CONCURRENCY CONTROL IN DISTRIBUTED RING COMPUTER SYSTEMS
Abstract
This thesis addresses the question of how to provide mutually-exclusive access to objects in a distributed computer system configured as a ring. By concentrating on a ring topology, we narrow the focus of the more general concurrency control problem and find that it is possible to design a specialized family of concurrency control algorithms that outperform general concurrency controllers. We develop transaction, object, and database access models for studying the problem. The object model extends the work of Reed and Svobodova at MIT in which objects are represented by chains of version records. We allow object version records to be distributed across the system. The transaction model assumes that transactions execute at their initiating sites and that objects are moved to transactions. Once this framework is built, we describe the design and implementation of a new distributed concurrency control algorithm calld the Distributed Queues (DQ) algorithm. DQ schedules access to shared data objects such that conflicting transactions are serialized and the amount of data flow on the ring is minimized. DQ is conceptually straightforward, allows substantial parallelism, and is easily implemented. Four versions of the basic algorithm are studied in detail using simulations. Extensions of the base algorithm for read-only transactions, for replicated objects, and for transactions that claim their objects as needed illustrate the fundamental soundness of the approach. We then systematically evaluate the new algorithm and several others--two centralized controllers (Basic CC and Garcia-Molina's MCLA-h Algorithm) and four decentralized controllers (Ellis' Ring Algorithm, Lelann's Control Token Method, Thomas' Majority Consensus Algorithm, and Milenkovic's Optimistic Protocol). Simulation studies compare DQ to two of these. These experiments confirm the superiority of DQ and support the thesis that a specialized concurrency control algorithm is an effective alternative to generalized concurrency controllers for a ring.
Degree
Ph.D.
Subject Area
Computer science
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