✅ Short Answer
You need a second-level cache (2LC) when your application frequently reads the same data across sessions and wants to avoid repeated database hits, especially for read-heavy, rarely changing data.
🔎 Detailed Explanation
🔹 When 2LC is useful
✅ When the same entities are read often by many users or processes, and don’t change frequently.
✅ When you want to reduce database load by caching data across sessions → improves scalability.
✅ When you have shared reference data (e.g., countries, currencies, product categories) → great candidates for 2LC.
✅ When your app needs fast response times → fetching from cache is much faster than hitting the DB.
🔹 When 2LC is not helpful (or even harmful)
❌ When entities are highly volatile → frequent updates would cause constant cache invalidations → adds overhead without benefit.
❌ When your application is write-heavy, since cache synchronization adds complexity.
❌ When strict real-time consistency is required → 2LC may serve stale data briefly depending on concurrency strategy.
🔹 Examples of good 2LC use cases
✅ Product catalogs in an e-commerce app → mostly reads, infrequent updates.
✅ User roles or permissions → rarely change, but read on every request.
✅ Static lookup tables → like states, time zones, or fixed configuration entities.
📊 Benefits of Second-Level Cache
✅ Reduce DB roundtrips → fewer selects for repeated entity loads.
✅ Improve scalability → lessen DB bottleneck, especially under high concurrent read load.
✅ Lower latency → fetch data quickly from memory.
📌 Key Takeaways
✅ Use second-level cache when you have read-mostly, shared data that benefits from reuse across sessions.
✅ Avoid it for frequently modified data → synchronization overhead can negate benefits.
✅ Always choose the right concurrency strategy (READ_ONLY, READ_WRITE, etc.) to match your data’s update patterns.