Optimistic Concurrency Control

Optimistic Concurrency Control

The optimistic method of concurrency control is based on the assumption that conflicts of database operations are rare and that it is better to let transactions run to completion and only check for conflicts before they commit.

An optimistic concurrency control method is also known as validation or certification methods. No checking is done while the transaction is executing. The optimistic method does not require locking or time stamping techniques. Instead, a transaction is executed without restrictions until it is committed.

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

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

Read phase: In this phase, the transaction T is read and executed. It is used to read the value of various data items and stores them in temporary local variables. It can perform all the write operations on temporary variables without an update to the actual database.

Validation phase: In this phase, the temporary variable value will be validated against the actual data to see if it violates the serializability.

Three rules to enforce serializability in validation phase −

            Rule 1 − Given two transactions T_i and T_j, if T_i is reading the data item which T_j is            writing, then T_i’s execution phase cannot overlap with T_j’s commit phase. T_j can commit        only after T_i has finished execution.

            Rule 2 − Given two transactions T_i and T_j, if T_i is writing the data item that T_j is reading, then T_i’s commit phase cannot overlap with T_j’s execution phase. T_j can start executing         only after T_i has already committed.

            Rule 3 − Given two transactions T_i and T_j, if T_i is writing the data item which T_j is also     writing, then T_i’s commit phase cannot overlap with T_j’s commit phase. T_j can start to            commit only after T_i has already committed.

Write phase: If the validation of the transaction is validated, then the temporary results are written to the database or system otherwise the transaction is rolled back.