How To Increment The Version Of The Locked Entity Even If This Entity Was Not Modified PESSIMISTIC_FORCE_INCREMENT

Description: This application is a sample of how PESSIMISTIC_FORCE_INCREMENT works in MySQL. This is useful when you want to increment the version of the locked entity even if this entity was not modified. Via PESSIMISTIC_FORCE_INCREMENT the version is updated (incremented) immediately (the entity version update is guaranteed to succeed immediately after acquiring the row-level lock). The incrementation takes place before the entity is returned to the data access layer.

Key points:

• use a root entity, Chapter (which uses @Version)
• several editors load a chapter and perform modifications mapped via the Modification entity
• between Modification (child-side) and Chapter (parent-side) there is a lazy unidirectional @ManyToOne association
• for each modification, Hibernate will trigger an INSERT statement against the modification table, therefore the chapter table will not be modified by editors
• but, Chapter entity version is needed to ensure that modifications are applied sequentially (each editor is notified if a modificaton was added since his chapter copy was loaded and he must re-load the chapter)
• the version is forcibly increased at each modification (this is materialized in an UPDATE triggered against the chapter table immediately after aquiring the row-level lock)
• set PESSIMISTIC_FORCE_INCREMENT in the corresponding repository
• rely on two concurrent transactions to shape two scenarios: one that will lead to an exception of type OptimisticLockException and one that will lead to QueryTimeoutException

Note: Pay attention to the MySQL dialect: MySQL5Dialect (MyISAM) doesn't support row-level locking, MySQL5InnoDBDialect (InnoDB) acquires row-level lock via FOR UPDATE (timeout can be set), MySQL8Dialect (InnoDB) acquires row-level lock via FOR UPDATE NOWAIT.

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