Fernando Pedone

Database replication techniques: a three parameter classification

Data replication is an increasingly important topic as databases are more and more deployed over clusters of workstations. One of the challenges in database replication is to introduce replication without severely affecting performance. Because of this difficulty, current database products use lazy replication, which is very efficient but can compromise consistency. As an alternative, eager replication guarantees consistency but most existing protocols have a prohibitive cost. In order to clarify the current state of the art and open up new avenues for research, this paper analyses existing eager techniques using three key parameters (server architecture, server interaction and transaction termination). In our analysis, we distinguish eight classes of eager replication protocols and, for each category, discuss its requirements, capabilities and cost. The contribution lies in showing when eager replication is feasible and in spelling out the different aspects a database replication protocol must account for.

The Database State Machine Approach

Database replication protocols have historically been built on top of distributed database systems, and have consequently been designed and implemented using distributed transactional mechanisms, such as atomic commitment. We present the Database State Machine approach, a new way to deal with database replication in a cluster of servers. This approach relies on a powerful atomic broadcast primitive to propagate transactions between database servers, and alleviates the need for atomic commitment. Transaction commit is based on a certification test, and abort rate is reduced by the reordering certification test. The approach is evaluated using a detailed simulation model that shows the scalability of the system and the benefits of the reordering certification test.

Processing transactions over optimistic atomic broadcast protocols

Atomic broadcast primitives allow fault-tolerant cooperation between sites in a distributed system. Unfortunately, the delay incurred before a message can be delivered makes it difficult to implement high performance, scalable applications on top of atomic broadcast primitives. A new approach has been proposed which, based on optimistic assumptions about the communication system, reduces the average delay for message delivery. We develop this idea further and present a replicated database architecture that employs the new atomic broadcast primitive in such a way that the coordination phase of the atomic broadcast is fully overlapped with the execution of transactions, providing high performance without relaxing transaction correctness.

Using optimistic atomic broadcast in transaction processing systems

Atomic broadcast primitives are often proposed as a mechanism to allow fault-tolerant cooperation between sites in a distributed system. Unfortunately, the delay incurred before a message can be delivered makes it difficult to implement high performance, scalable applications on top of atomic broadcast primitives. Recently, a new approach has been proposed for atomic broadcast which, based on optimistic assumptions about the communication system, reduces the average delay for message delivery to the application. We develop this idea further and show how applications can take even more advantage of the optimistic assumption by overlapping the coordination phase of the atomic broadcast algorithm with the processing of delivered messages. In particular, we present a replicated database architecture that employs the new atomic broadcast primitive in such a way that communication and transaction processing are fully overlapped, providing high performance without relaxing transaction correctness.

Merlin: A Language for Provisioning Network Resources

This paper presents Merlin, a new framework for managing resources in software-defined networks. With Merlin, administrators express high-level policies using programs in a declarative language. The language includes logical predicates to identify sets of packets, regular expressions to encode forwarding paths, and arithmetic formulas to specify bandwidth constraints. The Merlin compiler maps these policies into a constraint problem that determines bandwidth allocations using parameterizable heuristics. It then generates code that can be executed on the network elements to enforce the policies. To allow network tenants to dynamically adapt policies to their needs, Merlin provides mechanisms for delegating control of sub-policies and for verifying that modifications made to sub-policies do not violate global constraints. Experiments demonstrate the expressiveness and effectiveness of Merlin on real-world topologies and applications. Overall, Merlin simplifies network administration by providing high-level abstractions for specifying network policies that provision network resources.

Exploiting Atomic Broadcast in Replicated Databases

Database replication protocols have historically been built on top of distributed database systems, and have consequently been designed and implemented using distributed transactional mechanisms, such as atomic commitment. We argue in this paper that this approach is not always adequate to efficiently support database replication and that more suitable alternatives, such as atomic broadcast primitives, should be employed instead. More precisely, we show in this paper that fully replicated database systems, based on the deferred update replication model, have better throughput and response time if implemented with an atomic broadcast termination protocol than if implemented with atomic commitment.

Optimistic Atomic Broadcast

This paper presents an Optimistic Atomic Broadcast algorithm (OPT-ABcast) that exploits the spontaneous total order message reception property experienced in local area networks, in order to allow fast delivery of messages. The OPT-ABcast algorithm is based on the Optimistic Consensus problem (OPT-Consensus) that allows processes to decide optimistically or conservatively. A process optimistically decides if it knows that the spontaneous total order message reception property holds, otherwise it decides conservatively. We evaluate the efficiency of the OPT-ABcast and the OPT-Consensus algorithms using the notion of latency degree.

P-Store: Genuine Partial Replication in Wide Area Networks

Partial replication is a way to increase the scalability of replicated systems: updates only need to be applied to a subset of the systems sites, thus allowing replicas to handle independent parts of the workload in parallel. In this paper, we propose P-Store, a partially replicated key-value store for wide area networks. In P-Store, each transaction T optimistically executes on one or more sites and is then certified to guarantee serializability of the execution. The certification protocol is genuine, it only involves sites that replicate data items read or written by T, and incorporates a mechanism to minimize a convoy effect. P-Store makes a thrifty use of an atomic multicast service to guarantee correctness: no messages need to be multicast during Ts execution and a single message is multicast to certify T. In case T is global, that is, Ts execution is distributed at different geographical locations, an extra vote phase is required. Our approach may offer better scalability than previously proposed solutions that either require multiple atomic multicast messages to execute T or are non-genuine. Experimental evaluations reveal that the convoy effect plays an important role even when one percent of the transactions are global. We also compare the scalability of our approach to a fully replicated solution when the proportion of global transactions and the number of sites vary.

Handling message semantics with Generic Broadcast protocols

Summary. Message ordering is a fundamental abstraction in distributed systems. However, ordering guarantees are usually purely “syntactic,” that is, message “semantics” is not taken into consideration despite the fact that in several cases semantic information about messages could be exploited to avoid ordering messages unnecessarily. In this paper we define the Generic Broadcast problem, which orders messages only if needed, based on the semantics of the messages. The semantic information about messages is introduced by conflict relations. We show that Reliable Broadcast and Atomic Broadcast are special instances of Generic Broadcast. The paper also presents two algorithms that solve Generic Broadcast.

Partial replication in the Database State Machine

This paper investigates the use of partial replication in the Database State Machine approach introduced earlier for fully replicated databases. It builds on the order and atomicity properties of group communication primitives to achieve strong consistency and proposes two new abstractions: Resilient Atomic Commit and Fast Atomic Broadcast. Even with atomic broadcast, partial replication requires a termination protocol such as atomic commit to ensure transaction atomicity, With Resilient Atomic Commit our termination protocol allows the commit of a transaction despite the failure of some of the participants. Preliminary performance studies suggest that the additional cost of supporting partial replication can be mitigated through the use of Fast Atomic Broadcast.