Pessimistic Locking and Optimistic Locking are concurrency control mechanisms used to prevent data conflicts when multiple users try to access or update the same data simultaneously in Appian applications.

In Pessimistic Locking, the system assumes that conflicts are likely to occur. Therefore, when one user starts editing a record or task, the data gets locked immediately, preventing other users from modifying it until the lock is released. This type of locking is commonly seen in Appian process tasks, approval workflows, banking systems, and critical transaction-based applications where data consistency is very important. The main advantage is strong data integrity, but it can reduce performance and scalability because users may have to wait for locks to be released.

In Optimistic Locking, the system assumes conflicts are rare. Multiple users are allowed to edit the same data simultaneously without locking it initially. The conflict is checked only when the data is saved. If another user has already updated the same record, the system detects a version mismatch and throws a conflict error. Optimistic locking is commonly used in Appian record types, editable forms, integrations, and high-traffic enterprise applications because it improves scalability and user experience.

In Appian, optimistic locking is usually implemented using fields such as updatedOn, modifiedDate, or versionNumber to compare the latest database state before saving changes. Pessimistic locking is often handled internally in process models and task management where only one user can complete a task at a time.

The major difference is that pessimistic locking prevents conflicts before they happen, while optimistic locking detects conflicts after they happen. Optimistic locking provides better performance and scalability, whereas pessimistic locking provides stronger consistency and security for critical operations