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Hibernate / EclipseLink Interview questions

What is DDL generation in EclipseLink?

DDL generation is EclipseLink's ability to automatically create (or drop and recreate) database tables based on an application's entity mappings, controlled through the eclipselink.ddl-generation persistence unit property.

<property name="eclipselink.ddl-generation" value="create-tables"/>
<property name="eclipselink.ddl-generation.output-mode" value="database"/>

Common values include none (do nothing, the default for production use), create-tables (create tables that don't already exist), and drop-and-create-tables (wipe and recreate the schema on every startup). The output-mode property controls whether the generated DDL is applied directly to the database, written to a SQL script file, or both.

DDL generation is convenient for local development and quick prototyping, but production systems almost always manage schema changes through a dedicated migration tool instead, since automatic generation doesn't handle data migration, indexes, or constraints beyond what the entity mappings directly imply.

The property controlling automatic schema creation in EclipseLink is:
Why do production systems usually avoid relying on automatic DDL generation?

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What is EclipseLink? What is JPA (Jakarta Persistence API)? What is the relationship between EclipseLink and Jakarta Persistence? What is a Persistence Unit in EclipseLink? What is persistence.xml used for? What is an EntityManager in EclipseLink? What is an EntityManagerFactory? What is a JPA entity? What are EclipseLink's core components? What is EclipseLink MOXy? What is class weaving in EclipseLink? What is the EclipseLink shared cache (L2 cache)? What is a fetch group in EclipseLink? What is optimistic locking in EclipseLink and JPA? What is the @Convert annotation used for? What is a Customizer in EclipseLink? What is JPQL? What are EclipseLink query hints? What is DDL generation in EclipseLink? What is a UnitOfWork in EclipseLink's native API? What is EclipseLink's native Session API? What is static weaving in EclipseLink? What are EclipseLink descriptors? What is EclipseLink DBWS? What is EclipseLink SDO? Explain the lifecycle of a JPA entity managed by EclipseLink? Why is class weaving important for lazy loading in EclipseLink? How does EclipseLink's shared cache differ from Hibernate's second-level cache? What is the difference between static weaving and dynamic weaving? How do you configure batch fetching using EclipseLink query hints? When should you use pessimistic locking instead of optimistic locking? How do you troubleshoot lazy loading failures caused by missing weaving in EclipseLink? What is the difference between EAGER and LAZY fetch strategies, and how does EclipseLink implement lazy loading without a live session? How does EclipseLink implement change tracking for detecting entity modifications? Explain the internal working of EclipseLink's cache coordination? What is the difference between EclipseLink and Hibernate? How do you implement multitenancy in EclipseLink? Why use fetch groups instead of always fetching the full entity? What is the difference between attribute change tracking and deferred change detection? How does EclipseLink cache query results, and what invalidation strategies are available? When would you choose isolated cache over shared cache for a multitenant entity? How do you configure a Customizer to add native EclipseLink mappings not expressible via annotations? What is the difference between JPQL and the EclipseLink native Expression/Criteria API? Explain the internal working of EclipseLink's Unit of Work commit process? How do you optimize EclipseLink performance for a high-throughput application? What is the difference between EclipseLink's @Convert and JPA's standard AttributeConverter? How does EclipseLink decide when to use join fetch versus batch fetch? Why should you use @Version for optimistic locking instead of manual version checks? What is the difference between EntityManager persistence context types: transaction-scoped vs extended? How do you troubleshoot stale data issues caused by EclipseLink's shared cache?
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