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13.1 Redis-Based Global Caching and Distributed Session Sharing

Redis transcends being a simple in-memory database — it is the foundational distributed systems infrastructure for synchronizing shared state (cache, sessions, locks) across multiple server instances.

1. Spring Cache Abstraction (@Cacheable) with Redis​

Spring's caching abstraction uses @Cacheable, @CacheEvict, and @CachePut annotations to automatically store and retrieve method results from Redis, completely decoupling this logic from your business code.

implementation 'org.springframework.boot:spring-boot-starter-data-redis'
spring:
  data:
    redis:
      host: localhost
      port: 6379
      # password: Always set in production environments
@Configuration
@EnableCaching  // Activates all cache annotation functionality
public class CacheConfig {
    @Bean
    public RedisCacheManager cacheManager(RedisConnectionFactory factory) {
        RedisCacheConfiguration config = RedisCacheConfiguration.defaultCacheConfig()
                .entryTtl(Duration.ofMinutes(30))           // 30-minute cache TTL
                .disableCachingNullValues()                  // Don't cache null values
                .serializeValuesWith(RedisSerializationContext.SerializationPair
                        .fromSerializer(new GenericJackson2JsonRedisSerializer()));
        return RedisCacheManager.builder(factory)
                .cacheDefaults(config)
                .build();
    }
}
@Service
@RequiredArgsConstructor
public class ProductService {
    private final ProductRepository productRepository;

    // Returns cached result from Redis if available — skips DB query entirely
    @Cacheable(value = "products", key = "#productId")
    public ProductDto getProduct(Long productId) {
        return productRepository.findById(productId)
                .map(ProductDto::from)
                .orElseThrow(() -> new NotFoundException("Product not found."));
    }

    // Evicts the cache on update so next read fetches fresh data from DB
    @CacheEvict(value = "products", key = "#productId")
    public void updateProduct(Long productId, ProductUpdateDto dto) {
        // ...update logic
    }
}

2. Distributing HttpSession via Redis (Spring Session)​

In a scale-out environment, server instance A cannot read sessions created by instance B. With Spring Session, you can switch the session store to Redis without changing a single line of existing HttpSession code.

implementation 'org.springframework.session:spring-session-data-redis'
spring:
  session:
    store-type: redis
    timeout: 30m

With this single configuration, all session R/W via request.getSession() routes to Redis instead of local WAS memory — enabling seamless session sharing across any number of servers behind a load balancer.

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