Schedules

A Schedule in the context of transaction management is a sequence that represents the chronological order in which the instructions of concurrent transactions are executed. The database system's scheduler is responsible for creating and managing these schedules. Schedules are a way to model and analyze the execution of concurrent transactions to ensure that they maintain the database's consistency. There are two main types of schedules. A Serial Schedule is one where the operations of one transaction are executed completely before the operations of the next transaction begin. There is no interleaving of operations. For example, if we have Transaction 1 (T1) and Transaction 2 (T2), a serial schedule would be either all of T1 followed by all of T2, or all of T2 followed by all of T1. Serial schedules are always considered correct and consistent because they lack any concurrency, but they result in very poor performance and system throughput. A Concurrent Schedule (or non-serial schedule) is one where the operations of multiple transactions are interleaved. This allows for much better system utilization and performance, as the CPU can execute operations from one transaction while another is waiting for a disk read. The challenge with concurrent schedules is to ensure they are 'correct'. A concurrent schedule is considered correct if it is 'serializable', meaning it produces the same result and leaves the database in the same state as some serial schedule. The goal of a DBMS's concurrency control mechanism is to allow only serializable concurrent schedules.