Interleaved Executions

the DBMS interleaves the operations (reads / writes of database objects) of different user programs, as concurrent execution is fundamental for the performance of the application
the disk is frequently accessed (and access times are relatively slow)
overlap I/O and CPU activity
if concurrent execution is allowed, one needs to tackle the effects of interleaving user programs

Consider the two transactions below:

T1: BEGIN A ← A-100 B ← B+100 END

T2: BEGIN A ← 1.05A B ← 1.05B END

T1 is transferring 100 lei from account A to account B
T2 is adding 5% interest to both accounts
T1 and T2 are submitted together to the DBMS
- serial execution:
  - T1 can execute before or after T2 on a database DB, resulting in one of the following database states: State( (T1T2), DB) or State( (T2T1), DB)
- interleaved execution:
  - the net effect of executing T1 and T2 must be identical to State( (T1 T2), DB) or State( (T2 T1), DB)

Consider the following interleaving of operations (schedule):

T1: BEGIN A ← A-100 B ← B+100 END

T2: BEGIN A ← 1.05A B ← 1.05B END

T1: BEGIN A ← A-100 B ← B+100 END