Stéphane Gançarski

The leganet system: Freshness-aware transaction routing in a database cluster

We consider the use of a database cluster for Application Service Provider (ASP). In the ASP context, applications and databases can be update-intensive and must remain autonomous. In this paper, we describe the Leganet system which performs freshness-aware transaction routing in a database cluster. We use multi-master replication and relaxed replica freshness to increase load balancing. Our transaction routing takes into account freshness requirements of queries at the relation level and uses a cost function that takes into account the cluster load and the cost to refresh replicas to the required level. We implemented the Leganet prototype on an 11-node Linux cluster running Oracle8i. Using experimentation and emulation up to 128 nodes, our validation based on the TPC-C benchmark demonstrates the performance benefits of our approach.

Mobile databases: a selection of open issues and research directions

This paper reports on the main results of a specific action on mobile databases conducted by CNRS in France from October 2001 to December 2002. The objective of this action was to review the state of progress in mobile databases and identify major research directions for the French database community. Rather than provide a survey of all important issues in mobile databases, this paper gives an outline of the directions in which the action participants are now engaged, namely: copy synchronization in disconnected computing, mobile transactions, database embedded in ultra-light devices, data confidentiality, P2P dissemination models and middleware adaptability.

Refresco: Improving Query Performance Through Freshness Control in a Database Cluster

We consider the use of a cluster system for managing autonomous databases. In order to improve the performance of read-only queries, we strive to exploit user requirements on replica freshness. Assuming mono-master lazy replication, we propose a freshness model to help specifying the required freshness level for queries. We propose an algorithm to optimize the routing of queries on slave nodes based on the freshness requirements. Our approach uses non intrusive techniques that preserve application and database autonomy. We provide an experimental validation based on our prototype Refresco. The results show that freshness control can help increase query throughput significantly. They also show significant improvement when freshness requirements are specified at the relation level rather than at the database level.

DTR: Distributed Transaction Routing in a Large Scale Network

Grid systems provide access to huge storage and computing resources at large scale. While they have been mainly dedicated to scientific computing for years, grids are now considered as a viable solution for hosting data-intensive applications. To this end, databases are replicated over the grid in order to achieve high availability and fast transaction processing thanks to parallelism. However, achieving both fast and consistent data access on such architectures is challenging at many points. In particular, centralized control is prohibited because of its vulnerability and lack of efficiency at large scale. In this article, we propose a novel solution for the distributed control of transaction routing in a large scale network. We leverage a cluster-oriented routing solution with a fully distributed approach that uses a large scale distributed directory to handle routing metadata. Moreover, we demonstrate the feasibility of our implementation through experimentation: results expose linear scale-up, and transaction routing time is fast enough to make our solution eligible for update intensive applications such as world wide online booking.

TransPeer: adaptive distributed transaction monitoring for Web2.0 applications

In emerging Web2.0 applications such as virtual worlds or social networking websites, the number of users is very important (tens of thousands), hence the amount of data to manage is huge and dependability is a crucial issue. The large scale prevents from using centralized approaches or locking/two-phase-commit approach. Moreover, Web2.0 applications are mostly interactive, which means that the response time must always be less than few seconds. To face these problems, we present a novel solution, TransPeer, that allows applications to scale-up without the need to buy expensive resources at a data center. To this end, databases are replicated over a P2P system in order to achieve high availability and fast transaction processing thanks to parallelism. A distributed shared dictionary, implemented on top of a DHT, contains metadata used for routing transactions efficiently. Both metadata and data are accessed in an optimistic way: there is no locking on metadata and transactions are executed on nodes in a tentative way. We demonstrate the feasibility of our approaches through experimentation.

Trading Freshness for Performance in a Cluster of Replicated Databases

In the Application Service Provider (ASP) model, applications and databases are hosted at the provider site and are accessed by customers, typically through the Internet, who are no longer concerned with data and application maintenance tasks. Through replication of customers’ databases at several nodes, a cluster system can yield high-availability and high-performance at low cost. Our objective is to demonstrate the viability of the ASP model using a cluster system for pharmacy applications in France. In particular, we must support mixed workloads composed of front-o.ce update-intensive transactions (e.g. drug sales) and back-o.ce read-intensive queries (e.g. statistics on drugs sold). Our approach is to capture application semantics for optimizing load balancing within the cluster system. We use a mono-master replication scheme: updating transaction are all sent to a single master node while read-only queries may be sent to any node. Slave nodes are updated asynchronously through refresh transactions and consistency reduces to freshness. Application semantics mainly consist of a freshness level, which express how much the result of a given query may di.er from the expected results if the query was sent to the master node.

Failure-Tolerant Transaction Routing at Large Scale

Emerging Web2.0 applications such as virtual worlds or social networking websites strongly differ from usual OLTP applications. First, the transactions are encapsulated in an API such that it is possible to know which data a transaction will access, before processing it. Second, the simultaneous transactions are very often commutative since they access distinct data. Anticipating that the workload of such applications will quickly reach thousands of transactions per seconds, we envision a novel solution that would allow these applications to scale-up without the need to buy expensive resources at a data center. To this end, databases are replicated over a P2P infrastructure for achieving high availability and fast transaction processing thanks to parallelism. However, achieving both fast and consistent data access on such architectures is challenging at many points. In particular, centralized control is prohibited because of its vulnerability and lack of efficiency at large scale. Moreover dynamic behavior of nodes, which can join and leave the system at anytime and frequently, can compromise mutual consistency. In this article, we propose a failure-tolerant solution for the distributed control of transaction routing in a large scale network. We leverage a fully distributed approach relying on a DHT to handle routing metadata, with a suitable failure management mechanism that handles nodes dynamicity and nodes failures. Moreover, we demonstrate the feasibility of our transaction routing implementation through experimentation and the effectiveness of our failure management approach through simulation.

Optimistic path-based concurrency control over XML documents

We present a new approach for concurrency control over XML documents. Unlike most of other approaches, we use an optimistic scheme, since we believe that it is better suited for Web applications. The originality of our solution resides in the fact that we use path expressions associated with operations to detect conflicts between transactions. This makes our approach scalable since conflict detection except in few cases does not depend on the database size nor on the amount of modified fragments. In this paper, we describe and motivate our concurrency mechanism architecture, we describe the conflict detection algorithm which is the core of our proposal and exhibit first experimental results.

Routage décentralisé de transactions avec gestion des pannes dans un réseau à large échelle

Emerging Web 2.0 applications such as social networking websites deal with a heavy workload. We envision a novel solution that would allow these applications to scale-up without the need to use a data center. To this end, databases are replicated over a grid, thus, availability and fast transaction processing are achieved. However, achieving both fast and consistent data access challenging at many points: I) centralized control leads to bottleneck source, and 2) dynamic behaviour of nodes can compromise mutual consistency. In this article, we propose a distributed transaction routing algorithm with a suitable fault-management mechanism to deal with node dynamicity and/or node failures. Moreover, we demonstrate the feasibility of our approaches through experimentation and simulation.

Replica refresh strategies in a database cluster

Relaxing replica freshness has been exploited in database clusters to optimize load balancing. However, in most approaches, refreshment is typically coupled with other functions such as routing or scheduling, which makes it hard to analyze the impact of the refresh strategy itself on performance. In this paper, we propose to support routing-independent refresh strategies in a database cluster with monomaster lazy replication. First, we propose a model for capturing existing refresh strategies. Second, we describe the support of this model in Refresco, a middleware prototype for freshness-aware routing in database clusters. Third, we describe an experimental validation to test some typical strategies against different workloads. The results show that the choice of the best strategy depends not only on the workload, but also on the conflict rate between transactions and queries and on the level of freshness required by queries. Although there is no strategy that is best in all cases, we found that one strategy is usually very good and could be used as default strategy.