Optimistic Concurrency Control (OCC)

Optimistic concurrency control (OCC) is a concurrency control method that was first proposed in 1981 by Kung et al.H. T. Kung and J. T. Robinson, “On optimistic methods for concurrency control,” ACM Transactions on Database Systems, Volume 6, Issue 2, 1981., where transactions can access data items without acquiring locks on them.

In this method, transactions execute in the following three phases:

• Begin
• Read & modify
• Validate & commit/rollback

Begin phase

In this phase, transactions are assigned a unique timestampA timestamp is a monotonically increasing number, often based on the system clock. that marks the beginning of the transaction referred to as the start timestamp.

Read & modify phase

During this phase, transactions execute their read and write operations tentatively. This means that when an item is modified, a copy of the item is written to a temporary, local storage location. A read operation first checks for a copy of the item in this location and returns this one, if it exists. Otherwise, it performs a regular read operation from the database.

Validate & commit/rollback phase

The transaction enters this phase when all operations have been executed.

During this phase, the transaction checks whether there are other transactions that have modified the data this transaction has accessed, and have started after this transaction’s start time. If there are, then the transaction is aborted and restarted from the beginning, acquiring a new timestamp. Otherwise, the transaction can be committed.