Where is cache stored in JPA when second-level caching is enabled?

When second-level caching (L2 cache) is enabled in JPA, the cache stores entity data outside the scope of a single `EntityManager` or persistence context. Unlike the first-level cache, which is tied to the `EntityManager` and lives in its memory, the second-level cache is a shared cache designed to be accessible across multiple `EntityManager` instances and potentially multiple application instances in a clustered environment.

Understanding Second-Level Cache Storage

The exact storage location of the second-level cache is not directly managed by the JPA specification itself. Instead, it is determined by the specific JPA provider (e.g., Hibernate, EclipseLink) and the underlying caching solution that is integrated and configured for the L2 cache.

The L2 cache is typically configured as a shared cache region, and its data persistence can vary widely based on the chosen cache provider and its configuration.

Common Storage Mechanisms

• In-memory (JVM Heap): This is the most common storage mechanism for smaller to medium-sized caches. The cache provider (e.g., EHCache, Infinispan, Caffeine) allocates a portion of the application's JVM heap to store cached entities. This is still within the application's process but managed by the cache provider, distinct from the EntityManager's heap for the first-level cache.
• Off-heap memory: Some advanced caching solutions (e.g., EHCache 3, Infinispan) offer the ability to store cache data outside the JVM heap but still within the same process. This can help reduce garbage collection overhead for large caches.
• Distributed Caches: For clustered or highly scalable applications, the second-level cache can be stored across multiple JVMs or dedicated cache servers. This allows different application instances to share the same cache data. Examples include Redis, Memcached, Coherence, Apache Ignite, or Infinispan configured in a clustered mode.
• Disk/File System: To provide persistence across application restarts or to handle very large caches that exceed available memory, some cache providers can overflow data to a local disk or file system. This often involves serialization of entities to files.
• Database: While less common for the primary storage of a high-performance L2 cache, some caching solutions might use a database for cache persistence or to maintain cache cluster state, especially if strong durability guarantees are required.

Key Factors Influencing Storage

• JPA Provider: Different providers have preferred or built-in caching integrations (e.g., Hibernate often uses EHCache or Infinispan by default).
• Underlying Cache Provider: The specific caching library chosen (e.g., EHCache, Infinispan, Redis, Caffeine) dictates the available storage options.
• Configuration: The persistence.xml and the cache provider's configuration files (e.g., ehcache.xml) explicitly define the storage strategy, size, eviction policies, and other parameters.
• Application Architecture: A standalone application might use in-memory or disk-based storage, while a clustered application typically requires a distributed cache.

In summary, the JPA second-level cache is a shared resource whose storage location is managed by a separate caching solution integrated with the JPA provider, offering flexibility from in-memory to distributed and disk-based options depending on the application's performance and scalability requirements.