Transaction

• A unit of program execution that accesses and possibly updates various data items
• Executing multiple transactions concurrently has significant benefits
• Transactions should satisfy the ACID properties (Atomicity, Consistency, Isolation, Durability)
• Isolation: That is, for every pair of transactions Ti and Tj, it appears to Ti that Tj either finished execution before Ti started, or started execution after Ti finished.

begin
	INSERT INTO student VALUES ('12345', 'Alice', 'CS', 40);
  UPDATE student SET tot_cred = tot_cred + 3 WHERE ID = '12345';
rollback 
commit

Schedule

• A sequence of instructions that specify the chronological order in which instructions of concurrent transactions are executed.

Serial schedule

• Transactions are executed one after another, without overlapping.

Serializable schedule

• Produces the same final result as some serial execution of the same transactions.

• Conflict equivalent: If a schedule S can be transformed into a schedule S’ by a series of swaps of non-conflicting instructions

• Conflict Serializable: The schedule can be transformed into a serial one by swapping non-conflicting operations. All conflict-serializable schedules are also view-serializable.

• View Serializable: The schedule is equivalent to a serial one in terms of the read/write relationships (but may not be conflict serializable).

  Equivalent to executing T1 before T2, which makes the schedule view serializable.
  T1:       W(A)
  T2: W(A),     ,  W(A)
  

Recoverable schedule

• If a transaction reads data written by another transaction, it must not commit before that transaction commits.

Cascadeless schedule

• A transaction can only read data written by another transaction after that transaction has committed.
• Reading uncommitted (dirty) data violates cascadelessness.