Schedule and its types
- The order in which the operations of multiple transactions appear for execution is called a schedule.
- In DBMS, schedules may be classified as-
- Serial Schedules
- In serial schedules, All the transactions execute serially one after the other.
- When one transaction executes, no other transaction is allowed to execute.
- Serial schedules are always-Consistent, Recoverable, Cascade-less, Strict
- Example:
- In this schedule,
- There are two transactions T1 and T2 executing serially one after the other.
- Transaction T1 executes first.
- After T1 completes its execution, transaction T2 executes.
- So, this schedule is an example of a Serial Schedule.
- Non-Serial Schedules
- In non-serial schedules, Multiple transactions execute concurrently.
- Operations of all the transactions are interleaved or mixed with each other.
- Non-serial schedules are NOT always-Consistent, Recoverable, Cascade-less, Strict
- Example:
- In this schedule,
- There are two transactions T1 and T2 executing concurrently.
- The operations of T1 and T2 are interleaved.
- So, this schedule is an example of a Non-Serial Schedule.
- How to Finding Number Of Schedules?
- Consider there are n number of transactions T1, T2, T3 …. , Tn with N1, N2, N3 …. , Nn number of operations respectively.
- Total Number of Schedules:
- Total number of possible schedules (serial + non-serial) is given by:
- Total Number of Serial Schedules-
- = Number of different ways of arranging n transactions
- = n!
- Total Number of Non-Serial Schedules-
- = Total number of schedules – Total number of serial schedules
- Real-life example:
- Consider there are three transactions with 2, 3, 4 operations respectively, find-
- How many total number of schedules are possible?
- How many total number of serial schedules are possible?
- How many total number of non-serial schedules are possible?
- Solution-
- Total Number of Schedules-
- Using the above formula, we have-
- Total number of serial schedules
- = Number of different ways of arranging 3 transactions
- = 3!
- = 6
- Total number of non-serial schedules
- = Total number of schedules – Total number of serial schedules
- = 1260 – 6
- = 1254
Tags:
DBMS
