Design and Implementation of a Concurrency Control Mechanism in an Object-Oriented Database System

Transcription

1 Vol.ll No.4 J. of Comput. Sci. & Techol. Jul 1996 Desig ad Implemetatio of a Cocurrec Cotrol Mechaism i a Object-Orieted Database Sstem Qu Yuao (~~), Tia Zegpig (~il~), Wag Yuju (t-~n) ad Shi Baile (N ~ ~) Departmet of Computer Sciece, Fuda Uiversit, Shaghai Received Ma 10, 1995; revised Jauar 24, Abstract This paper presets a practical cocurrec cotrol mechaism ~ Object- Lockig i OODBMS. Object-Lockig ca schedule trasactios, each of them ca be cosidered as a sequece of high level operatios defied o classes. B the properties of parallelit ad commutativit betwee high level operatios, proper lock modes for each operatio axe desiged ad the compatibilit matrix is costructed. With these lock modes, phatoms are kept awa from databases ad a high degree of cocurrec is achieved. Kewords: Cocurrec cotrol, object-orieted database, trasactio maagemet. Itroductio Recet ears have see dramatic research ad developmet progress i the area of OODB (e.g., Orio, 02, Iris, Gemstoe, etc.[]'2]). Much effort has bee devoted to models, laguages, architecture, ad data maagemet [3], but few publishmet have appeared o desigig cocurrec cotrol mechaisms. For the ext geeratio iformatio sstem, cocurrec cotrol mechaisms are required to hadle High level operatios o complex objects ad to meet high throughput demads. For example, ma applicatios such as Office Iformatio Sstem, CAD/CAM, Software Egieerig, etc., require high performace ad support of complex iformatio. Traditioal cocurrec cotrol mechaisms i DBMS where the operatios are just Read ad Write, do ot support these applicatios well. Sematics-based cocurrec cotrol techiques have bee widel studied [4-7]. It could exploit richer sematic iformatio from objects ad high level operatios to get a higher degree of cocurrec. Hece, ma researchers believe that this kid of cocurrec cotrol is best suited for the trasactio maagemet of OODB sstems [3-5'7-9,]. Nevertheless a umber of sematics-based cocurrec cotrol schemes proposed for object-orieted sstems cosidered ol some special Abstract Data Tpes, e.g., Stack ad Queue [4,5'7]. Noe of them deal with a practical sstem. I this paper, we demostrate the desig ad implemetatio of a sematicsbased cocurrec cotrol mechaism i a OODB sstem ~ FOODB. FOODB is

2 338 J. of Comput. Sci. & Techol. Vol.11 a OODBMS, which was developed b the Computer Sciece Departmet of Fuda Uiversit. The cocurrec cotrol mechaism is desiged to schedule trasactios composed of high level operatios. There are three kids of operatios i FOODB: Schema operatios, Statistical operatios, ad Istace operatios. Schema operatios iclude querig or updatig the structure of a class. Statistical operatios iclude querig or updatig a set of objects (istaces) satisfig some coditios. Istace operatios iclude fidig, updatig or deletig a istace from a class, or isertig a istace to a class. 2 The FOODB Sstem 2.1 Architecture FOODB is a object-orieted database sstem. It has a cliet/server architecture. Database is stored o the server. All database operatios of the cliets are delivered to the server, where those operatios are executed. The uit of data trasfer betwee cliet ad server is object. The trasactio maager is resposible for cocurrec cotrol ad recover. The object maager is resposible for maagig objects i buffer, swappig objects betwee disks ad memor, ad packig or upackig objects for trasfer. Trasferrig objects betwee cliet ad server is the task of the commuicatio sstem. GUI(Graphical User Iterface) icludes a iteractive iterface ad a programmig iterface ad a set of sstem maiteace tools. Users ma use Object SQL (OSQL) or Exteded C++ to quer or update database i iteractive or programmig eviromet respectivel. 2.2 Data Model FOODB supports fudametal object-orieted cocepts: class, attribute, method, object, object idetit (OID), ecapsulatio, iheritace(sigle or multiple iheritace), polmorphism, etc. Complex objects are costructed through OID refereces betwee objects. Three tpes of refereces are used: (1) shared referece: the object ma be refereced b several objects; (2) exclusive referece: the object ma be refereced b ol oe object; (3) depedet referece: the referece is exclusive ad if X refereces Y the the existece of object Y depeds o the existece of object X. I FOODB, we also regard a class as a object. Schema operatios ad statistical operatios are set to this kid of objects. Operatios i the sstem are classified as follows. (1) Schema operatios 9 Schema quer: quer the structure of a class; 9 Schema evolutio: modif the structure of a class, e.g., add or drop a class, chage the superclass/subclass relatioship betwee a pair of classes, etc. We stipulate that the structure of a superclass caot be modified i its subclasses.

3 No.4 Cocurrec Cotrol Mechaism i a O-O Database Sstem 339 (2) Statistical operatios Statistical operatios operate o a set of istaces satisfig some give coditios. For example, selectig emploees (from a class) aged over 60 is a statistical operatio. (3) Istace operatios Istace operatios iclude: isert a object to a class, delete a object from a class, quer or update a object i a class. 2.3 Trasactio Model A trasactio is a sequece of database operatios. Users iteract with database b ivokig trasactios. A trasactio ca access a object or modif the state of a object ol b ivokig operatios (methods) defied for that object. Operatios scheduled b the trasactio maager are ot ol simple read or write, but also various high level operatios of classes. Users' trasactios i a sstem axe a successio of method ivocatios. Begi Oidl.M1... Oidi.Mi... Oid.M~ Ed Oidi: object idetifier, Mi: ame of a method 3 Commutativit ad Parallelit Parallelism allowed b a cocurrec cotrol mechaism depeds to a large extet o the properties of parallelit ad commutativit betwee the operatios defied o objects. Let X be a object that is usable through operatios Oi ad Oj. The pair of operatios (O~, O) has the propert of Parallellt if whatever is the iitim state of object X, for the cocurret executios of the two operatios b two differet trasactios, the effects o object X ad the trasactios are the same. I other words, two parallel operatios ma be performed cocurretl without havig to be cotrolled. For example, two quer operatios are parallel. Whe two operatios axe ot parallel, Commutativit ma be used, which is a weaker propert. The pair of operatios (Oi, Oj) has the propert of Commutativit if, whatever is the state of object X, executio of operatio O~ b trasactio Ti followed b executio of operatio Oj b trasactio Tj has the same fial effects o object X ad the same results for the trasactios as the executio i the reverse order (Oj followed b Oi). Let S be a database state, O be a operatio. State (O, S) is a state that is produced b the executio O o state S. Retur (O, S) is the retur value of operatio O executed o state S. We assume that ever operatio will retur a value, at least a status or coditio code (e.g. read operatio ma retur a value read b the operatio, write operatio ma retur a status idicatig success or failure of its executio). The above operatio O ma also be a sequece of operatios. Thus,

4 340 J. of Comput. Sci. & Techol. Vol.ll i this case, state (O, S) is the state produced after the executio of the operatios i O;.retur (O, S) is the retur value set of operatios i O executed o S. Let 01//02 mea that operatios O1 ad 02 execute i parallel. O102 meas that operatio 02 is executed followig the executio of O1. Defiitio 1. Two operatios 01 ad 02 are parallel if 01 ad 02 ca be executed cocurretl without a cotrol o the order of executio of their lower level operatios, ad/or all states S, state(01//02, S)=state(0102,S), state(01// 02,S)=state(0201,S), retur(01////02,s)=retur(ol02, S), retur(o1//// O2,S)= retur(0201,s). Defiitio 2. Two operatios O1 ad 02 are commutative if, for all states S, state(olo2,s)=state(0201,s), retur(0102, S)=retur(0201, S). It is obvious that if two operatios are parallel the the are commutative. I this paper, for simplicit, we schedule parallel operatios ad commutative operatios i the same maer (i fact, we ca exploit higher cocurrec from parallelit tha from commutativit). We call two operatios to be Coditioall Commutative, if the must satisf some coditios to commute. For example, for database state S = {x = 1,.-.}, there is a itegrit costrait: x > 0. Two operatios, O1: x = x+c, 02 : x = x-d. O1 ad 02 are commutative if the coditio 1 > d is true, otherwise the do ot commute. B coditioal commutativit, the cocurrec cotrol mechaism ca exploit more sematic iformatio for cocurrec. I OODBS, there are ma such sematic data i operatios ad objects [4'51. Therefore, how to exploit them is importat for the efficiec of a sstem. Defiitio 3. Two operatios are coflict iff the are ot commutative. 4 Lockig Techiques 4.1 Class Hierarch Lockig From the operatioal sematics i OODB, we kow that if a operatio operates o class C, the it ma also operate o the subclasses of C. The itetio lock mode ad simple graularit lockig protocol are satisfactor, if ol sigle iheritace relatioships are permitted. But this does ot work for a class hierarch i which a class ma have more tha oe superclass. Fig.1 is a multiple iheritace class hierarch. Class C has two superclasses C1, C2, ad oe subclass S C. C1 ad C2 have superclasses SC1 ad SC2 respectivel. Suppose a trasactio T1 sets a readlock o C1 ad a itetio lock o SC1, implicitl lockig subclasses C ad SC i readlock mode. Now aother trasactio T2 sets a writelock o C2 ad a itetio lock o SC2. T2 will also lock class C ad SC i writelock mode implicitl. The coflict o class C ad SC betwee T1 ad T2 is ot detected. This meas that settig itetio lock o the superclasses of the class beig locked is ot sufficiet to detect all coflictig requests from differet trasactios. To solve the above problems ad make lockig mechaisms more efficiet, for ever class C, we maitai a list L that records all C's subclasses that have more tha oe superclass.

5 No.4 Cocurrec Cotrol Mechaism i a 0-O Database Sstem 341 We ow give the class hierarch lockig protocol. (1) Lock class C. (2) Lock those of C's superclasses alog a oe superclass chai i itetio lock mode. (3) Fid the set S~ of C's subclasses with more tha oe superclass through L, for ever e 6 Si, lock e. Fig.1 Fig Complex Object Lockig I OODB, the uit of access to a database is a object, that is, the smallest uits of lockig are objects. Objects are usuall complex, i.e. the ca be costructed from other objects. Moreover, these objects ma also be shared b several differet objects. So, object lockig is a complicated task. The geeral techiques of complex object lockig have bee studied i [7,10]. I this paper, i order to exploit structural sematics of objects, we use referetial sematics to improve cocurrec i additio to usig commutativit propert betwee operatios. Now let's see the impacts of complex object lockig. X, Y, ad Z are objects costructed from subobjects x,, z as show i Fig.2. Object is shared b X, Y, Z. Trasactios are TI: O1(X)O2(Z) Is: O3(V) Suppose oe schedule S for TI ad T2 is S: OI(X)O3(Y)O2(Z) If we ol lock X, Y, ad Z, but do ot lock their subobjects x,, z, the it seems that T1 ad T2 do ot coflict, so, S is permitted. I fact, the high level operatios 01(X), O2(Z), ad O3(Y) must be coverted ito suboperatios o x, ad z: o (x) o2(z) o2()o (z) o3(v) o3() So, the schedule S becomes S t. s': If both operatio pairs (01(),o3()) ad (03(),o2()) do ot commute, the S' is ot serializable. Hece S is ot correct.

6 342 J. of Comput. Sci. & Techol. Vol.11 The problem here is that the coflicts at subobjects are igored! So, whe lockig complex objects, ot ol should we cosider the objects, but also their subobjects. If, i the above example, the referetial tpe of Y to is exclusive, that is, caot be refereced b X ad Z, the S must be serializable. I order to lock complex objects correctl ad efficietl, we ma use the referetial sematics i the lockig protocol. We stipulate that if object X refereces object Y exclusivel, the, for ever object Z refereced b Y, the referetial tpe of Y to Z is also exclusive. Now we give the complex object lockig protocol. (1) Lock object X. (2) If X refereces object Y i shared tpe, the lock Y. B this complex object lockig protocol, it is uecessar to lock the subobjects that are refereced exclusivel. Operatios ad Lock Modes Operatios i classes are the uits of schedulig b cocurrec cotrol mechaism i FOODB. For ever operatio, there is a correspodig lock mode. Whe a operatio begis to operate o object X, the cocurrec cotrol mechaism should lock X i its correspodig lock mode before it is executed. All operatios i FOODB are classified as three tpes. 1) Schema operatio This tpe of operatios icludes querig or updatig class hierarch. (1) quer the structure of a class (2) create or drop a class (3) add (drop) a attribute to (from) a class (4) add (drop) a iheritace relatioship to (from) a class (5) add (drop) a method to (from) a class 2) Statistical operatio This tpe of operatios icludes querig or updatig a set of istaces that satisf some give coditios. (1) quer a set of istaces that satisf a give coditio (2) update a set of istaces that satisf a give coditio (3) delete a set of istaces that satisf a give coditio 3) Istace operatio (1) fid a istace from a class (2) update a istace i a class (3) isert a istace to a class (4) delete a istace from a class The lock modes are desiged as follows. 1) Class lock (1) c-qh class quer lock (2) c-uh class update lock 2) Istace lock (1) i-sl: istace set quer lock (2) i-ql(ob): istace quer lock

7 No.4 Cocurrec Cotrol Mechaism i a O-O Database Sstem 343 (3) i-ul(ob): istace update lock (4) i-il(ob): istace isert lock (5) i-dl(ob): istace delete lock The i-ql, i-ul, i-il ad i-dl have a parameter ob idicatig the Oid of the object beig locked. Lock Mode c-ql c-ul i-sl i-ql(ob) i-ul(ob) i-il(ob) i-dl(ob) Operatios schema operatio(l) schema operatio(2)--(5), statistical operatio(2)--(3) statistical operatio(l) istace operatio(l) istace operatio(2) istace operatio(3) istace operatio(4) Fig.3 c-ql c-ul i-sl "i-ql(ob) i-ul(ob) i-il(ob) i-dl(ob) c-ql c-ul i-sl ob=obq i-ql(ob') i-ul(ob') i-il(ob') i-dl(ob') Y Y Y Y c c c C C C C c c c C C C C c="", ob # ob': c--", Fig.4 Ever operatio has a correspodig lock mode. The operatios ad their correspodig lock modes are described i Fig.3. I order to reduce the umber of locks set o objects to oe at a time, statistical operatios (2) ad (3) use the same lock mode as schema evolutio operatios. To get a higher degree of cocurrec, we desig a i-sl lock mode for statistical operatio (1). B the sematics of operatios ad commutativit betwee them, we summarize the compatibilit matrix of lock modes as Fig.4. If two lock modes are compatible, the their correspodig operatios ca be executed cocurretl. The "d ~ i matrix meas that two lock modes are coditioall compatible, that is, their correspodig operatios are coditioall commutative. For example, if two deletio operatios delete two differet objects the the ca be executed cocurretl. If a object, which is iserted or deleted b oe trasactio T1, ma also be operated b aother trasactio T2, the we sa that the object is a Phatom for T2, otherwise it is ot a phatom object for T2. I Orio, phatom problems

8 344 J. of Comput. Sci. & Techol. Vol.ll were ot cosidered, whereas i [3], i order to avoid phatoms, isert or delete operatio coflicted with all operatios. B our scheme, phatoms are avoided, ad the operatio pairs (isert, isert), (isert, delete) ad (delete, delete) ca be executed i parallel or coditioall parallel. 6 Desig ad Implemetatio of Cocurrec Cotrol Mechaism -- Object-Lockig I this sectio, we desig ad implemet the cocurrec cotrol mechaism -- Object-Lockig o the basis of above discussios. Fig.5 is the descriptio of Object-Lockig. It is a two-phase lockig procedure, that is, oce a trasactio releases a lock, it ever obtais locks agai. Ol whe trasactio's commit or abort operatios are received, ca the trasactio's locks be released. Sice deadlock detectig procedure is used, Object-Lockig is deadlockfree. Object-Lockig Iput: trasactio's operatio Oi Output: decisio of whether Oi ca be executed or ot Method: Do Case Case Oi # commit ad Oi #abort lock the object i a proper lock mode depedig o the tpe of O/ if lockig succeeds the retur(execute O/) else begi put Oi i wait-queue call WFG-adjust/*adjust WFG */ call deadlock-detect if there is a circle i WFG the begi call trasactio abort(ti) call WFG-adjust awake some trasactios i wait-queue retur ed ed susped Ti util T/is awaked Case Oi=abort call trasactio abort(ti) call WFG-adjust awake some trasactios i wait queue Case Oi =coxmit call buffer-write(t/) call WFG-adjust call lock-release(t/) awake some trasactios i wait queue EdCase Fig.5

9 No.4 Cocurrec Cotrol Mechaism i a O-O Database Sstem 345 A operatio Oi o a give object coflicts if it is ot commutative with other operatios executed o this object b still active trasactios, i.e. those trasactios that have ot bee committed or aborted. Whe a operatio coflicts, the trasactio requests that the operatio be blocked, ad deadlock detectio eeds to be iitiated. The process of checkig for deadlock is achieved b usig a walt-for-graph. Whe a trasactio issues a request to execute a operatio, the cocurrec cotrol mechaism, b usig the compatibilit matrix ad lockig process, determies whether the operatio request coflicts or ot. If the request coflicts, the trasactio is made to wait. The correspodig wait-for edges are added ad a circle detectio algorithm is iitiated. If a circle is foud, the trasactio makig the request is aborted. Whe a trasactio termiates successfull or usuccessfull, the ode that correspods to the termiatig trasactio together with the edges associated with the ode is removed from the WFG. The results of the trasactio are writte back to database or discarded simpl. All locks used b the trasactio are released. 7 Discussio (1) High level operatios as schedulig uits More ad more researchers believe that high level operatios as schedulig uits of cocurrec cotrol are preferable i OODB or other egieerig eviromets [4-s]. Because rich sematics of the operatios ca be exploited to improve cocurrec, the cocurrec cotrol mechaism ca be made more efficiet. I this paper, there are three kids of operatios: schema operatios, statistical operatios, ad istace operatios. Usig the sematics of these operatios, we ca schedule them i a more appropriate wa. For example, two isert operatios ca alwas be executed cocurretl, deletio ad isertio operatios ca be executed coditioall cocurretl, etc. Moreover, the phatoms ca be described clearl ad avoided ratioall, whereas the simple lower level operatio-based (read, write) trasactio model caot achieve this so clearl ad coveietl. (2) Recover The high level operatio- based trasactio model ecessitates further researches i recover methods[s,9]. Covetioal recover techiques based o log, for example, are ot adequate for the ew trasactio model. Whe simpl istallig the database, the "before image" of update doe b aborted trasactio is ot correct sometimes. I [8], the authors defied the otio of "strictess" for histories cotaiig high level operatios, ad gave a recover rule b simpl executig their "iverse" operatios. I [9], the authors developed a uified trasactio model icludig both data operatios ad termiatio operatios (commit, abort), which allows reasoig about the correctess of cocurrec cotrol ad recover withi the same framework. The future importat work is how to embed these techiques to real OODB sstems.

10 346 J. of Comput. Sci. & Techol. Vol.ll 8 Coclusio We have developed a practical cocurrec cotrol mechaism i OODBMS ad give the trasactio model based o three kids of high level operatios: schema operatios, statistical operatios, istace operatios. B the properties of parallelit ad commutativit betwee these operatios, the proper lock mode for each operatio has bee desiged ad the compatibilit matrix is costructed. With these lock modes, phatoms are preveted ad a high degree of cocurrec is achieved. Refereces [1] Berstei P A, Hazilacos V, Goodma N. Cocurrec Cotrol ad Recover i Database Sstem. Addiso-Wesle, Readig, MA, [2] Wo Kim. Itroductio to Object-Orieted Database. MIT Press, Cambridge, [3] Cart M, Feeri J. Itegratig Cocurrec Cotrol. I Buildig a Object-Orieted Database/The Stor of OP, Bacilho F, Claude Delobel, Paris Kaellakis, (eds.), Morga Kaufla Publishers, Sa Mateo, Califoria, [4] Paos K Chrsathis et al. Extractig cocurrec from objects: A methodolog. ACM. PODS [5] Badriath B, Ramamritham K. Sematics-based cocurrec cotrol: Beod commutativit. ACM PODS, [6] Badriath B, Ramamritham K. Performace evaluatio of sematics-based multilevel cocurrec cotrol protocols. I Proc. of ACM SIGMOD, [7] Weihl W. Commutativit-based cocurrec cotrol for abstract data tpes. IEEE Tras. o Computers, Dec [8] Rajeev Rastogi, Korth H F, Avi Silberschatz. Strict histories i object-based database sstem. ACM PODS, [9] Has-Jorg Schek, Gerhard Weikum, Haia Ye. Towards a uified theor of cocurrec cotrol ad recover. ACM PODS, [10] Jorge F Garza, Wo Kim. Trasactio maagemet i object orieted database sstem. I Proc. of ACM SIGMOD, Qu Yuao is a Associate Professor i Departmet of Computer Sciece, Fuda Uiversit. His curret research iterests iclude trasactio processig ad distributed Database. Tia Zegpig is a Ph.D. cadidate i Departmet of Computer Sciece, Fuda Uiversit. His curret research iterests iclude i Multimedium Database. Wag Yuju is a Ph.D. cadidate i Departmet of Computer Sciece, Fuda Uiversit. His curret research iterests are data model ad quer laguage. Shi Baile is a Professor i Departmet of Computer Sciece, Fuda Uiversit. His curret research iterests iclude database theor ad applicatio, software egieerig.