Virtual time synchronization in distributed database systems

Distributed systems synchronized by Virtual Time have been topics of recent interest. Virtual Time follows an optimistic philosophy relying on rollback for synchronization instead of abortion or blocKing Although many applications have been suggested as candidates for Virtual Time, few were simulated or implemented. This research reports on the first implementation and results of a Distributed Database Management System synchronized by virtual time. We argue that virtual time is a viable alternate concurrency control method for distributed database systems if its memory overhead can be absorbed

Ensuring Serializable Executions with Snapshot Isolation DBMS

Snapshot Isolation (SI) is a multiversion concurrency control that has been implemented by open source and commercial database systems such as PostgreSQL and Oracle. The main feature of SI is that a read operation does not block a write operation and vice versa, which allows higher degree of concurrency than traditional two-phase locking. SI prevents many anomalies that appear in other isolation levels, but it still can result in non-serializable execution, in which database integrity constraints can be violated. Several techniques have been proposed to ensure serializable execution with engines running SI; these techniques are based on modifying the applications by introducing conflicting SQL statements. However, with each of these techniques the DBA has to make a difficult choice among possible transactions to modify. This thesis helps the DBA’s to choose between these different techniques and choices by understanding how the choices affect system performance. It also proposes a novel technique called ’External Lock Manager’ (ELM) which introduces conflicts in a separate lock-manager object so that every execution will be serializable. We build a prototype system for ELM and we run experiments to demonstrate the robustness of the new technique compare to the previous techniques. Experiments show that modifying the application code for some transactions has a high impact on performance for some choices, which makes it very hard for DBA’s to choose wisely. However, ELM has peak performance which is similar to SI, no matter which transactions are chosen for modification. Thus we say that ELM is a robust technique for ensure serializable execution

Ensuring Serializable Executions with Snapshot Isolation DBMS

Snapshot Isolation (SI) is a multiversion concurrency control that has been implemented by open source and commercial database systems such as PostgreSQL and Oracle. The main feature of SI is that a read operation does not block a write operation and vice versa, which allows higher degree of concurrency than traditional two-phase locking. SI prevents many anomalies that appear in other isolation levels, but it still can result in non-serializable execution, in which database integrity constraints can be violated. Several techniques have been proposed to ensure serializable execution with engines running SI; these techniques are based on modifying the applications by introducing conflicting SQL statements. However, with each of these techniques the DBA has to make a difficult choice among possible transactions to modify. This thesis helps the DBA’s to choose between these different techniques and choices by understanding how the choices affect system performance. It also proposes a novel technique called ’External Lock Manager’ (ELM) which introduces conflicts in a separate lock-manager object so that every execution will be serializable. We build a prototype system for ELM and we run experiments to demonstrate the robustness of the new technique compare to the previous techniques. Experiments show that modifying the application code for some transactions has a high impact on performance for some choices, which makes it very hard for DBA’s to choose wisely. However, ELM has peak performance which is similar to SI, no matter which transactions are chosen for modification. Thus we say that ELM is a robust technique for ensure serializable execution

Serializable Isolation for Snapshot Databases

Many popular database management systems implement a multiversion concurrency control algorithm called snapshot isolation rather than providing full serializability based on locking. There are well-known anomalies permitted by snapshot isolation that can lead to violations of data consistency by interleaving transactions that would maintain consistency if run serially. Until now, the only way to prevent these anomalies was to modify the applications by introducing explicit locking or artificial update conflicts, following careful analysis of conflicts between all pairs of transactions. This thesis describes a modification to the concurrency control algorithm of a database management system that automatically detects and prevents snapshot isolation anomalies at runtime for arbitrary applications, thus providing serializable isolation. The new algorithm preserves the properties that make snapshot isolation attractive, including that readers do not block writers and vice versa. An implementation of the algorithm in a relational database management system is described, along with a benchmark and performance study, showing that the throughput approaches that of snapshot isolation in most cases

SQL Access Patterns for Optimistic Concurrency Control

Transaction processing is of growing importance for mobile and web applications. Booking tickets, flight reservation, e-Banking, e-Payment, and booking holiday arrangements are just a few examples. Due to temporarily disconnected situations the synchronization and consistent transaction processing are key issues. To avoid difficulties with blocked transactions or communication loss several authors and technology providers have recommended to use Optimistic Concurrency Control (OCC) to solve the problem. However most vendors of Relational Database Management Systems (DBMS) implemented only locking schemes for concurrency control which prohibit the immediate use of OCC. We propose Row Version Verifying (RVV) discipline to avoid lost updates and achieve a kind of OCC for those DBMS not providing an adequate non-blocking concurrency control. Moreover, the different mechanisms are categorized as access pattern in order to provide programmers with a general guideline for SQL databases. The proposed SQL access patterns are relevant for all transactional applications with unreliable communication and low conflicting situations. We demonstrate the proposed solution using mainstream database systems like Oracle, DB2, and SQLServer.Comment: 5 pages, 1 figur