Yuri Breitbart

Parameter free bursty events detection in text streams

A multidatabase system (MDBS) is a facility that allows users access to data located in multiple autonomous database management systems (DBMSs). In such a system,global transactions are executed under the control of the MDBS. Independently,local transactions are executed under the control of the local DBMSs. Each local DBMS integrated by the MDBS may employ a different transaction management scheme. In addition, each local DBMS has complete control over all transactions (global and local) executing at its site, including the ability to abort at any point any of the transactions executing at its site. Typically, no design or internal DBMS structure changes are allowed in order to accommodate the MDBS. Furthermore, the local DBMSs may not be aware of each other and, as a consequence, cannot coordinate their actions. Thus, traditional techniques for ensuring transaction atomicity and consistency in homogeneous distributed database systems may not be appropriate for an MDBS environment. The objective of this article is to provide a brief review of the most current work in the area of multidatabase transaction management. We first define the problem and argue that the multidatabase research will become increasingly important in the coming years. We then outline basic research issues in multidatabase transaction management and review recent results in the area. We conclude with a discussion of open problems and practical implications of this research.

Topology discovery in heterogeneous IP networks

Knowledge of the up-to-date physical topology of an IP network is crucial to a number of critical network management tasks, including reactive and proactive resource management, event correlation, and root-cause analysis. Given the dynamic nature of todays IP networks, keeping track of topology information manually is a daunting (if not impossible) task. Thus, effective algorithms for automatically discovering physical network topology are necessary. Earlier work has typically concentrated on either: (a) discovering logical (i.e., layer-3) topology, which implies that the connectivity of all layer-2 elements (e.g., switches and bridges) is ignored; or (b) proprietary solutions targeting specific product families. In this paper, we present novel algorithms for discovering physical topology in heterogeneous (i.e., multi-vendor) IP networks. Our algorithms rely on standard SNMP MIB information that is widely supported by modern IP network elements and require no modifications to the operating system software running on elements or hosts. We have implemented the algorithms presented in this paper in the context of a topology discovery tool that has been tested on Lucents own research network. The experimental results clearly validate our approach, demonstrating that our tool can consistently discover the accurate physical network topology in time that is roughly quadratic in the number of network elements.

On rigorous transaction scheduling

The class of transaction scheduling mechanisms in which the transaction serialization order can be determined by controlling their commitment order, is defined. This class of transaction management mechanisms is important, because it simplifies transaction management in a multidatabase system environment. The notion of analogous execution and serialization orders of transactions is defined and the concept of strongly recoverable and rigorous execution schedules is introduced. It is then proven that rigorous schedulers always produce analogous execution and serialization orders. It is shown that the systems using the rigorous scheduling can be naturally incorporated in hierarchical transaction management mechanisms. It is proven that several previously proposed multidatabase transaction management mechanisms guarantee global serializability only if all participating databases systems produce rigorous schedules. >

Topology discovery in heterogeneous IP networks: the NetInventory system

Knowledge of the up-to-date physical topology of an IP network is crucial to a number of critical network management tasks, including reactive and proactive resource management, event correlation, and root-cause analysis. Given the dynamic nature of todays IP networks, keeping track of topology information manually is a daunting (if not impossible) task. Thus, effective algorithms for automatically discovering physical network topology are necessary. Earlier work has typically concentrated on either 1) discovering logical (i.e., layer-3) topology, which implies that the connectivity of all layer-2 elements (e.g., switches and bridges) is ignored, or 2) proprietary solutions targeting specific product families. In this paper, we present novel algorithms for discovering physical topology in heterogeneous (i.e., multi-vendor) IP networks. Our algorithms rely on standard SNMP MIB information that is widely supported by modern IP network elements and require no modifications to the operating system software running on elements or hosts. We have implemented the algorithms presented in this paper in the context of the NetInventory topology-discovery tool that has been tested on Lucents own research network. The experimental results clearly validate our approach, demonstrating that our tool can consistently discover the accurate physical network topology with reasonably small running-time requirements even for fairly large network configurations.

Database integration in a distributed heterogeneous database system

This paper describes the approach to database integration in a heterogeneous distributed database environment utilized by the Amoco Distributed Database System (ADDS). We start with the definition of the extended relational data model that is used by ADDS for database integration. We demonstrate various aspects of resolving possible data conflicts occurring in the database integration process. The ADDS query and data definition languages are defined and their expressive power is discussed. We conclude with presentation of a query conversion algorithm to convert a user data request into a set of queries against supported physical databases.

Update propagation protocols for replicated databates

Replication is often used in many distributed systems to provide a higher level of performance, reliability and availability. Lazy replica update protocols, which propagate updates to replicas through independent transactions after the original transaction commits, have become popular with database vendors due to their superior performance characteristics. However, if lazy protocols are used indiscriminately, they can result in non-serializable executions. In this paper, we propose two new lazy update protocols that guarantee serializability but impose a much weaker requirement on data placement than earlier protocols. Further, many naturally occurring distributed systems, like distributed data warehouses, satisfy this requirement. We also extend our lazy update protocols to eliminate all requirements on data placement. The extension is a hybrid protocol that propagates as many updates as possible in a lazy fashion. We implemented our protocols on the Datablitz database system product developed at Bell Labs. We also conducted an extensive performance study which shows that our protocols outperform existing protocols over a wide range of workloads.

Reliable transaction management in a multidatabase system

A model of a multidatabase system is defined in which each local DBMS uses the two-phase locking protocol Locks are released by a global transaction only after the transaction commits or aborts at each local site. Failures may occur during the processing of transactions. We design a fault tolerant transaction management algorithm and recovery procedures that retain global database consistency. We also show that our algorithms ensure freedom from global deadlocks of any kind.

Multidatabase update issues

A formal model of data updates in a multidatabase environment is developed, and a theory of concurrency control in such an environment is presented. We formulate a correctness condition for the concurrency control mechanism and propose a protocol that allows concurrent execution of a set of global transactions in presence of local ones. This protocol ensures the consistency of the multidatabase and deadlock freedom. We use the developed theory to prove the protocols correctness and discuss complexity issues of implementing the proposed protocol.

Efficiently monitoring bandwidth and latency in IP networks

Effective monitoring of network utilization and performance indicators is a key enabling technology for proactive and reactive resource management, flexible accounting, and intelligent planning in next-generation IP networks. In this paper, we address the challenging problem of efficiently monitoring bandwidth utilization and path latencies in an IP data network. Unlike earlier approaches, our measurement architecture assumes a single point-of-control in the network (corresponding to the network operations center) that is responsible for gathering bandwidth and latency information using widely-deployed management tools, like SNMP, RMON/NetFlow, and explicitly-routed IP probe packets. Our goal is to identify effective techniques for monitoring (a) bandwidth usage for a given set of links or packet flows, and (b) path latencies for a given set of paths, while minimizing the overhead imposed by the management tools on the underlying production network. We demonstrate that minimizing overheads under our measurement model gives rise to new combinatorial optimization problems, most of which prove to be NP-hard. We also propose novel approximation algorithms for these optimization problems and prove guaranteed upper bounds on their worst-case performance. Our simulation results validate our approach, demonstrating the effectiveness of our novel monitoring algorithms over a wide range of network topologies.

Replication and consistency: being lazy helps sometimes

The issue of data replication is considered in the context of a restricted system model motivated by certain distributed data-warehousing applications. A new replica management protocol is defined for this model in which gIobaI serializability is ensured, while message overhead and deadlock frequency are less than in previously published work. The advantages of the protocol arise from its use of a lazy approach to update of secondary copies of replicated data and the use of a new concept, virtual sites, to reduce the potential for conflict among global transactions.