Storage Backends

/**
 * Implementation of KVStore that keeps data deserialized in memory.
 * Does not persist data; suitable for testing and development.
 */
public class InMemoryStore implements KVStore {
    
    /**
     * Creates a new in-memory store instance.
     */
    public InMemoryStore();
    
    // Inherits all KVStore interface methods
}

/**
 * Implementation of KVStore that uses LevelDB as the underlying data store.
 * Provides persistent storage with compression and efficient indexing.
 */
public class LevelDB implements KVStore {
    
    /**
     * Store version for compatibility checking.
     */
    public static final long STORE_VERSION = 1L;
    
    /**
     * Key used to store version information.
     */
    public static final byte[] STORE_VERSION_KEY;
    
    /**
     * Creates a LevelDB store with default serializer.
     * @param path Directory path where LevelDB files will be stored
     * @throws Exception if store creation fails
     */
    public LevelDB(File path) throws Exception;
    
    /**
     * Creates a LevelDB store with custom serializer.
     * @param path Directory path where LevelDB files will be stored
     * @param serializer Custom serializer for data transformation
     * @throws Exception if store creation fails
     */
    public LevelDB(File path, KVStoreSerializer serializer) throws Exception;
    
    // Inherits all KVStore interface methods
}

/**
 * Implementation of KVStore that uses RocksDB as the underlying data store.
 * Provides high-performance persistent storage optimized for modern hardware.
 */
public class RocksDB implements KVStore {
    
    /**
     * Store version for compatibility checking.
     */
    public static final long STORE_VERSION = 1L;
    
    /**
     * Key used to store version information.
     */
    public static final byte[] STORE_VERSION_KEY;
    
    /**
     * Creates a RocksDB store with default serializer.
     * @param path Directory path where RocksDB files will be stored
     * @throws Exception if store creation fails
     */
    public RocksDB(File path) throws Exception;
    
    /**
     * Creates a RocksDB store with custom serializer.
     * @param path Directory path where RocksDB files will be stored
     * @param serializer Custom serializer for data transformation
     * @throws Exception if store creation fails
     */
    public RocksDB(File path, KVStoreSerializer serializer) throws Exception;
    
    // Inherits all KVStore interface methods
}

import org.apache.spark.util.kvstore.*;
import java.io.File;

// In-memory store for testing
KVStore memoryStore = new InMemoryStore();

// LevelDB for moderate production use
KVStore levelStore = new LevelDB(new File("/data/app/leveldb"));

// RocksDB for high-performance scenarios
KVStore rocksStore = new RocksDB(new File("/data/app/rocksdb"));

// All stores implement the same interface
public void processData(KVStore store) {
    // Same code works with any backend
    store.write(new MyData("key1", "value1"));
    MyData data = store.read(MyData.class, "key1");
    long count = store.count(MyData.class);
}

// Create custom serializer by extending KVStoreSerializer
public class CustomKVStoreSerializer extends KVStoreSerializer {
    public CustomKVStoreSerializer() {
        super();
        // Configure the inherited ObjectMapper for custom needs
        mapper.registerModule(new com.fasterxml.jackson.datatype.jsr310.JavaTimeModule());
    }
}

// Use with any persistent backend
KVStore store = new LevelDB(new File("/data/store"), new CustomKVStoreSerializer());
// or
KVStore store = new RocksDB(new File("/data/store"), new CustomKVStoreSerializer());

// Store objects with Java 8 time types
public class Event {
    @KVIndex
    public String id;
    
    @KVIndex("timestamp")  
    public java.time.Instant timestamp;
    
    public Event(String id, java.time.Instant timestamp) {
        this.id = id;
        this.timestamp = timestamp;
    }
}

store.write(new Event("event1", java.time.Instant.now()));

// LevelDB with larger write buffer for bulk operations
System.setProperty("leveldb.writebuffer.size", "67108864"); // 64MB
KVStore levelStore = new LevelDB(new File("/data/leveldb"));

// RocksDB automatically uses optimized settings for:
// - Bloom filters for faster key lookups
// - Multi-level compression (LZ4 + ZSTD)  
// - Index compression disabled for faster access
// - Optimized block restart intervals
KVStore rocksStore = new RocksDB(new File("/data/rocksdb"));

// Batch operations for better performance
List<MyData> largeBatch = generateLargeDataset();
for (MyData item : largeBatch) {
    store.write(item);  // Each backend optimizes batch writes internally
}

/**
 * Choose storage backend based on requirements:
 */

// Development and Testing
if (environment.equals("test") || dataSize < 100_000) {
    return new InMemoryStore();
}

// Production with moderate data volumes (< 10GB)
if (dataSize < 10_000_000 && !requiresHighThroughput) {
    return new LevelDB(dataPath);
}

// High-performance production (large data, high throughput)
if (hasSSDs && requiresHighThroughput) {
    return new RocksDB(dataPath);
}

// Default safe choice for production
return new LevelDB(dataPath);

// All stores should be properly closed
try (KVStore store = new LevelDB(new File("/data/store"))) {
    
    // Perform operations
    store.write(data);
    MyData result = store.read(MyData.class, key);
    
    // Store is automatically closed due to try-with-resources
    
} catch (Exception e) {
    logger.error("Store operation failed", e);
}

// Manual resource management
KVStore store = new RocksDB(new File("/data/store"));
try {
    // Use store
    processData(store);
} finally {
    store.close();  // Always close to release resources
}

/**
 * Utility to migrate data between different storage backends.
 */
public class StoreMigration {
    
    public static void migrate(KVStore source, KVStore target, Class<?>... types) 
            throws Exception {
        
        for (Class<?> type : types) {
            // Copy metadata
            Object metadata = source.getMetadata(type);
            if (metadata != null) {
                target.setMetadata(metadata);
            }
            
            // Copy all instances of each type
            KVStoreView<?> view = source.view(type);
            for (Object item : view) {
                target.write(item);
            }
        }
    }
}

// Example: Upgrade from LevelDB to RocksDB
KVStore oldStore = new LevelDB(new File("/data/old"));
KVStore newStore = new RocksDB(new File("/data/new"));

try {
    StoreMigration.migrate(oldStore, newStore, 
        Employee.class, Task.class, Project.class);
} finally {
    oldStore.close();
    newStore.close();
}

// Store statistics and health checks
public class StoreMonitoring {
    
    public static void checkStoreHealth(KVStore store, Class<?>... types) 
            throws Exception {
        
        System.out.println("Store Health Check:");
        
        for (Class<?> type : types) {
            long count = store.count(type);
            System.out.println(type.getSimpleName() + ": " + count + " items");
        }
        
        // Check if store is responsive
        long startTime = System.currentTimeMillis();
        store.getMetadata(String.class);  // Simple read operation
        long responseTime = System.currentTimeMillis() - startTime;
        
        System.out.println("Response time: " + responseTime + "ms");
        
        if (responseTime > 1000) {
            System.out.println("WARNING: Store is responding slowly");
        }
    }
}