Java Project: Designing an In-Memory Key-Value Store

1. The Architecture: The Storage Engine

The heart of our database is a ConcurrentHashMap<String, Entry<V>>.

We use a composite Entry object to store:
1. The Value.
2. The Expiry Timestamp (Current time + TTL).

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2. Implementing Generics: Type-Safe Storage

We want the caller to use our database like this: Database<User> db = new Database<>(). By using Bounded Generics, we can ensure that the database only accepts types that meet our criteria (e.g., must be Serializable).

3. The "Cleaner" Thread: Handling Expiry

Expired items don't delete themselves! We need a background thread to "Scan" the memory and delete items after their TTL has passed.

The Wrong Way: A thread that sleeps for 1 second and then checks 1,000,000 items. (Eats 100% CPU).
The Professional Way: Use a ScheduledExecutorService to only check a small "Sample" of the database every few seconds, or use a DelayQueue for precise, low-CPU deletions.

4. Concurrency: Solving the "Static Lock"

Since it's an in-memory database, multiple threads will be writing at once.

We use computeIfAbsent() and remove() from the ConcurrentHashMap to ensure that no two threads can update the same key at the same time.
This provides "Lock-Free" performance for most operations, allowing our database to handle $100,000$ operations per second on a standard server.

Frequently Asked Questions

Why not just use a standard HashMap? A standard HashMap is not thread-safe. If two people try to add a user simultaneously, the internal structure of the map can become corrupted, leading to an infinite loop that crashes your whole application.

Can I persist this to a file? Yes! For an "Advanced" version, use the Serializable interface or turn the data into JSON and write it to disk whenever the app shuts down. This gives you a "Snapshot" of your data that survives a restart.

Key Takeaway

Building a database is the ultimate "Mind-Expander." By designing a thread-safe, generic storage engine, you move from someone who "Uses tools" to an engineer who "Builds tools." You gain the low-level understanding of data structures required to optimize massive, global enterprise systems.

Part of the Java Enterprise Mastery — engineering the store.