Changed database implementation. Removed static objects.

caching/src/main/java/com/iluwatar/caching/App.java

@@ -23,6 +23,8 @@
 
 package com.iluwatar.caching;
 
+import com.iluwatar.caching.database.DbManager;
+import com.iluwatar.caching.database.DbManagerFactory;
 import lombok.extern.slf4j.Slf4j;
 
 /**
@@ -55,12 +57,26 @@ import lombok.extern.slf4j.Slf4j;
  * <i>{@literal App --> AppManager --> CacheStore/LRUCache/CachingPolicy --> DBManager} </i>
  * </p>
  *
+ * <p>
+ *     To run the application with MongoDb, just start it with parameter --mongo
+ *     Example: java -jar app.jar --mongo
+ * </p>
+ *
  * @see CacheStore
  * @see LruCache
  * @see CachingPolicy
  */
 @Slf4j
 public class App {
+    private static final String USE_MONGO_DB = "--mongo";
+    private DbManager dbManager;
+    private AppManager appManager;
+
+    public App(boolean isMongo) {
+        dbManager = DbManagerFactory.initDb(isMongo);
+        appManager = new AppManager(dbManager);
+        appManager.initDb();
+    }
 
     /**
      * Program entry point.
@@ -68,31 +84,44 @@ public class App {
      * @param args command line args
      */
     public static void main(String[] args) {
-    AppManager.initDb(false); // VirtualDB (instead of MongoDB) was used in running the JUnit tests
+        // VirtualDB (instead of MongoDB) was used in running the JUnit tests
         // and the App class to avoid Maven compilation errors. Set flag to
         // true to run the tests with MongoDB (provided that MongoDB is
         // installed and socket connection is open).
-    AppManager.initCacheCapacity(3);
+        App app = new App(isDbMongo(args));
-    var app = new App();
         app.useReadAndWriteThroughStrategy();
         app.useReadThroughAndWriteAroundStrategy();
         app.useReadThroughAndWriteBehindStrategy();
         app.useCacheAsideStategy();
     }
 
+    /**
+     * Check the input parameters. if
+     * @param args input params
+     * @return true if there is "--mongo" parameter in arguments
+     */
+    private static boolean isDbMongo(String[] args) {
+        for (String arg : args) {
+            if (arg.equals(USE_MONGO_DB)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
     /**
      * Read-through and write-through.
      */
     public void useReadAndWriteThroughStrategy() {
         LOGGER.info("# CachingPolicy.THROUGH");
-    AppManager.initCachingPolicy(CachingPolicy.THROUGH);
+        appManager.initCachingPolicy(CachingPolicy.THROUGH);
 
         var userAccount1 = new UserAccount("001", "John", "He is a boy.");
 
-    AppManager.save(userAccount1);
+        appManager.save(userAccount1);
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
-    AppManager.find("001");
+        appManager.find("001");
-    AppManager.find("001");
+        appManager.find("001");
     }
 
     /**
@@ -100,21 +129,21 @@ public class App {
      */
     public void useReadThroughAndWriteAroundStrategy() {
         LOGGER.info("# CachingPolicy.AROUND");
-    AppManager.initCachingPolicy(CachingPolicy.AROUND);
+        appManager.initCachingPolicy(CachingPolicy.AROUND);
 
         var userAccount2 = new UserAccount("002", "Jane", "She is a girl.");
 
-    AppManager.save(userAccount2);
+        appManager.save(userAccount2);
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
-    AppManager.find("002");
+        appManager.find("002");
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
-    userAccount2 = AppManager.find("002");
+        userAccount2 = appManager.find("002");
         userAccount2.setUserName("Jane G.");
-    AppManager.save(userAccount2);
+        appManager.save(userAccount2);
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
-    AppManager.find("002");
+        appManager.find("002");
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
-    AppManager.find("002");
+        appManager.find("002");
     }
 
     /**
@@ -122,23 +151,23 @@ public class App {
      */
     public void useReadThroughAndWriteBehindStrategy() {
         LOGGER.info("# CachingPolicy.BEHIND");
-    AppManager.initCachingPolicy(CachingPolicy.BEHIND);
+        appManager.initCachingPolicy(CachingPolicy.BEHIND);
 
         var userAccount3 = new UserAccount("003", "Adam", "He likes food.");
         var userAccount4 = new UserAccount("004", "Rita", "She hates cats.");
         var userAccount5 = new UserAccount("005", "Isaac", "He is allergic to mustard.");
 
-    AppManager.save(userAccount3);
+        appManager.save(userAccount3);
-    AppManager.save(userAccount4);
+        appManager.save(userAccount4);
-    AppManager.save(userAccount5);
+        appManager.save(userAccount5);
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
-    AppManager.find("003");
+        appManager.find("003");
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
         UserAccount userAccount6 = new UserAccount("006", "Yasha", "She is an only child.");
-    AppManager.save(userAccount6);
+        appManager.save(userAccount6);
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
-    AppManager.find("004");
+        appManager.find("004");
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
     }
 
     /**
@@ -146,20 +175,20 @@ public class App {
      */
     public void useCacheAsideStategy() {
         LOGGER.info("# CachingPolicy.ASIDE");
-    AppManager.initCachingPolicy(CachingPolicy.ASIDE);
+        appManager.initCachingPolicy(CachingPolicy.ASIDE);
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
 
         var userAccount3 = new UserAccount("003", "Adam", "He likes food.");
         var userAccount4 = new UserAccount("004", "Rita", "She hates cats.");
         var userAccount5 = new UserAccount("005", "Isaac", "He is allergic to mustard.");
-    AppManager.save(userAccount3);
+        appManager.save(userAccount3);
-    AppManager.save(userAccount4);
+        appManager.save(userAccount4);
-    AppManager.save(userAccount5);
+        appManager.save(userAccount5);
 
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
-    AppManager.find("003");
+        appManager.find("003");
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
-    AppManager.find("004");
+        appManager.find("004");
-    LOGGER.info(AppManager.printCacheContent());
+        LOGGER.info(appManager.printCacheContent());
     }
 }

caching/src/main/java/com/iluwatar/caching/AppManager.java

@@ -23,8 +23,9 @@
 
 package com.iluwatar.caching;
 
-import java.text.ParseException;
 import java.util.Optional;
+
+import com.iluwatar.caching.database.DbManager;
 import lombok.extern.slf4j.Slf4j;
 
 /**
@@ -35,11 +36,15 @@ import lombok.extern.slf4j.Slf4j;
  * CacheStore class.
  */
 @Slf4j
-public final class AppManager {
+public class AppManager {
 
   private static CachingPolicy cachingPolicy;
+  private final DbManager dbManager;
+  private final CacheStore cacheStore;
 
-  private AppManager() {
+  public AppManager(DbManager dbManager) {
+    this.dbManager = dbManager;
+    this.cacheStore = new CacheStore(dbManager);
   }
 
   /**
@@ -47,41 +52,29 @@ public final class AppManager {
    * data storage or a simple Java data structure to (temporarily) store the data/objects during
    * runtime.
    */
-  public static void initDb(boolean useMongoDb) {
+  public void initDb() {
-    if (useMongoDb) {
+     dbManager.connect();
-      try {
-        DbManager.connect();
-      } catch (ParseException e) {
-        LOGGER.error("Error connecting to MongoDB", e);
-      }
-    } else {
-      DbManager.createVirtualDb();
-    }
   }
 
   /**
    * Initialize caching policy.
    */
-  public static void initCachingPolicy(CachingPolicy policy) {
+  public void initCachingPolicy(CachingPolicy policy) {
     cachingPolicy = policy;
     if (cachingPolicy == CachingPolicy.BEHIND) {
-      Runtime.getRuntime().addShutdownHook(new Thread(CacheStore::flushCache));
+      Runtime.getRuntime().addShutdownHook(new Thread(cacheStore::flushCache));
     }
-    CacheStore.clearCache();
+    cacheStore.clearCache();
-  }
-
-  public static void initCacheCapacity(int capacity) {
-    CacheStore.initCapacity(capacity);
   }
 
   /**
    * Find user account.
    */
-  public static UserAccount find(String userId) {
+  public UserAccount find(String userId) {
     if (cachingPolicy == CachingPolicy.THROUGH || cachingPolicy == CachingPolicy.AROUND) {
-      return CacheStore.readThrough(userId);
+      return cacheStore.readThrough(userId);
     } else if (cachingPolicy == CachingPolicy.BEHIND) {
-      return CacheStore.readThroughWithWriteBackPolicy(userId);
+      return cacheStore.readThroughWithWriteBackPolicy(userId);
     } else if (cachingPolicy == CachingPolicy.ASIDE) {
       return findAside(userId);
     }
@@ -91,38 +84,38 @@ public final class AppManager {
   /**
    * Save user account.
    */
-  public static void save(UserAccount userAccount) {
+  public void save(UserAccount userAccount) {
     if (cachingPolicy == CachingPolicy.THROUGH) {
-      CacheStore.writeThrough(userAccount);
+      cacheStore.writeThrough(userAccount);
     } else if (cachingPolicy == CachingPolicy.AROUND) {
-      CacheStore.writeAround(userAccount);
+      cacheStore.writeAround(userAccount);
     } else if (cachingPolicy == CachingPolicy.BEHIND) {
-      CacheStore.writeBehind(userAccount);
+      cacheStore.writeBehind(userAccount);
     } else if (cachingPolicy == CachingPolicy.ASIDE) {
       saveAside(userAccount);
     }
   }
 
-  public static String printCacheContent() {
+  public String printCacheContent() {
-    return CacheStore.print();
+    return cacheStore.print();
   }
 
   /**
    * Cache-Aside save user account helper.
    */
-  private static void saveAside(UserAccount userAccount) {
+  private void saveAside(UserAccount userAccount) {
-    DbManager.updateDb(userAccount);
+    dbManager.updateDb(userAccount);
-    CacheStore.invalidate(userAccount.getUserId());
+    cacheStore.invalidate(userAccount.getUserId());
   }
 
   /**
    * Cache-Aside find user account helper.
    */
-  private static UserAccount findAside(String userId) {
+  private UserAccount findAside(String userId) {
-    return Optional.ofNullable(CacheStore.get(userId))
+    return Optional.ofNullable(cacheStore.get(userId))
         .or(() -> {
-          Optional<UserAccount> userAccount = Optional.ofNullable(DbManager.readFromDb(userId));
+          Optional<UserAccount> userAccount = Optional.ofNullable(dbManager.readFromDb(userId));
-          userAccount.ifPresent(account -> CacheStore.set(userId, account));
+          userAccount.ifPresent(account -> cacheStore.set(userId, account));
           return userAccount;
         })
         .orElse(null);

caching/src/main/java/com/iluwatar/caching/CacheStore.java

@@ -26,6 +26,8 @@ package com.iluwatar.caching;
 import java.util.List;
 import java.util.Optional;
 import java.util.stream.Collectors;
+
+import com.iluwatar.caching.database.DbManager;
 import lombok.extern.slf4j.Slf4j;
 
 /**
@@ -33,16 +35,20 @@ import lombok.extern.slf4j.Slf4j;
  */
 @Slf4j
 public class CacheStore {
+  private static final int CAPACITY = 3;
 
   private static LruCache cache;
+  private DbManager dbManager;
 
-  private CacheStore() {
+  public CacheStore(DbManager dbManager) {
+    this.dbManager = dbManager;
+    initCapacity(CAPACITY);
   }
 
   /**
    * Init cache capacity.
    */
-  public static void initCapacity(int capacity) {
+  public void initCapacity(int capacity) {
     if (cache == null) {
       cache = new LruCache(capacity);
     } else {
@@ -53,13 +59,13 @@ public class CacheStore {
   /**
    * Get user account using read-through cache.
    */
-  public static UserAccount readThrough(String userId) {
+  public UserAccount readThrough(String userId) {
     if (cache.contains(userId)) {
       LOGGER.info("# Cache Hit!");
       return cache.get(userId);
     }
     LOGGER.info("# Cache Miss!");
-    UserAccount userAccount = DbManager.readFromDb(userId);
+    UserAccount userAccount = dbManager.readFromDb(userId);
     cache.set(userId, userAccount);
     return userAccount;
   }
@@ -67,11 +73,11 @@ public class CacheStore {
   /**
    * Get user account using write-through cache.
    */
-  public static void writeThrough(UserAccount userAccount) {
+  public void writeThrough(UserAccount userAccount) {
     if (cache.contains(userAccount.getUserId())) {
-      DbManager.updateDb(userAccount);
+      dbManager.updateDb(userAccount);
     } else {
-      DbManager.writeToDb(userAccount);
+      dbManager.writeToDb(userAccount);
     }
     cache.set(userAccount.getUserId(), userAccount);
   }
@@ -79,30 +85,30 @@ public class CacheStore {
   /**
    * Get user account using write-around cache.
    */
-  public static void writeAround(UserAccount userAccount) {
+  public void writeAround(UserAccount userAccount) {
     if (cache.contains(userAccount.getUserId())) {
-      DbManager.updateDb(userAccount);
+      dbManager.updateDb(userAccount);
       cache.invalidate(userAccount.getUserId()); // Cache data has been updated -- remove older
       // version from cache.
     } else {
-      DbManager.writeToDb(userAccount);
+      dbManager.writeToDb(userAccount);
     }
   }
 
   /**
    * Get user account using read-through cache with write-back policy.
    */
-  public static UserAccount readThroughWithWriteBackPolicy(String userId) {
+  public UserAccount readThroughWithWriteBackPolicy(String userId) {
     if (cache.contains(userId)) {
       LOGGER.info("# Cache Hit!");
       return cache.get(userId);
     }
     LOGGER.info("# Cache Miss!");
-    UserAccount userAccount = DbManager.readFromDb(userId);
+    UserAccount userAccount = dbManager.readFromDb(userId);
     if (cache.isFull()) {
       LOGGER.info("# Cache is FULL! Writing LRU data to DB...");
       UserAccount toBeWrittenToDb = cache.getLruData();
-      DbManager.upsertDb(toBeWrittenToDb);
+      dbManager.upsertDb(toBeWrittenToDb);
     }
     cache.set(userId, userAccount);
     return userAccount;
@@ -111,11 +117,11 @@ public class CacheStore {
   /**
    * Set user account.
    */
-  public static void writeBehind(UserAccount userAccount) {
+  public void writeBehind(UserAccount userAccount) {
     if (cache.isFull() && !cache.contains(userAccount.getUserId())) {
       LOGGER.info("# Cache is FULL! Writing LRU data to DB...");
       UserAccount toBeWrittenToDb = cache.getLruData();
-      DbManager.upsertDb(toBeWrittenToDb);
+      dbManager.upsertDb(toBeWrittenToDb);
     }
     cache.set(userAccount.getUserId(), userAccount);
   }
@@ -123,7 +129,7 @@ public class CacheStore {
   /**
    * Clears cache.
    */
-  public static void clearCache() {
+  public void clearCache() {
     if (cache != null) {
       cache.clear();
     }
@@ -132,18 +138,18 @@ public class CacheStore {
   /**
    * Writes remaining content in the cache into the DB.
    */
-  public static void flushCache() {
+  public void flushCache() {
     LOGGER.info("# flushCache...");
     Optional.ofNullable(cache)
         .map(LruCache::getCacheDataInListForm)
         .orElse(List.of())
-        .forEach(DbManager::updateDb);
+        .forEach(dbManager::updateDb);
   }
 
   /**
    * Print user accounts.
    */
-  public static String print() {
+  public String print() {
     return Optional.ofNullable(cache)
         .map(LruCache::getCacheDataInListForm)
         .orElse(List.of())
@@ -155,21 +161,21 @@ public class CacheStore {
   /**
    * Delegate to backing cache store.
    */
-  public static UserAccount get(String userId) {
+  public UserAccount get(String userId) {
     return cache.get(userId);
   }
 
   /**
    * Delegate to backing cache store.
    */
-  public static void set(String userId, UserAccount userAccount) {
+  public void set(String userId, UserAccount userAccount) {
     cache.set(userId, userAccount);
   }
 
   /**
    * Delegate to backing cache store.
    */
-  public static void invalidate(String userId) {
+  public void invalidate(String userId) {
     cache.invalidate(userId);
   }
 }

caching/src/main/java/com/iluwatar/caching/DbManager.java

@@ -1,171 +0,0 @@
-/*
- * The MIT License
- * Copyright © 2014-2021 Ilkka Seppälä
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in
- * all copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
- * THE SOFTWARE.
- */
-
-package com.iluwatar.caching;
-
-import com.iluwatar.caching.constants.CachingConstants;
-import com.mongodb.MongoClient;
-import com.mongodb.client.MongoDatabase;
-import com.mongodb.client.model.UpdateOptions;
-import java.text.ParseException;
-import java.util.HashMap;
-import java.util.Map;
-import lombok.extern.slf4j.Slf4j;
-import org.bson.Document;
-
-/**
- * <p>DBManager handles the communication with the underlying data store i.e. Database. It contains
- * the implemented methods for querying, inserting, and updating data. MongoDB was used as the
- * database for the application.</p>
- *
- * <p>Developer/Tester is able to choose whether the application should use MongoDB as its
- * underlying data storage (connect()) or a simple Java data structure to (temporarily) store the
- * data/objects during runtime (createVirtualDB()).</p>
- */
-@Slf4j
-public final class DbManager {
-
-  private static MongoClient mongoClient;
-  private static MongoDatabase db;
-  private static boolean useMongoDB;
-
-  private static Map<String, UserAccount> virtualDB;
-
-  private DbManager() {
-  }
-
-  /**
-   * Create DB.
-   */
-  public static void createVirtualDb() {
-    useMongoDB = false;
-    virtualDB = new HashMap<>();
-  }
-
-  /**
-   * Connect to DB.
-   */
-  public static void connect() throws ParseException {
-    useMongoDB = true;
-    mongoClient = new MongoClient();
-    db = mongoClient.getDatabase("test");
-  }
-
-  /**
-   * Read user account from DB.
-   */
-  public static UserAccount readFromDb(String userId) {
-    if (!useMongoDB) {
-      if (virtualDB.containsKey(userId)) {
-        return virtualDB.get(userId);
-      }
-      return null;
-    }
-    if (db == null) {
-      try {
-        connect();
-      } catch (ParseException e) {
-        LOGGER.error("Error connecting to MongoDB", e);
-      }
-    }
-    var iterable = db
-        .getCollection(CachingConstants.USER_ACCOUNT)
-        .find(new Document(CachingConstants.USER_ID, userId));
-    if (iterable == null) {
-      return null;
-    }
-    Document doc = iterable.first();
-    String userName = doc.getString(CachingConstants.USER_NAME);
-    String appInfo = doc.getString(CachingConstants.ADD_INFO);
-    return new UserAccount(userId, userName, appInfo);
-  }
-
-  /**
-   * Write user account to DB.
-   */
-  public static void writeToDb(UserAccount userAccount) {
-    if (!useMongoDB) {
-      virtualDB.put(userAccount.getUserId(), userAccount);
-      return;
-    }
-    if (db == null) {
-      try {
-        connect();
-      } catch (ParseException e) {
-        LOGGER.error("Error connecting to MongoDB", e);
-      }
-    }
-    db.getCollection(CachingConstants.USER_ACCOUNT).insertOne(
-        new Document(CachingConstants.USER_ID, userAccount.getUserId())
-            .append(CachingConstants.USER_NAME, userAccount.getUserName())
-            .append(CachingConstants.ADD_INFO, userAccount.getAdditionalInfo())
-    );
-  }
-
-  /**
-   * Update DB.
-   */
-  public static void updateDb(UserAccount userAccount) {
-    if (!useMongoDB) {
-      virtualDB.put(userAccount.getUserId(), userAccount);
-      return;
-    }
-    if (db == null) {
-      try {
-        connect();
-      } catch (ParseException e) {
-        LOGGER.error("Error connecting to MongoDB", e);
-      }
-    }
-    db.getCollection(CachingConstants.USER_ACCOUNT).updateOne(
-        new Document(CachingConstants.USER_ID, userAccount.getUserId()),
-        new Document("$set", new Document(CachingConstants.USER_NAME, userAccount.getUserName())
-            .append(CachingConstants.ADD_INFO, userAccount.getAdditionalInfo())));
-  }
-
-  /**
-   * Insert data into DB if it does not exist. Else, update it.
-   */
-  public static void upsertDb(UserAccount userAccount) {
-    if (!useMongoDB) {
-      virtualDB.put(userAccount.getUserId(), userAccount);
-      return;
-    }
-    if (db == null) {
-      try {
-        connect();
-      } catch (ParseException e) {
-        LOGGER.error("Error connecting to MongoDB", e);
-      }
-    }
-    db.getCollection(CachingConstants.USER_ACCOUNT).updateOne(
-        new Document(CachingConstants.USER_ID, userAccount.getUserId()),
-        new Document("$set",
-            new Document(CachingConstants.USER_ID, userAccount.getUserId())
-                .append(CachingConstants.USER_NAME, userAccount.getUserName())
-                .append(CachingConstants.ADD_INFO, userAccount.getAdditionalInfo())
-        ),
-        new UpdateOptions().upsert(true)
-    );
-  }
-}

caching/src/main/java/com/iluwatar/caching/database/DbManager.java

@@ -0,0 +1,16 @@
+package com.iluwatar.caching.database;
+
+import com.iluwatar.caching.UserAccount;
+
+/**
+ * <p>DBManager handles the communication with the underlying data store i.e. Database. It contains
+ * the implemented methods for querying, inserting, and updating data. MongoDB was used as the
+ * database for the application.</p>
+ */
+public interface DbManager {
+    void connect();
+    UserAccount readFromDb(String userId);
+    UserAccount writeToDb(UserAccount userAccount);
+    UserAccount updateDb(UserAccount userAccount);
+    UserAccount upsertDb(UserAccount userAccount);
+}

caching/src/main/java/com/iluwatar/caching/database/DbManagerFactory.java

@@ -0,0 +1,10 @@
+package com.iluwatar.caching.database;
+
+public class DbManagerFactory {
+    public static DbManager initDb(boolean isMongo){
+        if(isMongo){
+            return new MongoDb();
+        }
+        return new VirtualDb();
+    }
+}

caching/src/main/java/com/iluwatar/caching/database/MongoDb.java

@@ -0,0 +1,81 @@
+package com.iluwatar.caching.database;
+
+import com.iluwatar.caching.UserAccount;
+import com.iluwatar.caching.constants.CachingConstants;
+import com.mongodb.MongoClient;
+import com.mongodb.client.MongoDatabase;
+import com.mongodb.client.model.UpdateOptions;
+import org.bson.Document;
+
+/**
+ * Implementation of DatabaseManager.
+ * implements base methods to work with MongoDb.
+ */
+public class MongoDb implements DbManager {
+    private MongoDatabase db;
+
+    @Override
+    public void connect() {
+        MongoClient mongoClient = new MongoClient();
+        db = mongoClient.getDatabase("test");
+    }
+
+    @Override
+    public UserAccount readFromDb(String userId) {
+        if (db == null) {
+            connect();
+        }
+        var iterable = db
+                .getCollection(CachingConstants.USER_ACCOUNT)
+                .find(new Document(CachingConstants.USER_ID, userId));
+        if (iterable == null) {
+            return null;
+        }
+        Document doc = iterable.first();
+        String userName = doc.getString(CachingConstants.USER_NAME);
+        String appInfo = doc.getString(CachingConstants.ADD_INFO);
+        return new UserAccount(userId, userName, appInfo);
+    }
+
+    @Override
+    public UserAccount writeToDb(UserAccount userAccount) {
+        if (db == null) {
+            connect();
+        }
+        db.getCollection(CachingConstants.USER_ACCOUNT).insertOne(
+                new Document(CachingConstants.USER_ID, userAccount.getUserId())
+                        .append(CachingConstants.USER_NAME, userAccount.getUserName())
+                        .append(CachingConstants.ADD_INFO, userAccount.getAdditionalInfo())
+        );
+        return userAccount;
+    }
+
+    @Override
+    public UserAccount updateDb(UserAccount userAccount) {
+        if (db == null) {
+            connect();
+        }
+        db.getCollection(CachingConstants.USER_ACCOUNT).updateOne(
+                new Document(CachingConstants.USER_ID, userAccount.getUserId()),
+                new Document("$set", new Document(CachingConstants.USER_NAME, userAccount.getUserName())
+                        .append(CachingConstants.ADD_INFO, userAccount.getAdditionalInfo())));
+        return userAccount;
+    }
+
+    @Override
+    public UserAccount upsertDb(UserAccount userAccount) {
+        if (db == null) {
+            connect();
+        }
+        db.getCollection(CachingConstants.USER_ACCOUNT).updateOne(
+                new Document(CachingConstants.USER_ID, userAccount.getUserId()),
+                new Document("$set",
+                        new Document(CachingConstants.USER_ID, userAccount.getUserId())
+                                .append(CachingConstants.USER_NAME, userAccount.getUserName())
+                                .append(CachingConstants.ADD_INFO, userAccount.getAdditionalInfo())
+                ),
+                new UpdateOptions().upsert(true)
+        );
+        return userAccount;
+    }
+}

caching/src/main/java/com/iluwatar/caching/database/VirtualDb.java

@@ -0,0 +1,44 @@
+package com.iluwatar.caching.database;
+
+import com.iluwatar.caching.UserAccount;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Implementation of DatabaseManager.
+ * implements base methods to work with hashMap as database.
+ */
+public class VirtualDb implements DbManager {
+    private Map<String, UserAccount> virtualDB;
+
+    @Override
+    public void connect() {
+        virtualDB = new HashMap<>();
+    }
+
+    @Override
+    public UserAccount readFromDb(String userId) {
+        if (virtualDB.containsKey(userId)) {
+            return virtualDB.get(userId);
+        }
+        return null;
+    }
+
+    @Override
+    public UserAccount writeToDb(UserAccount userAccount) {
+        virtualDB.put(userAccount.getUserId(), userAccount);
+        return userAccount;
+    }
+
+    @Override
+    public UserAccount updateDb(UserAccount userAccount) {
+        virtualDB.put(userAccount.getUserId(), userAccount);
+        return userAccount;
+    }
+
+    @Override
+    public UserAccount upsertDb(UserAccount userAccount) {
+        return updateDb(userAccount);
+    }
+}

caching/src/test/java/com/iluwatar/caching/CachingTest.java

@@ -43,9 +43,7 @@ class CachingTest {
     // to avoid Maven compilation errors. Set flag to true to run the
     // tests with MongoDB (provided that MongoDB is installed and socket
     // connection is open).
-    AppManager.initDb(false);
+    app = new App(false);
-    AppManager.initCacheCapacity(3);
-    app = new App();
   }
 
   @Test