shared vs local cache mode in JPA

shared vs local cache mode in JPA

Cache in JPA :

In JPA (Java Persistence API), caching is a mechanism used to improve performance by reducing the number of database queries. Caching stores frequently accessed data in memory, allowing subsequent requests for the same data to be served faster.

shared vs local cache mode in JPA

Shared Cache Mode :

Shared cache mode in JPA allows multiple persistence contexts (typically across different EntityManager instances or even different applications) to share a common cache. This means that data fetched by one EntityManager instance can be reused by another instance, promoting data consistency across the application.

Local Cache Mode :

Local cache mode, on the other hand, restricts caching to a single persistence context or EntityManager instance. Each EntityManager manages its own cache independently, and data fetched by one EntityManager is not accessible to others. This can lead to potential inconsistencies if multiple EntityManager instances are involved.

Advantages and Disadvantages

Shared Cache Mode:

  • Advantages :
    • 1.  Improved Performance : Reduces database round-trips by serving frequently accessed data from memory.
    • 2.  Consistency : Ensures data consistency across different parts of the application or different applications sharing the same data.
    • 3.  Reduced Resource Usage : Optimizes memory usage by sharing cached data among EntityManager instances.
  • Disadvantages :
    • 1.  Concurrency Concerns : Requires careful management of concurrent access to ensure data integrity and avoid stale data issues.
    • 2.  Complexity : Introduces complexity in managing cache invalidation and synchronization across multiple persistence contexts.

Local Cache Mode:

  • Advantages :
    • 1.  Isolation : Ensures data isolation and avoids potential inconsistencies between different persistence contexts.
    • 2.  Simplicity : Simplifies cache management within a single persistence context or application.
  • Disadvantages :
    • 1.  Performance Overhead : May lead to increased database queries and slower performance, especially in applications with complex data access patterns.
    • 2.  Increased Memory Usage : Each EntityManager maintains its own cache, potentially leading to higher memory consumption in applications with many EntityManager instances.

Example in Java

Let’s demonstrate how to configure and use shared cache mode and local cache mode in JPA.

1. Shared Cache Mode Configuration
java
import javax.persistence.Cache;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class Main {

    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("my-persistence-unit");
        EntityManager em1 = emf.createEntityManager();
        EntityManager em2 = emf.createEntityManager();

        try {
            // Enable shared cache mode
            Cache cache = emf.getCache();
            cache.evictAll(); // Clear existing cache
            cache.evict(User.class); // Clear cache for specific entity

            // Fetch data using first EntityManager
            em1.getTransaction().begin();
            User user1 = em1.find(User.class, 1L);
            em1.getTransaction().commit();

            // Fetch data using second EntityManager
            em2.getTransaction().begin();
            User user2 = em2.find(User.class, 1L); // Retrieves from shared cache if enabled
            em2.getTransaction().commit();
        } finally {
            em1.close();
            em2.close();
            emf.close();
        }
    }
}

In this example, emf.getCache().evictAll() clears the shared cache, and emf.getCache().evict(User.class) clears the cache for a specific entity (User in this case). Both em1 and em2 instances can access data from the shared cache.

2. Local Cache Mode Configuration
java
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class Main {

    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("my-persistence-unit");
        EntityManager em1 = emf.createEntityManager();
        EntityManager em2 = emf.createEntityManager();

        try {
            // Fetch data using first EntityManager
            em1.getTransaction().begin();
            User user1 = em1.find(User.class, 1L); // Retrieves from local cache of em1
            em1.getTransaction().commit();

            // Fetch data using second EntityManager
            em2.getTransaction().begin();
            User user2 = em2.find(User.class, 1L); // Retrieves independently, not from em1's cache
            em2.getTransaction().commit();
        } finally {
            em1.close();
            em2.close();
            emf.close();
        }
    }
}

In this example, each EntityManager (em1 and em2) maintains its own local cache. Data fetched by one EntityManager is not accessible to the other, ensuring data isolation.