Database replication protocols based on group communication primitives have recently been the subject of a considerable body of research [1, 11, 13, 6, 8, 4]. The reason for this stems from the adequacy of the order and atomicity properties of group communication primitives to implement synchronous replication (i.e., strong consistent) strategies. Unlike database replication schemes based on traditional transactional

Moura, Francisco Coelho Soares

Oliveira, Rui Carlos Mendes de

Pereira, José

Sousa, António

Universidade do Minho: RepositoriUM

Semantic Reliability on the Database State Machine(Extended Abstract)  António SOUSA José PEREIRA Rui OLIVEIRA Francisco MOURA{als,jop,rco,fsm}@di.uminho.ptDepartamento de InformáticaUniversidade do MinhoBraga, Portugal1 IntroductionDatabase replication protocols based on group communication primitives have recently been the subject ofa considerable body of research [1, 11, 13, 6, 8, 4]. The reason for this stems from the adequacy of theorder and atomicity properties of group communication primitives to implement synchronous replication(i.e., strong consistent) strategies. Unlike database replication schemes based on traditional transactionalmechanisms, group-based replication mechanisms use atomic broadcast primitives to broadcast transac-tions to all replicas of the database. The approach allows the delegation of much of the synchronizationcomplexity to the group communication layer and can accommodate different replication strategies.We have been working on extending the Database State Machine protocol (DBSM) [8] to large-scalenetworks. In [12] we extended the protocol to handle partial replication and to use an optimistic atomicbroadcast protocol in order to mitigate the higher communication latency of large-scale settings.Optimistic atomic broadcast exposes preliminary message delivery orders to the application allowingit to tentatively antecipate processing before it is provided with a final and definitive order — an idea thatgeneralizes the broadcast protocol presented in [7]. In the DBSM, the optimistic atomic broadcast is mostuseful during low workload periods where processor idle time is used to obtain faster transaction executiontimes. During heavy workloads however it is of little help as the processor has no idle periods.To improve DBSM responsiveness during high workloads this paper proposes the use of a semanticallyreliable atomic broadcast protocol [9] in DBSM.2 Database State MachineIn this section we briefly describe the Database State Machine protocol. For a full description of theprotocol refer to [8].2.1 System ModelWe consider a system	of database sites. Sites communicate through message passing(i.e., no shared memory). The system is asynchronous in that there are no assumptions about the time ittakes for a site to execute a step nor the time it takes for messages to be transmitted.Sites may only fail by crashing (i.e., we do not consider Byzantine failures), and we do not rely on siterecovery for correctness. A site that never fails is said to be correct. We assume that our asynchronousmodel is augmented with a Failure Detector Oracle [3] so that Atomic Broadcast can be solved.A database is a finite set of data items and each database siteholds a copy of thedatabase.Research supported by FCT, ESCADA project (POSI/33792/CHS/2000).1A transaction is a sequence of read and write operations followed by a commit or abort operation, issuedby a client on behalf of the transaction. Every transaction belongs to the set  of all possible transactions.For each transaction ﬀﬂﬁﬃ ,  ﬀ"!#%$ﬀ& is defined as the set of data items read or written by ﬀ . '($ﬀ& denotesthe set of data items read by ﬀ and )*$ﬀ& the set of data items written by ﬀ .2.2 ProtocolA transaction ﬀ is entirely executed in some database siteﬁ. From the time ﬀ starts until it finishes, atransaction passes through three well-defined states. The starting state is the executing state, a state whereall operations are executed locally at the database site	. When the client that initiates the transactionrequests its commitment, transaction ﬀ passes to the committing state. At this point, transaction ﬀ , is sentto all database sites. A transaction received by a database site+is in the committing state until its fate isknown. The transaction then evolves to one of its final states committed or aborted.Using the deferred update replication technique, a transaction ﬀ is locally synchronized during its exe-cution at the database where it initiated according to some concurrency control mechanism [2] (e.g., two-phase locking). Interaction with other database sites on behalf of ﬀ only occurs when the client requests thetransaction commitment. At this time, ﬀ is propagated, i.e., the updates of transaction ﬀ and some controlstructures are propagated to all database sites using an atomic broadcast protocol. Each database site willthen certify and, if possible, commit the transaction.In order for a database site to certify a committing transaction ﬀ , it must be able to determine whichtransactions conflict with ﬀ . A transaction ﬀ-, conflicts with ﬀ if: (i) ﬀ and ﬀ, have conflicting operations and(ii) ﬀ,does not precede ﬀ .Two operations conflict when they are issued by different transactions, access the same data item andat least one of them is a write operation. The precedence relation between transactions ﬀ and ﬀ,is denotedﬀ,/.ﬀ (i.e., ﬀ,precedes ﬀ ) and defined as: (1) if ﬀ and ﬀ,execute at the same database site, ﬀ,precedes ﬀif ﬀ,enters the committing state before ﬀ ; or (2) if ﬀ and ﬀ,execute at different sites, for exampleand0+respectively, respectively, ﬀ,precedes ﬀ if ﬀ,commits atbefore ﬀ enters the committing state at.After receiving a transaction ﬀ delivered by the atomic broadcast protocol, the certification process isstarted. The certification of a transaction involves a deterministic certification test which when terminatedallows the database site executing the certification to immediately commit or abort the transaction.The certification test of ﬀ at database siteinvolves every data item accessed by ﬀ . Database sitecommits ﬀ if all committed transactions atprecede ﬀ , or if there is no committed transaction ﬀ,atthatconflicts with ﬀ ;aborts ﬀ otherwise. The certification test of siteon transaction ﬀ is formally describedas a transition from the committing to the committed state as follows:132#4#657ﬀ8ﬀ8579;:$ﬀ&=<132#6#457ﬀ8ﬀ"!>$ﬀ?&A@BCEDﬀ,F132#6#457ﬀ8ﬀ"!>$ﬀ,?&HGﬀ,.ﬀ;I$)($ﬀ,&KJ6'($ﬀ&ML&NO3 Semantically Reliable Multicast in the Database State MachineIn DBSM the certification test is assumed to be deterministic and thus, when a database site aborts atransaction then all database sites also abort it. Moreover, when the result of certifying a transaction ﬀ isabort then all the information received with ﬀ may be discarded. This means that if a site knows inantecipation that some transaction ﬀ will abort, thencan simply discard ﬀ and save itself from processingthe certification of ﬀ . Being able to notify in antecipation other database sites which transactions will abortcan be most valuable for overloaded database sites as they can simply discard those transactions.To implement this optimization efficiently we use a semantically reliable atomic broadcast protocolto dissiminate commiting transactions. Semantic reliability [9] is a correctness criterium for broadcastprotocols based on the concept of message obsolescence. A message becomes obsolete and can be readilydiscarded by the protocol whenever its purpose is superseded by a subsequent message. This informationis expressed in the protocol obsolescence relation encoded by the application.2In the DBSM the obsolescence relation is given by the knowledge a database site has about the trans-actions that aborted. That is, whenever a site broadcasts a message # to propagate a commiting transac-tion, encodes in # that # obsoletes all previous messages propagating commiting transactions that haveaborted at . With this information encoded in # , the broadcast protocol might be able to prevent thosemessages carrying transactions that will not pass the certification test from overloading some database sites.The obsolescence relation is a strict partial order and the fact that a message # is obsoleted by amessage #,is expressed as #QPR#,. A semantically reliable multicast protocol [10] is an algorithmfulfilling the following properties:Validity If a correct database site multicasts a message # and there is a time after which no database sitemulticast some #, ,such that #SPT#, ,, then it delivers some #,such that #SUV#,.Agreement If a correct database site delivers a message # and there is a time after which no database sitemulticasts # , , such that #RPW# , , , then all correct database sites deliver some # , such that #SUW# ,Integrity For every message # , every database site delivers # at most once and only if # was previouslymulticast by some database site.FIFO Order If a database site multicasts a message # before it multicasts a message #,, no database sitedelivers # after delivering #,.FIFO Completeness If a database site multicasts a message # before it multicasts a message #,and thereis a time after which no database site multicasts #X, , , such that #YPZ#ﬃ, , , , no correct database sitedelivers #,without delivering some #, ,such that #RPV#, ,.The DBSM correctness criterium relies on an atomic multicast protocol to ensure that all database sitesprocess the same set of transactions in the same order. An atomic multicast protocol is a reliable multicastprotocol satisfying a total order requirement [5]. Informally, an atomic multicast protocol must ensure thatif a database site delivers two messages # and #,in that order then no other database site delivers themdifferently. This is described by the following total order property:Total Order if correct database sitesand+both deliver messages # and #,, thendelivers # before#,if and only if+delivers # before #,.4 Conclusions and Future WorkIn [12] we extended the DBSM to support partial replication and the use of an optimistic atomic broadcastto improve performance mainly during low/moderate load periods.This paper describes ongoing research on improving DBSM performance during heavy load periods,allowing better responsiveness during those periods. This seems a promising research direction as DBSMpresents the required pattern for benefiting from semantically reliable protocols [9].Currently our obsolescence relation is based on aborted transactions. We believe that with resultsderived from the experiences we are conducting with this approach along with further analysis of the trans-actions we could enrich our obsolescence relation making it more effective. By reducing the processingrequirements at each database site, we might improve responsiveness to the client even in normal work-loads.In this paper we have focused in high workloads and dropped the use of an optimistic broadcast pro-tocol. We are studying how to combine semantic and optimistic protocols in order to improve DBSMperformance in all possible workloads.References[1] Y. Amir, D. Dolev, P. Melliar-Smith, and L. Moser. Robust and efficient replication using groupcommunication. Technical Report CS94-20, The Hebrew University of Jerusalem, November 1994.3[2] P. A. Bernstein, V. Hadzilacos, and N. Goodman. Concurrency control and recovery in DatabaseSystems. Addison-Wesley, 1987.[3] T. D. Chandra and S. Toueg. Unreliable failure detectors for reliable distributed systems. Journal ofthe ACM, 43(2), March 1996.[4] U. Fritzke and P. Ingels. Systéme transactionnel pour donnés partiellement dupliqués, fondé sur lacommunication de groupes. Technical Report 1322, INRISA, Rennes, France, April 2000.[5] V. Hadzilacos and S. Toueg. Fault-tolerant broadcasts and related problems. Technical Report 94-1425, Department of Computer Science, Cornell University, May 1994.[6] B. Kemme and G. Alonso. A suite of database replication protocols based on communication primi-tives. In Proceedings of the 18th International Conference on Distributed Computing Systems, Ams-terdam, The Netherlands, May 1998.[7] B. Kemme, F. Pedone, G. Alonso, and A. Schiper. Processing transactions over optimistic atomicbroadcast protocols. In 19th IEEE International Conference on Distributed Computing Systems(ICDCS ’99), pages 424–431. IEEE, May 1999.[8] F. Pedone, R. Guerraoui, and A. Schiper. The database state machine approach. Technical ReportSSC/1999/008, École Polytechnique Fédérale de Lausanne, Switzerland, March 1999.[9] J. Pereira, L. Rodrigues, and R. Oliveira. Semantically reliable multicast protocols. In Proceedingsof the 19th IEEE Symposium on Reliable Distributed Systems (SRDS’19), pages 60–69, Nurnberg,Germany, October 2000.[10] J. Pereira, L. Rodrigues, and R. Oliveira. Semantically reliable multicast: Definition implementa-tion and performance evaluation. Special Issue of IEEE Transactions on Computers on ReliableDistributed Systems, 2003. to appear.[11] A. Schiper and M. Raynal. From group communication to transactions in distributed systems. Com-munications of the ACM, 39:84–87, April 1996.[12] A. Sousa, F. Pedone, R. Oliveira, and F. Moura. Partial replication in the database state machine. InIEEE International Symposium on Network Computing and Applications, pages 298–309, Cambridge,MA, October 2001. IEEE Computer Science.[13] I. Stanoi, A. Agrawal, and E. Abbadi. Using broadcast primitives in replicated databases (abstract).In Proceeding of the Sixteen Annual ACM Symposium on Principles of Distributed Computing, page283, Santa Barbara, USA, August 1997.4

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