BACKGROUND: ACID AND CONSISTENCY

Database systems support the concept of a transaction, which is informally an execution of a program. While the systems execute multiple programs concurrently in inter-leaved fashion for high performance, they guarantee that the execution’s result leaves the database in the same state as some serial execution of the same transactions. The term ACID denotes that a transaction is atomic in that the system executes it completely or not at all; consis-tent in that the database remains unchanged; isolated in that the effects of incomplete execution are not exposed; and durable in that results from completed transactions survive failures. The transaction abstraction is one of the great achieve-ments of database management systems, freeing programmers from concern about other concurrently ex