Francesc D. Muñoz-Escoí

MADIS: a slim middleware for database replication

Data replication serves to improve the availability and performance of distributed systems. The price to be paid consists of costs caused by protocols by which a sufficient degree of consistency of replicated data is maintained. Different kinds of targeted applications require different kinds of replication protocols, each one requiring a different set of metadata. We discuss the middleware architecture used in the MADIS project for maintaining the consistency of replicated databases. Instead of reinventing wheels, MADIS makes use of basic resources provided by conventional database systems (e.g. triggers, views, etc) to achieve its purpose, to a large extent. So, the underlying databases can perform more efficiently many of the routines needed to support any consistency protocol, the implementation of which thus becomes much simpler and easier. MADIS enables the databases to simultaneously maintain different metadata needed for different replication protocols, so that the latter can be chosen, plugged in and exchanged on the fly as online-configurable modules, in order to fit the shifting needs of given applications best, at each moment.

SIPRe: a partial database replication protocol with SI replicas

Database replication has been researched as a solution to overcome the problems of performance and availability of distributed systems. Full database replication, based on group communication systems, is an attempt to enhance performance that works well for a reduced number of sites. If application locality is taken into consideration, partial replication, i.e. not all sites store the full database, also enhances scalability. On the other hand, it is needed to keep all copies consistent. If each DBMS provides SI, the execution of transactions has to be coordinated so as to obtain Generalized-SI (GSI). In this paper, a partial replication protocol providing GSI is introduced that gives a consistent view of the database, providing an adaptive replication technique and supporting the failure and recovery of replicas.

FOBr: a version-based recovery protocol for replicated databases

Within the field of databases that are deployed in distributed environments there is a need to guarantee consistency among replicas and availability among nodes despite of network disconnections and node crashes. A recovery protocol, such as FOBr, manages the database update of a recovering node as it might have missed many transactions during its absence. FOBr does so without stopping data access, and minimizing its interference with the active nodes, their memory usage, and the network traffic. The recovery protocol we propose is very suitable when a fast recovery of the missed data is required. It balances the recovery issues among nodes very fairly and due to this, out-dated data will promptly be recovered. Thus accesses to data that is not out-dated are not interrupted or delayed in any way. These characteristics allow regular transactions to be performed during the recovery process.

k -bound GSI: a flexible database replication protocol

Several previous works have proven that there is no way of guaranteeing a snapshot isolation level in symmetrical replicated database systems without blocking transactions when they are started. As a result of this, the generalized snapshot isolation (GSI) level was defined, relaxing a bit the freshness of the snapshot being taken when a transaction is initiated in its local replica. This enhances performance, since transactions do not need to get blocked, but in some cases will increase the abortion rate. This paper proposes a flexible protocol that is able to bound the degree of snapshot outdateness from a relaxed GSI to the strict one-copy equivalent SI. Additionally, it proposes an optimistic solution where transactions do not block, and only need to be re-initiated when their optimistic start fails. Such re-initialization is made very soon and only rolls back the first transaction accesses, without waiting for the transaction completion. Finally, if 1CSI is not enough, this protocol is also able to manage transactions with serializable isolation, if such a level is requested.

CLOB: communication support for efficient replicated database recovery

Replication protocols using an eager update propagation strategy commonly need a reliable broadcast service; i.e., a broadcast primitive with atomic delivery and, in some cases, also with total order. This communication service provides some appropriate features for the recovery tasks, although in some cases this leads to partial blocking of the replica taken as the source in the updating process. CLOB is a framework for reliable broadcast protocols that log the missed update messages in case of failure, being able to automatically resend these updates when the faulty destinations recover. This behaviour is easily configurable and allows an efficient recovery mechanism in case of short-term failures, which can be combined with other version-based recovery protocols in order to manage long-term outages.

Revisiting and improving a result on integrity preservation by concurrent transactions

We revisit a well-known result on the preservation of integrity by concurrent transactions. It says that the serializability of integrity-preserving transactions yields integrity-preserving histories. We improve it in two ways. First, we discuss divergent interpretations and restate them more precisely. Second, we make it applicable in the presence of inconsistency.

SIRC, a multiple isolation level protocol for middleware-based data replication

One of the weaknesses of database replication protocols, compared to centralized DBMSs, is that they are unable to manage concurrent execution of transactions at different isolation levels. In the last years, some theoretical works related to this research line have appeared but none of them has proposed and implemented a real replication protocol with support to multiple isolation levels. This paper takes advantage of our MADIS middleware and one of its implemented Snapshot Isolation protocols to design and implement SIRC, a protocol that is able to execute concurrently both generalized snapshot isolation (GSI) and generalized loose read committed (GLRC) transactions. We have also made a performance analysis to show how this kind of protocols can improve the system performance and decrease the transaction abortion rate in applications that do not require the strictest isolation level in every transaction.

Ensuring Progress in Amnesiac Replicated Systems

Replication is used for providing highly available and fault-tolerant information systems, which are constructed on top of replication and recovery protocols. Important aspects when designing these systems are the failure model assumed and the progress condition assumed. Replicated transactional systems usually assume the crash-recovery with partial amnesia failure model, and the majority partition progress condition. But, despite the large use of such combination most of these works do not handle accurately a very special phenomenon that can lead to diverging states in different replicas causing, when happening, critical situations.

Extending Middleware Protocols for Database Replication with Integrity Support

Current middleware database replication protocols take care of read-write conflict evaluation. If there are no such conflicts, protocols sanction transactions to commit. Other conflicts may arise due to integrity violation. So, if, in addition to the consistency of transactions and replicas, also the consistency of integrity constraints is to be supported, some more care must be taken. Some classes of replication protocols are able to seamlessly deal with the integrity support of the underlying DBMS, but others are not. In this paper, we analyze the support for integrity that can be provided in various classes of replication protocols. Also, we propose extensions for those that cannot directly manage certain kinds of constraints that are usually supported in DBMSs.

Implementing Network Partition-Aware Fault-Tolerant CORBA Systems

The current standard for fault-tolerance in the Common Object Request Broker Architecture (CORBA) does not support network partitioning. However, distributed systems, and those deployed on wide area networks in particular, are susceptible to network partitions. The contribution of this paper is the description of the design and implementation of a CORBA fault-tolerance add-on for partitionable environments. Our solution can be applied to an off-the-shelf Object Request Broker, without having access to the ORBs source code and with minimal changes to existing CORBA applications. The system distinguishes itself from existing solutions in the way different replication and reconciliation strategies can be implemented easily. Furthermore, we provide a novel replication and reconciliation protocol that increases the availability of systems, by allowing operations in all partitions to continue