Merge pull request #277 from waisuan/master

Issue #273: Caching Patterns [new pattern]

caching/.gitignore

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+/target/

caching/etc/caching.ucls

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+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.1.8" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true" 
+  realizations="true" associations="true" dependencies="false" nesting-relationships="true">  
+  <class id="1" language="java" name="main.java.com.wssia.caching.App" project="CachingPatterns" 
+    file="/CachingPatterns/src/main/java/com/wssia/caching/App.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="249" y="150"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="2" language="java" name="main.java.com.wssia.caching.AppManager" project="CachingPatterns" 
+    file="/CachingPatterns/src/main/java/com/wssia/caching/AppManager.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="502" y="163"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="3" language="java" name="main.java.com.wssia.caching.CacheStore" project="CachingPatterns" 
+    file="/CachingPatterns/src/main/java/com/wssia/caching/CacheStore.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="537" y="436"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <enumeration id="4" language="java" name="main.java.com.wssia.caching.CachingPolicy" project="CachingPatterns" 
+    file="/CachingPatterns/src/main/java/com/wssia/caching/CachingPolicy.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="789" y="162"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </enumeration>  
+  <class id="5" language="java" name="main.java.com.wssia.caching.DBManager" project="CachingPatterns" 
+    file="/CachingPatterns/src/main/java/com/wssia/caching/DBManager.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="1137" y="134"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="6" language="java" name="main.java.com.wssia.caching.LRUCache" project="CachingPatterns" 
+    file="/CachingPatterns/src/main/java/com/wssia/caching/LRUCache.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="884" y="435"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="7" language="java" name="main.java.com.wssia.caching.UserAccount" project="CachingPatterns" 
+    file="/CachingPatterns/src/main/java/com/wssia/caching/UserAccount.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="1140" y="405"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="8" language="java" name="test.java.com.wssia.caching.AppTest" project="CachingPatterns" 
+    file="/CachingPatterns/src/test/java/com/wssia/caching/AppTest.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="251" y="374"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <association id="9">    
+    <end type="SOURCE" refId="2" navigable="false">      
+      <attribute id="10" name="cachingPolicy"/>      
+      <multiplicity id="11" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="4" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <association id="12">    
+    <end type="SOURCE" refId="8" navigable="false">      
+      <attribute id="13" name="app"/>      
+      <multiplicity id="14" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="1" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <association id="15">    
+    <end type="SOURCE" refId="3" navigable="false">      
+      <attribute id="16" name="cache"/>      
+      <multiplicity id="17" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="6" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+    sort-features="false" accessors="true" visibility="true">    
+    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
+    <operations public="true" package="true" protected="true" private="true" static="true"/>  
+  </classifier-display>  
+  <association-display labels="true" multiplicity="true"/>
+</class-diagram>

caching/index.md

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+---
+layout: pattern
+title: Caching
+folder: caching
+permalink: /patterns/caching/
+categories: Other
+tags:
+ - Java
+---
+
+**Intent:** To avoid expensive re-acquisition of resources by not releasing
+the resources immediately after their use. The resources retain their identity, are kept in some
+fast-access storage, and are re-used to avoid having to acquire them again.
+
+![alt text](./etc/caching.png "Caching")
+
+**Applicability:** Use the Caching pattern(s) when
+
+* Repetitious acquisition, initialization, and release of the same resource causes unnecessary performance overhead.
+
+**Credits**
+
+* [Write-through, write-around, write-back: Cache explained](http://www.computerweekly.com/feature/Write-through-write-around-write-back-Cache-explained)
+* [Read-Through, Write-Through, Write-Behind, and Refresh-Ahead Caching](https://docs.oracle.com/cd/E15357_01/coh.360/e15723/cache_rtwtwbra.htm#COHDG5177)

caching/pom.xml

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+<?xml version="1.0"?>
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.7.0</version>
+  </parent>
+  <artifactId>caching</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.mongodb</groupId>
+      <artifactId>mongodb-driver</artifactId>
+      <version>3.0.4</version>
+    </dependency>
+    <dependency>
+      <groupId>org.mongodb</groupId>
+      <artifactId>mongodb-driver-core</artifactId>
+      <version>3.0.4</version>
+    </dependency>
+    <dependency>
+      <groupId>org.mongodb</groupId>
+      <artifactId>bson</artifactId>
+      <version>3.0.4</version>
+    </dependency>
+  </dependencies>
+  <!--
+    Due to the use of MongoDB in the test of this pattern, TRAVIS and/or MAVEN might fail if the DB connection is
+    not open for the JUnit test. To avoid disrupting the compilation process, the surefire plug-in was used
+    to SKIP the running of the JUnit tests for this pattern. To ACTIVATE the running of the tests, change the
+    skipTests (below) flag to 'false' and vice-versa.
+  -->
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-surefire-plugin</artifactId>
+        <version>2.19</version>
+        <configuration>
+          <skipTests>false</skipTests>
+        </configuration>
+      </plugin>
+    </plugins>
+  </build>
+</project>

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

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+package com.iluwatar.caching;
+
+/**
+ *
+ * The Caching pattern describes how to avoid expensive re-acquisition of resources by not releasing
+ * the resources immediately after their use. The resources retain their identity, are kept in some
+ * fast-access storage, and are re-used to avoid having to acquire them again. There are three main
+ * caching strategies/techniques in this pattern; each with their own pros and cons. They are:
+ * <code>write-through</code> which writes data to the cache and DB in a single transaction,
+ * <code>write-around</code> which writes data immediately into the DB instead of the cache, and
+ * <code>write-behind</code> which writes data into the cache initially whilst the data is only
+ * written into the DB when the cache is full. The <code>read-through</code> strategy is also
+ * included in the mentioned three strategies -- returns data from the cache to the caller <b>if</b>
+ * it exists <b>else</b> queries from DB and stores it into the cache for future use. These
+ * strategies determine when the data in the cache should be written back to the backing store (i.e.
+ * Database) and help keep both data sources synchronized/up-to-date. This pattern can improve
+ * performance and also helps to maintain consistency between data held in the cache and the data in
+ * the underlying data store.
+ * <p>
+ * In this example, the user account ({@link UserAccount}) entity is used as the underlying
+ * application data. The cache itself is implemented as an internal (Java) data structure. It adopts
+ * a Least-Recently-Used (LRU) strategy for evicting data from itself when its full. The three
+ * strategies are individually tested. The testing of the cache is restricted towards saving and
+ * querying of user accounts from the underlying data store ( {@link DBManager}). The main class (
+ * {@link App} is not aware of the underlying mechanics of the application (i.e. save and query) and
+ * whether the data is coming from the cache or the DB (i.e. separation of concern). The AppManager
+ * ({@link AppManager}) handles the transaction of data to-and-from the underlying data store
+ * (depending on the preferred caching policy/strategy).
+ *
+ * <i>App --> AppManager --> CacheStore/LRUCache/CachingPolicy --> DBManager</i>
+ * </p>
+ *
+ * @see CacheStore
+ * @See LRUCache
+ * @see CachingPolicy
+ *
+ */
+public class App {
+
+  /**
+   * Program entry point
+   *
+   * @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
+                              // 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();
+    app.useReadAndWriteThroughStrategy();
+    app.useReadThroughAndWriteAroundStrategy();
+    app.useReadThroughAndWriteBehindStrategy();
+  }
+
+  /**
+   * Read-through and write-through
+   */
+  public void useReadAndWriteThroughStrategy() {
+    System.out.println("# CachingPolicy.THROUGH");
+    AppManager.initCachingPolicy(CachingPolicy.THROUGH);
+
+    UserAccount userAccount1 = new UserAccount("001", "John", "He is a boy.");
+
+    AppManager.save(userAccount1);
+    System.out.println(AppManager.printCacheContent());
+    userAccount1 = AppManager.find("001");
+    userAccount1 = AppManager.find("001");
+  }
+
+  /**
+   * Read-through and write-around
+   */
+  public void useReadThroughAndWriteAroundStrategy() {
+    System.out.println("# CachingPolicy.AROUND");
+    AppManager.initCachingPolicy(CachingPolicy.AROUND);
+
+    UserAccount userAccount2 = new UserAccount("002", "Jane", "She is a girl.");
+
+    AppManager.save(userAccount2);
+    System.out.println(AppManager.printCacheContent());
+    userAccount2 = AppManager.find("002");
+    System.out.println(AppManager.printCacheContent());
+    userAccount2 = AppManager.find("002");
+    userAccount2.setUserName("Jane G.");
+    AppManager.save(userAccount2);
+    System.out.println(AppManager.printCacheContent());
+    userAccount2 = AppManager.find("002");
+    System.out.println(AppManager.printCacheContent());
+    userAccount2 = AppManager.find("002");
+  }
+
+  /**
+   * Read-through and write-behind
+   */
+  public void useReadThroughAndWriteBehindStrategy() {
+    System.out.println("# CachingPolicy.BEHIND");
+    AppManager.initCachingPolicy(CachingPolicy.BEHIND);
+
+    UserAccount userAccount3 = new UserAccount("003", "Adam", "He likes food.");
+    UserAccount userAccount4 = new UserAccount("004", "Rita", "She hates cats.");
+    UserAccount userAccount5 = new UserAccount("005", "Isaac", "He is allergic to mustard.");
+
+    AppManager.save(userAccount3);
+    AppManager.save(userAccount4);
+    AppManager.save(userAccount5);
+    System.out.println(AppManager.printCacheContent());
+    userAccount3 = AppManager.find("003");
+    System.out.println(AppManager.printCacheContent());
+    UserAccount userAccount6 = new UserAccount("006", "Yasha", "She is an only child.");
+    AppManager.save(userAccount6);
+    System.out.println(AppManager.printCacheContent());
+    userAccount4 = AppManager.find("004");
+    System.out.println(AppManager.printCacheContent());
+  }
+}

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

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+package com.iluwatar.caching;
+
+import java.text.ParseException;
+
+/**
+ *
+ * AppManager helps to bridge the gap in communication between the main class and the application's
+ * back-end. DB connection is initialized through this class. The chosen caching strategy/policy is
+ * also initialized here. Before the cache can be used, the size of the cache has to be set.
+ * Depending on the chosen caching policy, AppManager will call the appropriate function in the
+ * CacheStore class.
+ *
+ */
+public class AppManager {
+
+  private static CachingPolicy cachingPolicy;
+
+  /**
+   *
+   * Developer/Tester is able to choose whether the application should use MongoDB as its underlying
+   * data storage or a simple Java data structure to (temporarily) store the data/objects during
+   * runtime.
+   */
+  public static void initDB(boolean useMongoDB) {
+    if (useMongoDB) {
+      try {
+        DBManager.connect();
+      } catch (ParseException e) {
+        e.printStackTrace();
+      }
+    } else {
+      DBManager.createVirtualDB();
+    }
+  }
+
+  public static void initCachingPolicy(CachingPolicy policy) {
+    cachingPolicy = policy;
+    if (cachingPolicy == CachingPolicy.BEHIND) {
+      Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
+        @Override
+        public void run() {
+          CacheStore.flushCache();
+        }
+      }));
+    }
+    CacheStore.clearCache();
+  }
+
+  public static void initCacheCapacity(int capacity) {
+    CacheStore.initCapacity(capacity);
+  }
+
+  public static UserAccount find(String userID) {
+    if (cachingPolicy == CachingPolicy.THROUGH || cachingPolicy == CachingPolicy.AROUND) {
+      return CacheStore.readThrough(userID);
+    } else if (cachingPolicy == CachingPolicy.BEHIND) {
+      return CacheStore.readThroughWithWriteBackPolicy(userID);
+    }
+    return null;
+  }
+
+  public static void save(UserAccount userAccount) {
+    if (cachingPolicy == CachingPolicy.THROUGH) {
+      CacheStore.writeThrough(userAccount);
+    } else if (cachingPolicy == CachingPolicy.AROUND) {
+      CacheStore.writeAround(userAccount);
+    } else if (cachingPolicy == CachingPolicy.BEHIND) {
+      CacheStore.writeBehind(userAccount);
+    }
+  }
+
+  public static String printCacheContent() {
+    return CacheStore.print();
+  }
+}