In-Memory Database (Thread-Safe with TTL)

A high-performance, command-driven in-memory key-value database written in Java. Engineered with custom thread-safety primitives, lazy/proactive Time-To-Live (TTL) expiration algorithms, and a detached command parser architecture.

Features

• Command-Driven CLI Interface: Parses and executes PUT, GET, DELETE, START, STOP, and EXIT commands.
• TTL Entry Auto-Expiration: Dual-strategy expiration using Lazy Eviction (during GET lookups) and a Proactive Background Daemon (periodic sweeps).
• Fine-Grained Concurrency: Transitioned from coarse synchronized methods to ConcurrentHashMap for high-throughput multi-threaded operations.
• Volatile Lifecycle Controls: Instant thread-wide visibility for database START/STOP states using thread safety primitives without locking overhead.
• Fail-Fast Error Handling: Custom exceptions for syntax errors, invalid TTL constraints, missing keys, and illegal operations when stopped.

Tech Stack

• Language: Java 8+
• Concurrency Tools: ConcurrentHashMap, ScheduledExecutorService, volatile flags
• Design Patterns: Command Pattern, Separation of Concerns

Command Reference

Command	Syntax	Description
PUT	PUT <key> <value> [ttl_ms]	Inserts or updates an entry (optional TTL in milliseconds).
GET	GET <key>	Retrieves active entry value; purges key if expired.
DELETE	DELETE <key>	Explicitly removes a key-value entry.
START	START	Sets volatile state to active and enables writes.
STOP	STOP	Suspends write operations and rejects new mutations.
EXIT	EXIT	Terminates background cleanup threads and stops process.

How It Works

1. Input Parsing: The command parser tokenizes raw input into structured execution objects before reaching the database engine.
2. Concurrent Storage: Integer keys route to entry objects containing data values alongside epoch millisecond expiration timestamps.
3. Hybrid Expiration Sweeps:

• Lazy Check: Checks current epoch time against key metadata on GET.
• Proactive Cleanup: A dedicated thread sweeps the database periodically to reclaim memory from abandoned keys.

Getting Started

1. Build

javac Main.java

2. Run

java Main