Defines contracts for transactional and concurrent access to cached {@link org.hibernate.cache.access.EntityRegionAccessStrategy entity} and {@link org.hibernate.cache.access.CollectionRegionAccessStrategy collection} data. Transactions pass in a timestamp indicating transaction start time which is then used to protect against concurrent access (exactly how that occurs is based on the actual access-strategy impl used). Two different implementation patterns are provided for.

- A transaction-aware cache implementation might be wrapped by a synchronous access strategy, where updates to the cache are written to the cache inside the transaction.
- A non-transaction-aware cache would be wrapped by an asynchronous access strategy, where items are merely "soft locked" during the transaction and then updated during the "after transaction completion" phase; the soft lock is not an actual lock on the database row - only upon the cached representation of the item.

The asynchronous access strategies are: {@link org.hibernate.cache.access.AccessType.READ_ONLY read-only}, {@link org.hibernate.cache.access.AccessType.READ_WRITE read-write} and {@link org.hibernate.cache.access.AccessType.NONSTRICT_READ_WRITE nonstrict-read-write}. The only synchronous access strategy is {@link org.hibernate.cache.access.AccessType.TRANSACTIONAL transactional}.

Note that, for an asynchronous cache, cache invalidation must be a two step process (lock->unlock or lock->afterUpdate), since this is the only way to guarantee consistency with the database for a nontransactional cache implementation. For a synchronous cache, cache invalidation is a single step process (evict or update). Hence, these contracts ({@link org.hibernate.cache.access.EntityRegionAcessStrategy} and {@link org.hibernate.cache.access.CollectionRegionAccessStrategy}) define a three step process to cater for both models (see the individual contracts for details).

Note that query result caching does not go through an access strategy; those caches are managed directly against the underlying {@link org.hibernate.cache.QueryResultsRegion}.