Secondary Copies Research Articles

Evidence of an Ancient Archive? The Papyrus British Museum EA 9961

Used on both sides, the papyrus British Museum EA 9961 reproduces a marsh scene on the recto and a copy of the myth of Isis and her seven scorpions in cursive hieroglyphs on the verso. Although the high quality of the illustration is the most striking feature, the text has also been carefully laid out and written. This research seeks to investigate how, when and why this document, whose provenance and context are unknown, was composed. A detailed inspection of its surface using infrared photography helped to reveal many marks otherwise invisible and to retrace part of the history and deterioration of the roll. While the context of production will also be considered, we will examine if text and imagery were produced as models for the production of monuments or were secondary copies of older documents.

Replica Consistency in Lazy Master Replicated Databases

In a lazy master replicated database, a transaction can commit after updating one replica copy (primary copy) at some master node. After the transaction commits, the updates are propagated towards the other replicas (secondary copies), which are updated in separate refresh transactions. A central problem is the design of algorithms that maintain replica's consistency while at the same time minimizing the performance degradation due to the synchronization of refresh transactions. In this paper, we propose a simple and general refreshment algorithm that solves this problem and we prove its correctness. The principle of the algorithm is to let refresh transactions wait for a certain “deliver time” before being executed at a node having secondary copies. We then present two main optimizations to this algorithm. One is based on specific properties of the topology of replica distribution across nodes. In particular, we characterize the nodes for which the deliver time can be null. The other improves the refreshment algorithm by using an immediate update propagation strategy.

Replication and Consistency in a Distributed Environment

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 global 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 secondary copies of replicated data and the use of a new concept, virtual sites, to reduce the potential for conflict among global transactions.

A secure dynamic copy protocol in real-time secure database systems

Concurrency control for real-time secure database systems must satisfy not only logical data consistency but also timing constraints and security requirements associated with transactions. These conflicting natures between timing constraints and security requirements are often resolved by maintaining several versions (or secondary copies) on the same data items. In this paper, we propose a new lock-based concurrency control protocol, Secure Dynamic Copy Protocol, ensuring both conflicting requirements. Our protocol aims for reducing the storage overhead of maintaining secondary copies and minimizing the processing overhead of update history. The main idea of our protocol is to keep a secondary copy only when it is needed to resolve the conflicting read/write operations in real time secure database systems. For doing this, a secondary copy is dynamically created and removed during a transaction's read/write operations. While comparing the existing real-time security protocol, we have also examined the performance characteristics of our protocol through simulation under different workloads. The results show that our protocol consumed less storage and decreased the deadline missing transactions.

THE ADR REPLICATION MANAGER

ADR (Atomic Delayed Replication) is a controllable replication manager implemented on top of commercial distributed relational databases. ADR's goal is to enable various well-defined trade-offs between database coherence, throughput and response time in large database networks, e.g. for telecom applications. By combining a strategy for distributed database design with a specific replication protocol, ADR preserves the ACID properties with a controlled relaxation of coherence between primary and secondary copies. We first discuss formal characteristics of ADR, and present the implementation techniques required to realize these formal characteristics on top of commercial distributed database technology. Then, after reviewing a validated analytical performance model for the approach, we demonstrate its flexibility by summarizing experiences with two industrial ADR applications in telecommunications management, both jointly developed with Philips Laboratories. One is database support for the integrated operation and evolution of Intelligent Network telephone services, where secondary copies are held within a distributed database system optimized for throughput and availability during schema evolution. The other concerns database support for mobile phones in a City-wide DECT setting (Digital Enhanced Cordless Telecommunications), where secondary copies are held in main memory caches outside the DBMS.

Performance analysis of a random packet selection policy for multicast switching

This paper studies a random packet selection policy for multicast switching. An input packet generates a fixed number of primary copies plus a random number of secondary copies. Assuming a constant number of contending packets during a slot, the system is modeled as a discrete time birth process. A difference equation describing the dynamics of this process is derived, the solution of which gives a closed form expression for the distribution of the number of packets chosen. Then this result is extended to the steady state distribution through a Markov chain analysis. It is shown that the old packets have larger fanout than the fresh packets and the copy distribution of the mixed packets is determined. The packet and copy throughput taking into account the old packets have been obtained. We determined the mean packet delay as well as an upperbound for packet loss probabilities for finite buffer sizes. The asymptotic distribution of the number of packets is also given for large switch sizes under saturation by applying results from the renewal theory. Finally, simulations are done to determine the performance of the switch under mixed (unicast plus multicast) traffic.