Concurrency control

• Concurrency controlling techniques ensure that multiple transactions are executed simultaneously while maintaining the ACID properties of the transactions.

• Simultaneous execution of transactions overashared database can create several data integrity and consistency problems.

  • Lost Update
  • Dirty Read Problems
  • Incorrect Summary Problems
  • Unrepeatable Read Problem

Optimistic concurrency control

• Optimistic concurrency control isaconcurrency control method applied to transactional systems

  • The transaction is executed without restriction and before committing, each transaction verifies that no other transaction has R/W conflicts.
  • If the check reveals conflicting modifications, the committing transaction rolls back and can be restarted.

• In optimistic methods, each transaction moves through the following phases:

  • Read phase.
  • Validation or certification phase.
  • Write phase.

• Read phase:

  • The transaction reads the database, executes the needed computations and makes the updates toaprivate copy of the database values.
  • All update operations of the transactions are recorded ina temporary update file, which is not accessed by the remaining transactions.

• Validation phase:

  • The transaction is validated to establish whether or not its operations on objects conflict with operations of other transactions on the same objects.
  • If the validation test is positive, the transaction goes toa write phase. If the validation test is negative, the transaction is restarted and the changes are discarded.

• To pass the validation test one of the condition must be true,

  • The earlier transaction Ti finish before new transaction Tj started (no overlapping transactions)
  • The new transaction Tj starts before the earlier transaction Ti started (overlapping transactions)
  • The validation phase ofatransaction comes after the write phase of another transaction, then validation is true

• Two types of Validation,

  • Backward Validation
  • Forward Validation

• Backward validation:

  • algorithm checks the transaction in validation phase with other preceding overlapping transactions that have entered validation phaseO
  • Write operations are ok since Read operations of earlier transactions are done already (Rule1)
  • Check if Read operations have any conflict with Write operations of earlier overlapping transactions (Rule 2) => if yes, abort transaction

• Forward validation

  • algorithm checks transaction in validation phase with other overlapping active transactionsO
  • Read operations are ok since later transactions do not write until the transaction is done (Rule 2)
  • check if Write operations have any conflict with Read operations of overlapping active transactions (Rule 1) => if yes, abort transaction
• Validation uses the read-write conflict rules to ensure that the scheduling ofaparticular transaction is serially equivalent with respect to all other overlapping transactions
• Write Phase:
  • The changes are permanently applied to the database.