Hubert Naacke

The distributed information search component (Disco) and the World Wide Web

The Distributed Information Search COmponent (DISCO) is a prototype heterogeneous distributed database that accesses underlying data sources. The DISCO prototype currently focuses on three central research problems in the context of these systems. First, since the capabilities of each data source is different, transforming queries into subqueries on data source is difficult. We call this problem the weak data source problem. Second, since each data source performs operations in a generally unique way, the cost for performing an operation may vary radically from one wrapper to another. We call this problem the radical cost problem. Finally, existing systems behave rudely when attempting to access an unavailable data source. We call this problem the ungraceful failure problem. DISCO copes with these problems. For the weak data source problem, the database implementor defines precisely the capabilities of each data source. For the radical cost problem, the database implementor (optionally) defines cost information for some of the operations of a data source. The mediator uses this cost information to improve its cost model. To deal with ungraceful failures, queries return partial answers. A partial answer contains the part of the final answer to the query that was produced by the available data sources. The current working prototype of DISCO contains implementations of these solutions and operations over a collection of wrappers that access information both in files and on the World Wide Web.

Leveraging mediator cost models with heterogeneous data sources

Distributed systems require declarative access to diverse information sources. One approach to solving this heterogeneous distributed database problem is based on mediator architectures. In these architectures, mediators accept queries from users, process them with respect to wrappers, and return answers. Wrappers provide access to underlying sources. To efficiently process queries, the mediator must optimize the plan used for processing the query. In classical databases, cost-estimate based query optimization is effective. In a heterogeneous distributed databases, cost-estimate based query optimization is difficult to achieve because the underlying data sources do not export cost information. This paper describes a new method that permits the wrapper programmer to export cost estimates. For the wrapper programmer to describe all cost estimates may be impossible due to lack of information or burdensome due to the amount of information. We ease this responsibility of the wrapper programmer by leveraging the generic cost model of the mediator with specific cost estimates from the wrappers.

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.

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.

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.

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.