उन्नत कैश पैटर्न

import java.util.HashMap;
import java.util.Map;

// Simplified Read-Through Cache concept
class ProductCache {
  private Map<String, String> cache = new HashMap<>();
  private DatabaseService db = new DatabaseService();

  public String getProduct(String productId) {
    // 1. Check cache
    if (cache.containsKey(productId)) {
      System.out.println("Cache hit for " + productId);
      return cache.get(productId);
    }

    // 2. Cache miss, fetch from DB
    System.out.println("Cache miss for " + productId + ", fetching from DB.");
    String productData = db.fetchProductFromDB(productId);

    // 3. Store in cache and return
    cache.put(productId, productData);
    return productData;
  }
}

class DatabaseService {
  public String fetchProductFromDB(String productId) {
    // Simulate DB call
    return "Product_" + productId + "_Details";
  }
}

public class Main {
  public static void main(String[] args) {
    ProductCache productCache = new ProductCache();
    System.out.println(productCache.getProduct("P1")); // Miss, then hit
    System.out.println(productCache.getProduct("P1")); // Hit
  }
}

import java.util.HashMap;
import java.util.Map;

// Simplified Write-Back Cache concept
class DataCache {
  private Map<String, String> cache = new HashMap<>();
  private DatabaseService db = new DatabaseService();

  public void updateData(String key, String value) {
    // 1. Write to cache immediately
    cache.put(key, value);
    System.out.println("Data '" + key + "' updated in cache.");

    // 2. Schedule asynchronous write to DB
    // In a real system, this would be a separate thread/queue
    new Thread(() -> {
      try {
        Thread.sleep(100); // Simulate async DB write delay
        db.saveDataToDB(key, value);
        System.out.println("Data '" + key + "' written to DB asynchronously.");
      } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
      }
    }).start();
  }
}

class DatabaseService {
  public void saveDataToDB(String key, String value) {
    // Simulate DB write
    System.out.println("Saving '" + key + ":" + value + "' to database.");
  }
}

public class Main {
  public static void main(String[] args) {
    DataCache dataCache = new DataCache();
    dataCache.updateData("User1", "NewEmail@example.com");
    System.out.println("Application continues immediately...");
    // In a real app, you'd handle cache shutdown gracefully to ensure writes complete.
  }
}

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

// Simplified Refresh-Ahead Cache concept
class ItemCache {
  private ConcurrentHashMap<String, String> cache = new ConcurrentHashMap<>();
  private ConcurrentHashMap<String, Long> expirationTimes = new ConcurrentHashMap<>();
  private DatabaseService db = new DatabaseService();
  private ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();

  private final long CACHE_TTL_MS = 10000; // 10 seconds
  private final long REFRESH_THRESHOLD_MS = 2000; // Refresh 2 seconds before expiry

  public ItemCache() {
    // Simulate initial data load
    cache.put("ItemA", "DataA_V1");
    expirationTimes.put("ItemA", System.currentTimeMillis() + CACHE_TTL_MS);
  }

  public String getItem(String itemId) {
    if (cache.containsKey(itemId)) {
      long currentExpiry = expirationTimes.get(itemId);
      long timeToLive = currentExpiry - System.currentTimeMillis();

      // If item is nearing expiration, schedule a refresh
      if (timeToLive > 0 && timeToLive < REFRESH_THRESHOLD_MS) {
        System.out.println("Item " + itemId + " nearing expiry, scheduling refresh.");
        scheduler.schedule(() -> refreshItem(itemId), 0, TimeUnit.MILLISECONDS);
      }
      return cache.get(itemId);
    }
    // Fallback to read-through if not in cache (simplified)
    System.out.println("Item " + itemId + " not in cache, fetching fresh.");
    String data = db.fetchItemFromDB(itemId);
    cache.put(itemId, data);
    expirationTimes.put(itemId, System.currentTimeMillis() + CACHE_TTL_MS);
    return data;
  }

  private void refreshItem(String itemId) {
    System.out.println("Refreshing item " + itemId + " from DB...");
    String freshData = db.fetchItemFromDB(itemId + "_Refreshed"); // Simulate updated data
    cache.put(itemId, freshData);
    expirationTimes.put(itemId, System.currentTimeMillis() + CACHE_TTL_MS);
    System.out.println("Item " + itemId + " refreshed with " + freshData);
  }
}

class DatabaseService {
  public String fetchItemFromDB(String itemId) {
    // Simulate DB call
    return "Data for " + itemId + " from DB";
  }
}

public class Main {
  public static void main(String[] args) throws InterruptedException {
    ItemCache itemCache = new ItemCache();
    System.out.println("First access: " + itemCache.getItem("ItemA"));
    Thread.sleep(8500); // Wait until it's near expiration
    System.out.println("Second access (triggers refresh): " + itemCache.getItem("ItemA"));
    Thread.sleep(500); // Give refresh a chance to run
    System.out.println("Third access (should be refreshed): " + itemCache.getItem("ItemA"));
    // A real app would shut down the scheduler
  }
}