Fault Tolerance as an aspect using JReplica

Faul Tole ance as an aspec using JReplica José Luis He e o1, Fe nando Sánchez1, Miguel To o2 1 Compu e Science Depa men Uni e si y o Ex emadu a.Spain {jhe e o, e nando}@unex.es 2 Compu e Science Depa men Uni e si y o Se illa.Spain [email p o ec ed] Abs ac Reliabili y and a ailabili y a e e y impo an ends in he de elopmen p ocess o dis ibu ed sys ems. In o de o imp o e hese ea u es, objec eplica ion mechanisms ha e been in oduced. P og amming eplica ion policies o a gi en applica ion is no an easy ask, and his is he eason why anspa ency o he p og amme has been one o he mos impo an p ope ies o e ed by all eplica ion models. Howe e , his anspa ency o he p og amme is no always desi able. In his pape we p esen a eplica ion model, JReplica, based on Aspec O ien ed P og amming (AOP). JReplica allows he sepa a ed speci ica ion o he eplica ion code om he unc ional beha iou o objec s, p o iding no only a high deg ee o anspa ency, as done by p e ious models, bu also he possibili y o p og amme s o in oduce new beha iou o speci y di e en aul ole ance equi emen s. Mo eo e , he eplica ion aspec has been in oduced a design ime, and in his way, UML has been ex ended in o de o conside eplica ion issues sepa a ely when designing aul ole ance sys ems. 1: In oduc ion This wo k ies o in oduce eplica ion in objec o ien a ion by means o a new aspec . Fo his pu pose, a new language called JReplica has been de eloped. This language cap u es he ele an aspec s o eplica ion, and This wo k has been de eloped wi h he suppo o CICYT unde con ac TIC99-1083-C02-02 encapsula es hem in o a componen o g oup o ela ed componen s a ou ing he eusabili y and dynamic adap abili y o eplica ion policies. This language also a ou s he use and euse o eplica ion policies independen ly om he middlewa e used o communica e objec s (cu en ly se e al implemen a ions o CORBA and RMI). The wo k is no limi ed o he de ini ion o his new language. AOP ideas ha e been ansla ed o he design le el. In his way, he seman ic o UML has been ex ended in o de o ep esen eplica ion p ope ies. F om a gi en design, he same o wha e e middlewa e, a isual ool is able o gene a e code. The es o he pape is as ollows: sec ion 2 explains he di e en app oaches o in oduce eplica ion in objec o ien a ion. Ou p oposal is in oduced in sec ion 3. Sec ion 4 shows ela ed wo ks. Finally, u u e wo ks a e ou lined in sec ion 5. 2: Faul ole ance app oaches The e a e se e al di e en app oaches o in oduce aul ole ance in objec o ien ed sys ems. They can be ca ego ised in he ollowing way: 1. In eg a ion app oach: In his app oach, eplica ion is in eg a ed inside he model. Replica ion is coded inside he ORB so each ORB mus be modi ied in o de o p o ide aul ole ance. Elec a [1], O bix+Isis [2] a e wo models ha a e based on his app oach ( igu e 1). Clien SS’ Replica ion ORB Com Figu e 1. In eg a ion app oach 2. In e cep ion app oach: In his model, e e y message is in e cep ed and edi ec ed o a eplica ion oolki . This new ool is in cha ge o p o iding aul ole ance. The ORB mus be modi ied in oducing he in e cep ion mechanism. E e nal [3] is an example o his app oach ( igu e 2). Replica ion In e cep ion Clien S S’ ORB Com Figu e 2. In e cep ion App oach 3. Se ice app oach: A new eplica ion se ice is added o he ORB. This se ice p o ides mechanisms o objec eplica ion. OGS [4] and he new Co ba Faul Tole ance speci ica ion [5] a e based on his app oach ( igu e 3). Clien S S’ Replica ion ORB Com Figu e 3. Se ice App oach All hese models in oduce new elemen s o p o ide aul ole ance h ough eplica ion. T anspa ency is he mos impo an p ope y achie ed. In his way, p og amme s do no ha e o ake ca e abou eplica ion, and hey do no need o de ine any p o ocol o de elop aul ole ance applica ions because eplica ion is ob ained au oma ically by he model. Howe e , all hese models ha e wo main d awbacks in he ollowing sense: •Close: A o ally anspa en sys em doesn’ allow p og amme s o change eplica ion mechanisms. Replica ion p ope ies can no be es ablished, such as he eplica ion g anula i y, o he momen when eplica ion p o ocols mus be execu ed. These p ope ies a e de ined au oma ically by he model and hey a e he same o e e y sys em. In his way, p og amme s can no ake ad an age om sys em equi emen s. •ORB dependen : Replica ion depends on he ORB implemen a ion. Any eplica ion policy mus be coded in o an indi idual ORB, and i can no be eused in a di e en ORB. The e’s no way o po he same eplica ion policy o o he ORBs. Al hough anspa ency is a good p ope y o be achie ed, i is no always necessa y, mo eo e , some imes i is no desi able. Some imes he na u e o he p oblem may equi e es ablishing he eplica ion p ope ies and beha iou by he p og amme . E en mo e, i equi emen s guide he eplica ion beha iou , he sys em could ake ad an age o hem, and sys em pe o mance could be inc eased. I he eplica ion model is o ally anspa en , he e is no way o de ine aul ole ance applica ions acco ding wi h sys em equi emen s. 3: P oposal The model he e p oposed is based on he pa adigm o Aspec O ien ed P og amming (AOP). Ou esea ch g oup has gained expe ience wi h AOP du ing he las ew yea s wo king wi h he synch oniza ion, coo dina ion and dis ibu ion aspec s [6, 7]. He e we go one s ep u he in oducing he eplica ion aspec as a new non- unc ional p ope y o he objec . Wi h his, sepa a ion anspa ency is g an ed because eplica ion policies can be eused among applica ions wi h no changes. In addi ion, p og amme s can ge con ol o e he eplica ion policy using he speci ic eplica ion language p o ided: JReplica. 3.1: F amewo k The p oposed model keeps aspec s sepa a ed om he unc ional code o he objec . A e lexi e a chi ec u e is used in o de o in oduce wo di e en le els o execu ion: •Func ional Le el: Objec unc ionali y is de ined a his le el. Two new en i ies (in, ou ) ha e been a ached o each objec in o de o communica e objec s wi h i s aspec s. •Aspec Le el: Aspec s a e de ined a his le el. Each objec can be associa ed wi h one o mo e aspec s. The model is show in he igu e 4. Ou In Aspec 1 Aspec 2 Inpu Messages Ou pu messages Objec Figu e 4. The aspec model Each objec is composed o h ee di e en en i ies: •In: This en i y in e cep s all he inpu messages and edi ec s hem o he aspec le el. •Func ional Objec : This is he place whe e he basic beha iou o he objec is implemen ed. •Ou : This en i y in e cep s all he ou pu messages and ans o ms hem in o he igh middlewa e (CORBA o Ja aRMI). 3.2: Replica ion Aspec Acco ding o igu e 4, his wo k ies o ocus on he eplica ion aspec ( igu e 5). Passi e eplica ion is he eplica ion echnique ha has been conside ed. Al hough he e a e o he di e en eplica ion echniques such as ac i e eplica ion, passi e eplica ion allows eplica ing de e minis ic and non-de e minis ic objec s, while ac i e can no . Ou In Replica ion Aspec Inpu Messages Ou pu messages Objec Figu e 5. The Replica ion Aspec Unde his p e ious conside a ion, he eplica ion aspec mus pe o m he ollowing asks: •Decide when o ac i a e he eplica ion mechanisms: I ’s e y impo an o de ine when o send eplica ion messages because hey will a ec he pe o mance o he whole sys em. I hese messages a e sen e y o en, he sys em will collapse. While i no , copies can be inconsis en o a long ime. The e a e wo di e en ways o es ablish his ac i a ion: •Di ec : eplica ion is ac i a ed jus a e a me hod o he objec has been execu ed. •Indi ec : eplica ion is ac i a ed depending on he alue o some condi ions ha mus be checked pe iodically. •S a e Upda e: When he eplica ion mechanisms a e ac i a ed, he eplica ion aspec cap u es he s a e o he objec and sends i o e e y copy. In o de no o b eak objec encapsula ion, ge and se me hods a e called o ob ain and modi y he s a e. •Faul Checking: In o de o es aul s, a pin p o ocol is de eloped. This p o ocol ensu es ha i a aul happens, each copy will be no i ied. •Reco e y: When a aul is ound, ano he p o ocol will selec one o he eplicas o ake he con ol. Figu e 6 shows a ep esen a ion o a aul ole ance sys em using his aspec model. Objec Copy 1 Copy 2 S a e S a e Ge S a e Se S a e Se S a e Replica ion Aspec Replica ion Aspec Replica ion Aspec Figu e 6. Aspec sys em example The main ad an ages o his model a e he ollowings: •Those bene i s de i ed om he use o AOP, mainly modula i y, eusabili y o code and adap abili y o applica ions. •ORB Independence: Replica ion algo i hms a e independen om de ORB. In a p e ious wo k [7] di e en dis ibu ion p o ocols we e de ined as a sepa a ed aspec p o iding a dynamic, adap able and anspa en objec dis ibu ion. Now, as he eplica ion module is de ined ou side he ORB, he combina ion o dis ibu ion and eplica ion aspec s o e he possibili y o eusing he same eplica ion policy in di e en ORBs. •Open: Though eplica ion algo i hms a e hidden and sepa a ed om objec beha iou , eplica ion p ope ies and beha iou can be de ined. Re lec i e mechanisms can communica e he objec le el wi h he eplica ion le el. This communica ion p o ides he way o in oduce new eplica ion ac ions. 3.3: JReplica: Ja a Faul Tole ance Language JReplica is a language wi h he only pu pose o de ining eplica ion policies. I s syn ax is based on Ja a. I in oduces new p imi i es, which a e shown in igu e 7. This Ja a ex ension in oduces wo main elemen s: 1. Replica ion Policy: A new en i y called Disguise Replica ion de ines he eplica ion aspec . This en i y is di ided in o he ollowing pa s: •A ibu es: he in o ma ion ha de ines he eplica ion policy. •S a e: he se o eplica ion s a es. •Ope a ions: me hods ha can manipula e he eplica ion s a e. •Gua d: a condi ion ha mus be ue be o e eplica ion. I his condi ion is alse, eplica ion won’ be execu ed. •Be o e Replica ion: he se o ac ions ha mus be execu ed jus be o e eplica ion. •A e Replica ion: he se o ac ions ha mus be execu ed jus a e he eplica ion is execu ed. •E o : he se o ac ions ha mus be execu ed when a eplica ion e o appea s. 2. Composi ion: A class can be composed wi h di e en aspec s. In ou case, his means ha e e y objec will ex end i s unc ionali y wi h eplica ion mechanisms. 3.4: Rep esen ing Replica ion a Design Le el Replica ion policies now can be de ined wi h he JReplica language. This language helps p og amme s o de ine easily eplica ion p ope ies in objec o ien ed sys ems. Bu we conside ha eplica ion mus be in oduced a ea lie s ages o objec li e cycle, mo e conc e ely a design le el. In his way, UML [8] is used as he modelling language due o i being a s anda d. As UML does no p o ide mechanisms o ep esen eplica ion, i s seman ic has been ex ended in o de o exp ess eplica ion p ope ies and beha iou . Class <name> { ........ } ........................ x=new C1 Compose x wi h R; y=new Replica o x; ........................ Diguise Replica ion <name> { A ibu es: ..... Ope a ions: ..... S a e: ..... Gua d: ..... Be o e Replica ion: .... A e Replica ion: ..... E o : ..... } Figu e 7. JReplica eplica ion p imi i es UML seman ic can be ex ended wi h he in oduc ion o new s e eo ypes. A his poin , we ha e conside ed ha eplica ion policies can be designed sepa a ely and independen ly, in he same way as has been explained a he implemen a ion le el. As such, he aspec concep is in oduced in UML o exp ess he AOP philosophy. The eplica ion aspec is ep esen ed wi h a new s e eo ype, called <Replica ion>. This new s e eo ype is shown in igu e 8. The eplica ion s e eo ype ep esen s a pa icula eplica ion policy. In o ma ion is ep esen ed as ollows: •S e eo ype A ibu es: Rep esen he in o ma ion ha de ines he eplica ion policy. •S e eo ype Me hods: De ine he se o me hods ha can manipula e he eplica ion s a e. < Replica ion > Name A ibu es Me hods Class A ibu es Me hods <Replica ed> Figu e 8. UML ex ension As i can be shown, he e a e o he elemen s ha can no be ep esen ed in his s e eo ype. The dynamic beha iou o eplica ion can no be ep esen ed in a no mal class diag am. S a echa diag ams ep esen dynamic beha iou . So he solu ion goes by a aching a s a echa diag am o his eplica ion s e eo ype. In his way, eplica ion s a ic p ope ies and dynamic beha iou can be designed. The dynamic beha iou o eplica ion policies can be ep esen ed in a s a echa diag am as i is shown in igu e 9. S a e 1 Replica ion [Replica ion Gua d] En y : Ac ions be o e eplica ion Exi : Ac ions a e eplica ion Figu e 9. S a echa Rep esen a ion The elemen s ha a e ep esen ed in his s a echa diag am a e: •S a e: Each eplica ion s a e is ep esen ed by an s a e. The e is a special s a e called Replica ion ha ep esen s he momen when eplica ion is o be execu ed. •Gua d: Gua ds a e ep esen ed in he ansi ion o each s a e. •Be o e Replica ion: The se o ac ions ha is execu ed jus be o e he eplica ion begins is ep esen ed in he en y ac ions o he Replica ion s a e. •A e Replica ion: The se o ac ions ha a e execu ed jus a e he eplica ion ends a e ep esen ed in he exi ac ions o he Replica ion s a e. •E o : Replica ion e o s a e ep esen ed as a new s a e. A ool is being de eloped in o de o gene a e JReplica code s a ing om his ex ension o UML. In his way Replica ion aspec has been in oduced om design o implemen a ion le el. This ool is based on o he one we ha e de eloped o he synch onisa ion aspec [9]. 4: Rela ed wo ks The e a e se e al models ha p o ide eplica ion mechanism o achie e aul ole ance. In [10], a new in e cep ion mechanism called A oma is in oduced in he Ja a RMI a chi ec u e. O he models a e based on he in oduc ion o sepa a ed en i ies ha implemen eplica ion p o ocols. The Cadmium Model [11] de ines a couple o new en i ies called S ub and Scion, which a e a ached o a clien and a se e espec i ely and o e eplica ion mechanisms. In [12] a new eplica ion en i y and a consis ency manage a e in oduced, bo h sepa a ed om he objec . In Aspec IX [13] a single objec is di ided in o agmen s, all o which ha e a di e en pu pose. One o hese agmen s o e s eplica ion acili ies. The GARF [14] model de ines wo di e en en i ies in o de o in oduce eplica ion, hey a e called encapsula o and maile . A wo le el e lec i e a chi ec u e was de ined o Ja a in [15]; objec unc ionali y is de ined in he i s le el, while eplica ion p o ocols a e es ablished in he second one. All hese models only ake in o accoun he implemen a ion le el, hey a e ocused on eplica ion p o ocols and he de ini ion o a amewo k ha p o ides aul ole ance, igno ing he design phase. A new pa e n [16] has been de ined in o de o p o ide suppo o he ep esen a ion o eplica ed objec s. Mo eo e , a new language ha helps p og amme s o build aul ole ance sys ems has been de ined in [17, 18]. This p oposal is based on he concep o sepa a ion o conce ns and ex ends AspecJ language [19] wi h eplica ion p imi i es. I is possible o de ine he a ibu es ha need eplica ion and wha o do when a eplica ion e o happens. Bu he e is no way o exp ess new eplica ion ac ions o when eplica ion mus be execu ed. Al hough hese models help p og amme s o implemen aul ole ance sys ems, i is necessa y o in oduce mechanisms ha help so wa e enginee s o design his kind o equi emen s. 5: Fu u e wo ks Fu u e wo ks will conside ex ensions o JReplica in o de o exp ess mo e complex eplica ion mechanisms. The cu en e sion showed us he sui abili y o he model. Re e ences [1] S.Ma eis. Run-Time suppo o objec -o ien ed dis ibu ed p og amming. Phd Thesis, Uni e si y o Zu ich, 1995. [2] IONA and Isis. An In oduc ion o O bix+Isis. IONA Technologies L d. and Isis Dis ibu ed Sys ems, Inc., 1994. [3] L.E.Mose , P.M. Melia -Smi h and P. Na asimhan. Consis en objec eplica ion in he E e nal sys em. Theo y and P ac ice o Objec Sys ems, 81-92, 1998. [4] Pascal Felbe . The CORBA Objec G oup Se ice. A Se ice app oach o objec g oups in CORBA. Phd Thesis 1998. Uni e si y o Lausanne. [5] OMG TC documen p c/2000-03-04. Faul Tole an CORBA. D a Adop ed Speci ica ion. 2000. [6] J.M. Mu illo, J. He nández, F. Sánchez, L.A. Ál a ez. Coo dina ed Roles: P omo ing Reusabili y o Coo dina ed Ac i e Objec s Using E en s No i ica ion P o ocols. In Coo dina ion Languages and Models. Sp inge -Ve lag, LNCS 1594, Ap il, 1999. [7] F. Sánchez, J.He nández, J.M.Mu illo, J.L.He e o, R.Rod íguez. Adap abili y o Objec Dis ibu ion P o ocols Using he Disguises Model App oach. 2nd In l. Symposium, Dis ibu ed Objec s & Applica ions (DOA 2000). [8] Objec Managemen G oup. Uni ied Modeling Language, e sion 1.3. [9] J.L.He e o. In oducing sepa a ion o conce ns a design ime. PhDOOS Wo kshop, Eu opean Con e ence on Objec - O ien ed P og amming (ECOOP’2000). [10] N. Na asimhan, L.E. Mose and P. M. Mellia -Smi h. T anspa en Consis en Replica ion o Ja a RMI Objec s.2nd In l. Symposium, Dis ibu ed Objec s & Applica ions (DOA 2000). [11] Aline Baggio. Adap able and Mobile-Awa e Dis ibu ed Objec s. PhD Thesis, Uni e si é Pie e e Ma ie Cu ie and INRIA, Pa is, F ance, June 1999. [12] Geo ges B un-Co an and Mesaac Makpangou. 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ECOOP Wo kshops 1998: 424-425. [18] Johan Fab y. A F amewo k o eplica ion o objec s using Aspec -O ien ed P og amming. Phd Thesis 1998. Uni e si y o B ussel. [19] C.V. Lopes. D: A Language F amewo k o Dis ibu ed P og amming. Phd Thesis 1997. Uni e si y o No heas e n.