Optimistic concurrency control

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In computer science, in the field of databases, optimistic concurrency control, (OCC) is a concurrency control method used in relational databases without using locking. It is commonly referred to as optimistic locking, a misnomer.

Optimistic concurrency control is based on the assumption that most database transactions don't conflict with other transactions, allowing OCC to be as permissive as possible in allowing transactions to execute.

There are three phases in an OCC transaction:

1. Read: The client reads values from the database, storing them to a private sandbox or cache that the client can then edit.
2. Validate: When the client has completed editing of the values in its sandbox or cache, it initiates the storage of the changes back to the database. During validation, an algorithm checks if the changes to the data would conflict with either
   • already-committed transactions in the case of backward validation schemes, or
   • currently executing transactions in the case of forward validation schemes.
   If a conflict exists, a conflict resolution algorithm must be used to resolve the conflict somehow (ideally by minimizing the number of changes made by the user) or, as a last resort, the entire transaction can be aborted (resulting in the loss of all changes made by the user).
3. Write: If there is no possibility of conflict, the transaction commits.

When conflicts are rare, validation can be done efficiently, leading to higher throughput than other concurrency control methods.^{[citation needed]} However, if conflicts happen often, the cost of repeatedly restarting transactions, hurts performance significantly — other non-lock concurrency control methods have better performance when there are many conflicts.

[edit] Web usage

The stateless nature of HTTP makes locking infeasible for web user interfaces.^{[citation needed]} It's common for a user to start editing a record, then leave without following a "cancel" or "logout" link. If locking is used, other users who attempt to edit the same record must wait until the first user's lock expires.

OCC is a natural choice. It is simple to implement and avoids unnecessary waiting or silently overwritten records. Typically the form presented to the user includes a hidden field with the record's original content, a timestamp, a sequence number, or an opaque token. On submit, this is compared against the database. If it differs, the conflict resolution algorithm is invoked.

[edit] Examples

• MediaWiki's edit pages use OCC. The conflict resolution algorithm is described here.
• Bugzilla uses OCC; conflicts are called "mid-air collisions". [1]
• The Ruby on Rails framework has an API for OCC. [2]
• Most revision control systems support the "merge" model for concurrency, which is OCC.