Transaction Design for Databases with High Performance and Availability

When many concurrent transactions like ERP and E-commerce orders want to update the same stock records, long duration locking may reduce the availability of the locked data. Therefore, transactions are often designed without analyzing the consequences of loosing the traditional ACID (Atomicity, Consistency, Isolation and Durability) properties. In this paper, we will analyze how low isolation levels, optimistic concurrency control, short duration locks, and countermeasures against isolation anomalies can be used to design transactions for databases with high performance and availability. Long duration locks are defined as locks that are held until a transaction has been committed, i.e. the data of a record is locked from the first read to the last update of any data used by the transaction. This will decrease the availability of locked data for concurrent transactions, and, therefore, optimistic concurrency control and low isolation levels are often used. However, in systems with relatively many updates like ERP-systems and E-commerce systems, low isolation levels cannot solve the availability problem as all update locks must be exclusive. In such situations, we will recommend the use of short duration locks. Short duration locks are local locks that are released as soon as possible, i.e. data will for example not be locked across a dialog with the user. Normally, databases where only short duration locks are used do not have the traditional ACID properties as at least the isolation property is missing when locks are not hold across a dialog with the user. The problems caused by the missing ACID properties may be managed by using approximated ACID properties, i.e. from an application point of view the system should function as if all the traditional ACID properties had been implemented.