phi ti thng qua nh x thng k s gip cn bng mt cch hiu qu cc ti v cung cp kh nng p ng nhanh khi nhm server c thay i.

Cc server trong nhm cn bng ti pht i mt bn tin c bit thng bo trng thi hot ng ca n (gi l heartbeat message) ti cc host khc trong nhm ng thi nghe bn tin ny t cc khc host khc. Nu mt server trong nhm gp trc trc, cc host khc s iu chnh v ti phn phi li ti duy tr lin tc cc dch v cho cc client. Trong phn ln cc trng hp, phn mm client thng t ng kt ni li v ngi s dng ch cm thy tr mt vi giy khi nhn c p ng tr li. 1.3.3. Kin trc h thng cn bng ti ti a ho thng lng v kh dng, cng ngh cn bng ti s dng kin trc phn mm phn tn hon ton, trnh iu khin cn bng ti c ci t v chy song song trn tt c cc host trong nhm. Trnh iu khin ny sp xp tt c cc host trong nhm vo mt mng con pht hin ng thi lu lng mng n a ch IP chnh ca nhm (v cc a ch b sung ca cc host nhiu v tr khc nhau). Trn mi host, trnh iu khin hot ng nh mt b lc gia trnh iu khin card mng v chng giao thc TCP/IP, cho php mt phn lu lng mng n c nhn bi host . Nh , cc yu cu ca client s c phn vng v cn bng ti gia cc host trong nhm. Kin trc ny ti a ho dung lng nh vic s dng mng qung b phn phi lu lng mng n tt c cc host trong nhm v loi b s cn thit phi nh tuyn cc gi n tng host ring l. Do thi gian lc cc gi khng mong mun din ra nhanh hn thi gian nh tuyn cc gi (nh tuyn bao gm cc qu trnh nhn gi, kim tra, ng gi li v gi i), kin trc ny cung cp thng lng cao hn cc gii php da trn b iu phi. Khi tc ca mng v server tng ln, thng lng cng tng theo t l thun, do loi b c bt c s l thuc no vo vic nh tuyn da trn cc phn cng c bit. Trn thc t, b cn bng ti c th t thng lng 250Mbit/s trong cc mng Gigabit. Mt u im c bn khc ca kin trc phn tn hon ton l kh dng c tng cng vi (N-1) cch khc phc li trong mt nhm c N host. Cc gii php da trn b iu phi to ra mt im li k tha m ch c th c khc phc bng cch s dng mt b iu phi d phng v do ch cung cp mt cch khc phc li duy nht. Kin trc cn bng ti cng tn dng c nhng u im v kin trc cc thit b chuyn mch (switch) hoc cc b tp trung (hub) ca mng con trong vic ng thi phn phi lu lng mng n tt c cc host trong nhm. Tuy nhin, phng php ny lm tng "ti trng" trn cc chuyn mch do chim thm bng thng cng. y khng phi l vn trong phn ln cc ng dng nh dch v Web hay

streaming media, do t l lu lng n ch chim mt phn rt nh trong tng lu lng mng. Tuy nhin, nu cc kt ni mng pha client n thit b chuyn mch c tc nhanh hn nhiu cc kt ni pha server, lu lng c th chim mt t l ln qu mc cho php ca bng thng cng pha server. Vn tng t s gia tng nu nhiu nhm kt ni trn cng mt thit b chuyn mch v cc bin php thit lp cc mng LAN o cho tng nhm khng c thc hin. Trong qu trnh nhn gi, vic trin khai ca SLB l s kt hp gia vic phn phi cc gi ti tng TCP/IP v nhn cc gi khc qua trnh iu khin card mng. Vic ny gip tng tc x l chung v gim tr do TCP/IP c th x l gi trong khi trnh iu khin NDIS (Network Driver Interface Specification) nhn gi tip theo. Trong qu trnh gi gi, SLB cng tng cng thng lng, gim tr v ph ph (overhead) nh tng s lng gi m TCP/IP c th gi trong mt kt ni. c c nhng ci thin v hiu nng ny, SLB thit lp v qun l mt tp hp cc b m gi v cc k hiu (descriptor) c s dng phi hp cc hot ng ca TCP/IP v trnh iu khin NDIS. 1.3.4 Phn phi lu lng trong SLB Nh ta c th thy, lung d liu t ngi dng cui ti thit b cn bng ti, ti my ch thc s pha sau v sau quay ngc tr li ngi dng cui. Phn ny s phn chia qu trnh i ca gi tin trn tng on mng gip cho vic hiu r SLB hot ng nh th no. SLB hot ng bi vic iu khin gi tin trc v sau khi n ti server thc s pha sau. Vic ny thc hin mt cch n gin bi vic s dng a ch IP ch v ngun ti lp 3 trong hot ng s l c bit n nh l NAT (Network Address Translation).

Hnh 2.6: Hnh trnh ca mt gi d liu Trong Hnh 2.6, ta c th thy mt gi d liu c a ch ngun l 208.185.43.202 a ch ch l 192.168.0.200. B nh tuyn s dng cc thng tin ny chuyn tip d liu trn mng qua cc trm trung gian ti ch. Mt vn c tnh cht quan trng sng cn vi SLB ni ring v mng TCP/IP ni chung l, khi gi mt gi d liu ti mt a ch ch, gi d liu cn phi c bo nhn li vi cng a ch ngun v ch hay ni mt cch khc, khi gi mt gi tin ti mt my ch, my ch phi gi ngc tr li my gi vi a ch ch l my gi v a ch ngun l a ch ca chnh my nhn, nu t mt a ch khc gi d liu s b loi b. Nguyn tc ny khng c ngha quan trng vi gi tin UDP do UDP s dng giao thc khng hng ni (Connectionless). Mc d vy, ph bin cc SLB da trn giao thc hng ni TCP. sng t SLB hot ng c th nh th no cng xt mt v d c th v cch thc mt ngi dng cui truy cp ti my ch web t trong h thng SLB. Nh ta c th thy hnh 2.6. mt my khch c a ch 208.185.43.202, mt VIP c a ch 192.168.0.200 v my ch web thc s c a ch 192.168.0.100. duyt web, ngi dng cui s dng mt URL xc nh v tr ca website ci m nh x ti a ch ca VIP l 192.168.0.200. Gi d liu vi a ch ngun 208.185.43.202 v a ch ch l 192.168.0.200. Thit b cn bng ti thay v tr li yu cu t my khch, n lu li gi d liu v vit li thng tin iu khin trong gi d liu bng vic thay i a ch ch trong gi d liu thnh 192.168.0.100 sao cho chuyn tip c gi d liu ti my ch web tht c a ch 192.168.0.100. Nh vy trong giai on 2 ny a ngun l 208.185.43.202 v ch l 192.168.0.100. My ch thc khi nhn c yu cu s gi thng tin phn hi ti ngi dng cui. Trong giai on 3 ny a ch ngun tr thnh 192.168.0.100 v ch tr thnh 208.185.43.202, nh vy ny sinh mt vn . Ngi dng s b qua gi tin p li t a ch 192.168.0.100 bi kt ni khng c gi n my c a ch m n a ch 192.168.0.200. SLB gii quyt vn ny bng cch thay i ng i mc nh ca my ch thc v thay i a ch ngun ca gi tn thnh a ch ca VIP 192.168.0.200 trc khi gi tr li my khch. Trong giai on 4 ny a ch ngun l 192.168.0.200 v ch l 208.185.43.202. Vi ln thay i thng tin cui ny, gi d liu hon thnh chuyn i v khi to thnh cng mt kt ni. Nhn t pha my khch, n dng nh ch l mt kt ni thng thng ti ng mt my ch, m khng h bit trong thc t c th c mt vi n

hng trm my ch thc s pha sau tr li yu cu ca n. C th hnh dung cc bc theo m t bng di y.

Direct Server Return DSR l mt trong cc phng php phn phi lu lng ca cc thit b cn bng ti t cc kt ni bn ngoi. Phng php phn phi ny lm tng s thc thi ca thit b cn bng ti bi vic gim mt cch ng k lu lng i qua thit b v qu trnh x l vit li thng tin iu khin trong gi d liu nh cc bc phn trn. DSR lm iu bi vic b qua giai on 3 trong bng x l trn. Bi vic li dng mt my ch thc pha trong gi ra ngoi mt gi d liu vi a ch ngun c vit li l a ch ngun ca VIP (trong trng hp ca ny l 192.168.0.200). DSR thc hin iu bng cch iu khin khung d liu ti lp 2 thc hin SLB. X l c bit n nh l MAT (MAC Address Translation). hiu x l v cch DSR lm vic nh th no trc ht cn xem xt mt s c tnh ca packets ti lp 2 v mi lin quan ca n ti SLB. a ch vt l ca Card mng (Network Interface Card NIC): l mt s 48 bit, thng c biu din bng 12 s hexa (c s 16), trong 24 bit u l m s ca cng ty sn xut Card mng, cn 24 bit sau l s seri ca tng Card mng i vi mt hng sn xut. Nh vy ngi ta bo m khng c hai Card mng no trng nhau v a ch vt l, ni chnh xc hn l s Identification ca tng Card mng. Cc s Identification ny c lu trong mt chip ROM gn trn mi Card mng ngay t khi sn xut, nn cn gi l Burnt-in Address, do ngi dng khng th thay i c. Trn mt mng Ethernet, a ch MAC gip cho gi tin IP tm ng thit b vt l cn nhn gi tin. DSR s dng s kt hp ca MAT v mt my ch c cu hnh c bit thc thi SLB m khng i ra qua thit b cn bng ti. Mt my ch thc s c cu hnh vi mt a ch nh cch lm vic thng thng, nhng n cng c cu hnh vi a ch IP ca VIP. Theo nguyn tc hot ng ca mng th khng th tn ti hai my c cng mt a ch IP bi v hai a ch MAC khng th kt hp ti cng mt a ch IP. gii quyt vn ny, thay v vic cu hnh a ch IP ca

VIP ti giao din mng ca my ch thc th ta kt hp n vi giao din loopback. Giao din loopback l mt giao din o s dng cho vic truyn thng bn trong ca my ch v thng thng khng c tc ng ti cu hnh hay hot ng ca my ch. a ch IP ca giao din loopback thng l 127.0.0.1, mc d vy c th cu hnh nhiu a ch IP trn cng mt giao din (thng c bit n nh a ch IP b danh) v giao din loopback cng khng nm ngoi kh nng . Nh vy a ch ca VIP c th c cu hnh trn giao din loopback m khng nh hng ti hot ng ca mng, khng gy ra s xung t vi my ch khc trong mng. Trong trng hp SLB, my ch web hay cc dch v khc c cu hnh kt hp vi a ch ca VIP trn giao din loopback ch khng phi a ch trn giao din vt l ca my ch. Bc tip theo thc t yu cu c c lu lng ti giao din loopback ca my ch thc. gii quyt vn ny SLB s dng mt k thut gi l MAT (MAC address Translation). Nh trnh by trn mi my trong mng Ethernet u c mt a ch MAC xc nh v tr ca n trong mng. Thit b cn bng ti x l lu lng trn VIP v thay v vic thay i a ch IP ch nh phng php trn. DSR s dng MAT bin i a ch MAC ch. My ch thng thng s loi b lu lng khi n khng c a ch IP ca VIP, nhng trong gii php ca SLB th do a ch ca ca VIP c cu hnh trn giao din loopback ca ca my ch thc nn la c my ch chp nhn lu lng gi ti n. Qu trnh s l ny gip cho my ch khi gi bo nhn li cho my khch vn m bo c a ch IP ngun l a ch ca VIP do b qua c giai on 3 trong qu trnh gi d liu trong phng php trn. Ta c th hnh dung ton b qu trnh trn qua bng tm tt di y.

NLB s dng hai lp broadcast hoc multicast phn phi ng thi lu lng mng n tt c cc host trong nhm. Trong ch hot ng mc nh l unicast, NLB s gn a ch trm lm vic (a ch MAC) cho card mng card mng c th hot ng (card ny gi l card nhm ? cluster adapter), v tt c cc host trong nhm c gn cng mt a ch MAC. Cc gi n do c nhn bi tt c cc host trong nhm v chuyn gi ti trnh iu khin cn bng ti lc. m bo tnh duy nht, a ch MAC c dn xut t a ch IP chnh ca nhm. V d, vi a ch

IP chnh ca nhm l 1.2.3.4, a ch MAC unicast c t l 02-BF-1-2-3-4. Trnh iu khin cn bng ti s t ng sa a ch MAC ca card nhm bng cch thit lp mt thc th ng k v ti np trnh iu khin card nhm. H iu hnh khng cn phi khi ng li. Nu cc host trong cluster c gn vo mt thit b chuyn mch (swicth) ch khng phi mt b tp trung (hub), vic s dng chung mt a ch MAC s gy ra xung t do cc chuyn mch lp 2 ch c th hot ng khi cc a ch MAC ngun trn tt c cc cng ca thit b chuyn mch l duy nht. trnh iu ny, NLB sa a ch MAC ngun cho cc gi u ra l duy nht, a ch MAC ca nhm l 02-BF-1-2-3-4 c chuyn thnh 02-h-1-2-3-4, trong h l mc u tin ca host trong nhm. K thut ny ngn khng cho thit b chuyn mch tm ra a ch MAC thc s ca nhm v kt qu l cc gi n nhm c phn phi ti tt c cc cng ca thit b chuyn mch. Nu cc host trong nhm c kt ni trc tip vo mt hub, mt n a ch MAC ngun ca NLB trong ch unicast c th c v hiu ho trnh gy ra hin tng trn cho cc thit b chuyn mch ng ln (upstream). iu ny c th thc hin bng cch thit lp tham s ng k NLB l MaskSourceMAC=0. Vic s dng h thng chuyn mch ng ln ba mc cng c th hn ch trn cho cc thit b chuyn mch. Ch unicast ca NLB c th lm v hiu ho qu trnh trao i thng tin gia cc host trong nhm c s dng card nhm. Khi cc gi ca mt host c gi i vi a ch MAC ch ging a ch MAC ngun, cc gi ny s b quay vng (loop-back) gia cc tng giao thc mng bn trong h thng pha gi v khng bao gi ra n ng truyn. Hn ch ny c th trnh c bng cch thm mt card mng th hai cho mi host. Trong cu hnh ny, NLB s dng mt card mng trn mng con nhn cc yu cu ca client v mt card mng khc thng c t tch bit trn mng con cc b trao i thng tin gia cc host trong nhm v vi cc server c s d liu cng nh cc file server gc. NLB ch s dng card nhm truyn cc bn tin "heartbeat" v lu lng iu khin t xa. Ch rng, trao i thng tin gia cc host trong nhm v cc host ngoi nhm khng bao gi b nh hng bi ch unicast ca NLB. Lu lng mng n mt a ch IP dnh ring cho host (trong card nhm) c nhn bi tt c cc host trong nhm do chng s dng chung mt a ch MAC. Do NLB khng bao gi cn bng ti lu lng i vi cc a ch IP dnh ring, NLB s lp tc phn phi lu lng ny n TCP/IP trn host nh. Cc host khc trong nhm coi lu lng ny l lu lng c cn bng ti v s loi b lu lng ny. Ch , nu lu lng mng

n qu ln i vi cc a ch IP dnh ring c th nh hng n hiu nng khi h thng NLB hot ng trong ch unicast (tu theo s cn thit i vi TCP/IP trong vic loi b cc gi khng mong mun). Network Load Balancing (NLB) NLB cung cp ch th hai phn phi lu lng mng n cc host trong nhm, ch multicast. Ch ny gn a ch multicast 2 lp cho card nhm thay v thay i a ch trm lm vic ca card. V d, a ch MAC multicast s c gn l 03-BF-1-2-3-4 tng ng vi a ch IP chnh l 1.2.3.4. Do mi host trong nhm c mt a ch trm lm vic duy nht, ch ny khng cn mt b card mng th hai trao i thng tin gia cc host trong nhm v n cng khng c bt c nh hng no n hiu nng ca ton h thng do vic s dng cc a ch IP dnh ring. Ch unicast ca NLB gy ra trn trn switch do s phn phi ng thi lu lng mng trn tt c cc cng. Tuy nhin, ch multicast ca NLB a ra c hi hn ch trn switch ngi qun tr h thng c th cu hnh mt mng LAN o trn switch cho cc cng tng ng vi cc host. C th lm c iu ny bng cch lp trnh cho switch hoc s dng giao thc IGMP hoc giao thc GARP, GMRP. NLB cn trin khai chc nng ARP m bo rng a ch IP chnh ca nhm v cc a ch IP o khc c th phn gii sang a ch MAC multicast ca nhm. (a ch IP dnh ring s tip tc phn gii sang a ch trm lm vic ca card nhm). 1.3.4. Thut ton cn bng ti NLB s dng thut ton lc phn tn hon ton nh x cc client n cc host trong nhm. Thut ton ny cho php cc host trong nhm a ra cc quyt nh cn bng ti mt cch c lp v nhanh chng cho tng gi n. N c ti u ho cung cp kh nng cn bng ti mt cch thng k i vi mt s lng ln cc yu cu nh do v s client to ra, in hnh l i vi cc Web server. Nu s client v/hoc cc kt ni client to ra cc ti qu chnh lch nhau trn server, thut ton cn bng ti s t hiu qu. Tuy nhin, tnh n gin v tc ca thut ton cho php cung cp hiu nng rt cao bao gm c thng lng cao v thi gian p ng ngn trong mt di rng cc ng dng client/server thng dng. NLB x l cc yu cu ca client bng cch dn ng cho mt t l phn trm chn nhng yu cu mi cho tng host trong nhm. Thut ton khng p ng nhng thay i v ti trn mi host (chng hn nh ti CPU hay vn s dng b

nh). Tuy nhin, qu trnh nh x s c thay i khi quan h thnh vin trong nhm thay i v t l phn trm ti phn b s c ti cn bng. Khi xem xt mt gi n, tt c cc host thc hin ng thi vic nh x thng k xc nh nhanh chng host no s x l gi . Qu trnh nh x s dng mt hm ngu nhin tnh mc u tin ca host da trn a ch IP v cng n ca client cng cc thng tin trng thi khc ti u ho vic cn bng ti. Host tng ng s chuyn gi t cc tng di ln tng TCP/IP cn cc host khc s loi b gi ny. Qu trnh nh x khng thay i tr phi quan h gia cc host trong nhm thay i, m bo rng a ch IP v cng n ca client cho trc s lun c nh x n cng mt host trong nhm. Tuy nhin, host c th trong nhm m a ch IP v cng n ca client nh x ti khng th c xc nh trc do hm ngu nhin c tnh n quan h thnh vin trong nhm hin ti v qu kh ti thiu ho kh nng nh x li. Nhn chung, cht lng cn bng ti c xc nh mt cch thng k bi s lng client to ra yu cu. Nh kt cu tng gim v s lng client theo thng k, s u n v cht lng ca thut ton cn bng ti s thay i nh. hot ng cn bng ti c chnh xc cao trn mi host trong nhm, mt phn ti nguyn h thng s c s dng o v phn ng trc nhng thay i ca ti. S tr gi v hiu nng ny phi c cn nhc so vi li ch ca vic ti a ho kh nng s dng cc ti nguyn trong nhm (v c bn l CPU v b nh). Trong bt c trng hp no, vic s dng hp l cc ngun ti nguyn server phi c duy tr c th phc v cho cc ti client khc trong trng hp xy ra li. Khi mt host mi tham gia vo nhm, n s kch hot qu trnh hi t v mt quan h thnh vin mi trong nhm s c tnh ton. Khi qu trnh hi t hon thnh, mt phn ti thiu client s c nh x ti host mi. NLB d cc kt ni TCP trn mi host v sau khi kt ni TCP hin ti ca chng hon thnh, kt ni tip theo t cc client b nh hng s c x l bi host mi. Do , cc host nn c b sung vo nhm ti nhng thi im ti tng qu mnh nhm ti thiu ho hin tng ngt qung cc phin. trnh vn ny, trng thi phin phi c qun l bi ng dng server sao cho n c th c ti cu trc hay c tr li t bt k mt host no trong nhm. V d, trng thi phin c th c y n server c s d liu v lu trong cc cookies ca client.

1.3.5. Qu trnh hi t ca SLB Cc host trong nhm trao i nh k cc bn tin "heartbeat" multicast hoc broadcast vi nhau. iu ny cho php cc host c th gim st trng thi ca nhm. Khi trng thi ca nhm thay i (chng hn nh khi c host gp trc trc, ri khi hoc tham gia vo nhm), SLB kch hot mt chu trnh gi l hi t trong cc host trao i bn tin "heartbeat" nh ra mt trng thi mi, bn vng cho nhm. Khi tt c cc t c s "nht tr" trng thi mi ca chng s c thit lp v nhng thay i ny s c lu vo nht k s kin. Trong qu trnh hi t, cc host tip tc x l lu lng mng n nh mi khi ngoi tr lu lng n host b li khng nhn c dch v. Qu trnh hi t kt thc khi tt c cc host trong nhm c c mt quan h thnh vin n nh trong vng mt vi chu k heartbeat. Khi hon thnh qu trnh hi t, lu lng n host b li s c ti phn phi cho cc host cn li. Nu mt host c thm vo nhm, qu trnh hi t cho php host ny nhn c phn ti ca n trong lu lng c cn bng. Vic m rng nhm khng nh hng n cc hot ng ca nhm v theo mt cch hon ton trong sut i vi tt c cc Internet client cng nh trc cc chng trnh phn mm server. Tuy nhin, n c th nh hng n cc phin client v cc client c th phi ti nh x ti cc host khc trong nhm. 1.3.6. Hiu sut ca SLB Vai tr ca NLB tc ng n hiu nng ca h thng c th c nh gi da trn cc tiu ch chnh sau: CPU overhead trn cc host ca nhm - Phn trm CPU cn thit phn tch v lc cc gi ca mng (cng thp cng tt). Tt c cc gii php cn bng ti u cn s dng mt phn ti nguyn ca h thng xem xt gi n v a ra quyt nh cn bng ti v do t nhiu nh hng n hiu nng ca mng. Gii php cn bng ti da trn b iu phi cn kim tra, hiu chnh v truyn li gi ti cc host trong nhm (thng phi sa i li a ch IP ti nh tuyn gi t a ch IP o ti a ch IP ca tng host c th). i vi NLB, n phn phi ng thi cc gi n ti tt c cc host trong nhm v p dng mt thut ton lc loi b cc gi khng mong mun... Qu trnh lc gy nh hng t hn so vi qu trnh ti nh tuyn v kt qu l thi gian p ng nhanh hn vi thng lng ton h thng cao hn.

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Thng lng v thi gian p ng yu cu: NLB nng cao hiu nng h thng bng cch tng thng lng v ti thiu ho thi gian p ng ti cc yu cu ca client. Khi nng lc ca cc host trong nhm c khai thc ti a, n s khng th cung cp thm thng lng v thi gian p ng tng t bin tu theo tr hng i cc yu cu ca client. B sung thm host s cho php tng thng lng v gim thi gian p ng. Nu nhu cu ca khch hng tip tc tng, cc host s c thm vo cho n khi mng con bo ho. V nu ti tip tc tng, cn s dng nhiu nhm NLB v vic phn phi lu lng gia cc host c thc hin bng k thut Round Robin DNS. Bng thng s dng ca Switch (Switch occupancy): T l bng thng ca switch c s dng bi qu trnh lm trn cc yu cu ca client. Kin trc lc gi ca NLB da trn mng con broadcast phn phi cc yu cu ca client ti tt c cc host cng lc. Trong cc nhm nh, c th s dng hub kt ni cc host. Vi cc nhm ln hn, switch s l s la chn. V nh mc nh, NLB s to ra hin tng "trn" switch c th phn phi cc yu cu ca client ti tt c cc host cng lc. Cn chc chn rng hin tng "trn" switch khng c vt qu nng lc ca switch, c bit khi switch c chia s gia nhm v cc my tnh ngoi nhm. Bnh thng, bng thng s dng cho lu lng yu cu ca client ch chim mt t l nh trong tng lng bng thng cn thit cho qu trnh truyn thng gia server v client. Tuy nhin qu trnh "trn" switch s tr thnh vn trong nhng ng dng c t l phn trm ng k lu lng mng c dn ti nhm (chng hn nh qu trnh upload file trong cc ng dng FTP) hay khi nhiu nhm s dng chung mt switch. Trong nhng trng hp ny, chy NLB trong ch multicast v thit lp mng LAN o hn ch trn switch l bin php khc phc rt hiu qu khim khuyt ny. Ngoi ra, tnh kh m ca NLB quyt nh kh nng ci thin hiu nng ca h thng khi cc host c thm vo nhm.

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CHNG 2: TRIN KHAI V NH GI H THNG SERVER LOAD BALANCING Phn trn trnh by nhng vn v k thut c bn ca SLB: Cc m hnh trin khai SLB, cc k thut phn phi lu lng, x l gi d liu. Cc vn v k thut trn c nhiu hng trn th gii gii quyt trong cc sn phn ca h. Do cha c mt chun thng nht trn th gii cho cc thit b cn bng ti nn vn cn t ra l tnh tng thch gia cc sn phn ca cc hng khc nhau. Nh vy, cc nguyn tc v cc m hnh thc hin SLB y l cc nguyn tc v m hnh ni chung, cn c th i vi mi sn phn ca cc hng khc nhau li c cc gii php chi tit hn cho sn phn ca h. Vic la chn mt gii php SLB ph thuc vo mc ch v quy m ca ng dng m cc t chc nh trin khai. Trong phn ny s tin hnh thit k xy dng mt h thng cn bng ti my ch th nghim, qua s hiu su sc hn h thng SLB, cng nh c nhng b sung cho l thuyt. d dng cho vic th nghim y em xin trnh by mt h thng cn bng ti n gin trong cc ng dng web s dng phng php lun chuyn vng DNS. 2.1. Gii php SLB da trn lun chuyn vng DNS

DNS l vit tt ca Domain Name System. DNS Server l my ch phn gii tn min. Mi my tnh, thit b mng tham gia vo mng Internet u "ni chuyn" vi nhau bng a ch IP (Internet Protocol) . thun tin cho vic s dng v d nh ta dng tn (domain name) xc nh thit b . H thng tn min DNS (Domain Name System) c s dng nh x tn min thnh a ch IP. V vy, khi mun lin h ti cc my, chng ch cn s dng chui k t d nh (domain name) nh: www.coltech.vnu.edu.vn, www.vnu.edu.vn, thay v s dng a ch IP l mt dy s di kh nh. Hnh bn di m t qu trnh truy cp vo website ca trng i hc Cng Ngh c a ch l www.coltech.vnu.edu.vn

Khi nhp mt URL vo trong trnh duyt (v d nh www.coltech.vnu.edu.vn) th trnh duyt s gi mt yu cu n DNS yu cu n tr v a ch IP ca site. y c gi l vic tra cu DNS. Sau khi trnh duyt Web c c a ch IP cho site th n s lin h vi site bng a ch IP, v hin th trang va yu cu. My ch DNS thng c mt a ch IP c bn ha vi mt tn site no . Trong v d ny gi s th site l www.coltech.vnu.edu.vn bn ha thnh a ch IP l 203.113.130.201 cn bng ti bng DNS, my ch DNS phi duy trnh mt s a ch IP khc nhau cho cng mt tn site. Nhiu a ch IP th hin nhiu my trong mt cluster, tt c trong s chng u bn ha n mt tn site logic. Trong v d ny, www.coltech.vnu.edu.vn c th c cu hnh trn ba my ch trong mt cluster vi cc a ch IP di y: 203.113.130.201 203.113.130.202 203.113.130.203 Trong trng hp ny, my ch DNS c bn ha nh sau:

www.coltech.vnu.edu.vn

203.113.130.201 203.113.130.202 203.113.130.203

www.coltech.vnu.edu.vn

www.coltech.vnu.edu.vn

Khi yu cu u tin n c my ch DNS, n s tr v a ch IP 203.113.130.201, my u tin. Khi c yu cu th hai, n s tr v a ch IP th hai: 203.113.130.202. Tip tc nh vy, vi yu cu th t, a ch IP u tin li c lp li. Bng cch s dng lun chuyn vng DNS nh trn, tt c cc yu cu i vi mt site no u c phn phi u n tt c cc my trong cluster. Chnh v vy, vi phng php cn bng ti ny, tt c cc nt trong cluster u c s dng. 2.2. nh gi SLB da trn lun chuyn vng DNS 2.2.1. u im ca phng php lun chuyn vng DNS Khng t v d dng thit lp: Cc qun tr vin h thng ch cn to mt s thay i trong my ch DNS h tr c vic lun chuyn vng, v nhiu my ch DNS c s h tr ny. N khng yu cu n s thay i m ca ng dng Web; trong thc t, cc ng dng Web khng h bit v c ch cn bng ti m n b thc hin. n gin: Phng php ny khng yu cu n cc chuyn gia v mng trong vic thit lp hoc gi ri h thng trong trng hp c vn no xay ra. 2.2.2. Nhc im ca phng php ny C hai nhc im chnh ca phng php da trn phn mm ny l n khng cung cp s h tr mi quan h thi gian thc gia cc my ch vi nhau v khng h tr kh nng c sn cao. Khng h tr mi quan h thi gian thc gia cc my ch. Mi quan h thi gian thc gia cc my ch l kh nng ca h thng trong vic qun l cc yu cu ca ngi dng, my ch ny hoc bt k my ch no, ph thuc vo thng tin session c duy tr trn my ch hoc ti mc c s bn, mc c s d liu. Khng c c kh nng h tr mi quan h gia cc my ch, phng php lun chuyn vng DNS da vo mt trong ba phng php c a ra duy tr s

kim sot session hoc s nhn dng ngi dng i vi cc yu cu ang n trn HTTP. Cc cookie Cc trng n Vit li URL Khi mt ngi dng thc hin mt yu cu u tin, my ch Web s tr mt th bng vn bn duy nht phn bit ngi dng . Cc yu cu tip theo c th ny s dng cookie, vit li URL hoc cc trng n, cho php my ch xut hin duy tr mt session gia my khch v my ch. Khi ngi dng thit lp mt session vi mt my ch, th tt c cc yu cu n sau thng u i n cng mt my ch. Vn y l trnh duyt lu a ch IP ca my ch . Khi Cache ht hn, trnh duyt s thc hin mt yu cu khc i vi my ch DNS c c a ch IP c lin kt vi tn min. Nu my ch DNS tr v mt a ch IP khc, mt my ch khc trong cluster, th cc thng tin v session s b mt. Khng h tr cho kh nng c sn cao. Xem xt mt cluster c n nt. Nu mt nt no gp vn th c yu cu th n n my ch DNS u hng bn n mt nt hng ny. Mt router thng minh c th gii quyt c vn ny bng cch kim tra cc nt cc khong thi gian no , pht hin ra cc nt b hng v g b chng ra khi danh sch, chnh v vy s khng c yu cu no c gi n chng na. Tuy vy, vn y vn tn ti nu nt vn c nhng ng dng Web ang chy trn nt b hng. Thay i cluster s mt nhiu thi gian truyn b n ton b phn cn li ca Internet. Mt l do y l trong nhiu t chc ln cc ISP, cc cng ty, hay i l lu cc yu cu DNS ca h gim lu lng mng v thi gian request. Khi ngi dng bn trong cc t chc nh vy thc hin mt request th h thng s c kim tra danh sch cc tn DNS ca Cache c bn ha a ch IP. Nu h thng pht hin thy mt entry no th n s tr a ch IP v cho ngi dng. Nu n khng pht hin thy entry no trong Cache ni b th ISP s gi request DNS ny n my ch DNS v lu s p tr. Khi mt entry c lu ht hn, ISP s nng cp c s d liu ni b ca n bng cch lin h vi cc my ch DNS khc. Khi danh sch cc my ch ca bn thay i, n c th cn n mt khong thi gian ngn cho cc entry c lu trn mng ca

cc t chc khc ht hn v tm kim danh sch cc my ch c cp nht. Trong sut chu trnh ny, my khch vn c th thc hin hnh ng hit nt my ch b hng, nu ISP ca my khch vn c mt entry tr n n. Trong trng hp nh vy, mt s ngi dng ca ISP khng th truy cp vo site ca bn t nhng ln truy cp ban u, thm tr nu cluster ca bn c cc my ch d tha vn ang hot ng. Mt vn cn ln hn xut hin khi g b (removing) mt nt so vi vic b sung. Khi bn bt i mt nt, ngi dng c th ang thc hin hit mt my ch khng tn ti. Cn khi bn thm mt nt th my ch vn cha c s dng cho ti khi a ch IP ca n n c tt c cc my ch DNS. Mc d phng php ny c th cn bng c mt s lng ngi dng trn mi my ch, nhng n khng hon ton cn bng ti my ch. Mt s ngi dng c th yu cu mc ti cao hn trong sut mt session ca h so vi nhng ngi dng khc trn my ch khc, v phng php ny khng th bo m chng li c s bt cng bng .

KT LUN Cn bng ti Server l mt vn quan trng trong s pht trin ca cng ngh mng. Chnh v vy, vic thc hin ti Cng ngh cn bng ti server c nhng ng gp v mt l thuyt cng nh v mt cng ngh trong vic tm hiu v xy dng cc h thng cn bng ti my ch. Sau y l nhng kt qu chnh m ti t c: Trnh by tng quan v mng my tnh v cc giao thc mng. Trnh by c cc yu cu cp thit t ra cho cc t chc, doanh nghip trong vic xy dng mt h thng mng, my ch c kh nng phc v c cc yu cu ngy cng cao ca mnh nh kh nng m rng d dnh, tnh sn sng cao vn c kh nng hot ng khi h thng xy ra s c. Trnh by c chi tit cc cng ngh, cch thc phn phi lu lng trong cc h thng SLB v u nhc im ca tng cng ngh t gip cc nh qun tr c nhng quyt nh ng n trong vic xy dng gii php cho t chc ca mnh. Trong ti cng thc hin trin khai gii php cn bng ti da trn cng ngh DNS round robin. Nhn a ra mt ci nhn r rng hn v hot ng ca mt h thng cn bng ti. Trong phm vi mt lun vn, do hn ch v thi gian nghin cu v cc yu t khch quan khc, mt s vn lin quan ca ti vn cha gii quyt c trn vn. Qua kt qu t c ca lun vn cng nh cc vn cha gii quyt c, em xin xut cho hng nghin cu tip theo nh sau: Tip tc nghin cu v hon thin l thuyt v cc phng php phn phi lu lng gia cc nt. Nghin cu m rng v cc thut ton cn bng ti ti u. Thc hin ci t th nghim trn h thng mng thc ca cc cng ty, t chc.

Ti liu tham kho Ti liu ting vit [1]. V Duy Li (2002), Mng thng tin my tnh, NXB Th gii [2] . Vng o Vi, Mng truyn d liu. NXB i Hc Quc Gia H Ni. Ti liu ting Anh [3]. Paul Albitz, Cricket Liu, DNS and BIND, Fourth Edition, 2001 [4]. Tony Bourke, Server Load Balancing, O'Reilly & Associates [5].http://en.wikipedia.org/wiki/Load_balancing_(computing)