#590 add explanation for event aggregator

event-aggregator/README.md

@@ -9,6 +9,10 @@ tags:
  - Reactive
 ---
 
+## Name
+
+Event Aggregator
+
 ## Intent
 A system with lots of objects can lead to complexities when a
 client wants to subscribe to events. The client has to find and register for
@@ -17,6 +21,136 @@ requires a separate subscription. An Event Aggregator acts as a single source
 of events for many objects. It registers for all the events of the many objects
 allowing clients to register with just the aggregator.
 
+## Explanation
+
+Real-world example
+
+> King Joffrey sits on the iron throne and rules the seven kingdoms of Westeros. He receives most
+> of his critical information from King's Hand, the second in command. King's hand has many
+> close advisors himself, feeding him with relevant information about events occurring in the
+> kingdom.
+
+In Plain Words
+
+> Event Aggregator is an event mediator that collects events from multiple sources and delivers
+> them to registered observers.
+
+**Programmatic Example**
+
+In our programmatic example, we demonstrate the implementation of an event aggregator pattern. Some of
+the objects are event listeners, some are event emitters, and the event aggregator does both.
+
+```java
+public interface EventObserver {
+  void onEvent(Event e);
+}
+
+public abstract class EventEmitter {
+
+  private final Map<Event, List<EventObserver>> observerLists;
+
+  public EventEmitter() {
+    observerLists = new HashMap<>();
+  }
+
+  public final void registerObserver(EventObserver obs, Event e) {
+    ...
+  }
+
+  protected void notifyObservers(Event e) {
+    ...
+  }
+}
+```
+
+`KingJoffrey` is listening to events from `KingsHand`.
+
+```java
+@Slf4j
+public class KingJoffrey implements EventObserver {
+  @Override
+  public void onEvent(Event e) {
+    LOGGER.info("Received event from the King's Hand: {}", e.toString());
+  }
+}
+```
+
+`KingsHand` is listening to events from his subordinates `LordBaelish`, `LordVarys`, and `Scout`.
+Whatever he hears from them, he delivers to `KingJoffrey`.
+
+```java
+public class KingsHand extends EventEmitter implements EventObserver {
+
+  public KingsHand() {
+  }
+
+  public KingsHand(EventObserver obs, Event e) {
+    super(obs, e);
+  }
+
+  @Override
+  public void onEvent(Event e) {
+    notifyObservers(e);
+  }
+}
+```
+
+For example, `LordVarys` finds a traitor every Sunday and notifies the `KingsHand`.
+
+```java
+@Slf4j
+public class LordVarys extends EventEmitter implements EventObserver {
+  @Override
+  public void timePasses(Weekday day) {
+    if (day == Weekday.SATURDAY) {
+      notifyObservers(Event.TRAITOR_DETECTED);
+    }
+  }
+}
+```
+
+The following snippet demonstrates how the objects are constructed and wired together.
+
+```java
+    var kingJoffrey = new KingJoffrey();
+
+    var kingsHand = new KingsHand();
+    kingsHand.registerObserver(kingJoffrey, Event.TRAITOR_DETECTED);
+    kingsHand.registerObserver(kingJoffrey, Event.STARK_SIGHTED);
+    kingsHand.registerObserver(kingJoffrey, Event.WARSHIPS_APPROACHING);
+    kingsHand.registerObserver(kingJoffrey, Event.WHITE_WALKERS_SIGHTED);
+
+    var varys = new LordVarys();
+    varys.registerObserver(kingsHand, Event.TRAITOR_DETECTED);
+    varys.registerObserver(kingsHand, Event.WHITE_WALKERS_SIGHTED);
+
+    var scout = new Scout();
+    scout.registerObserver(kingsHand, Event.WARSHIPS_APPROACHING);
+    scout.registerObserver(varys, Event.WHITE_WALKERS_SIGHTED);
+
+    var baelish = new LordBaelish(kingsHand, Event.STARK_SIGHTED);
+
+    var emitters = List.of(
+        kingsHand,
+        baelish,
+        varys,
+        scout
+    );
+
+    Arrays.stream(Weekday.values())
+        .<Consumer<? super EventEmitter>>map(day -> emitter -> emitter.timePasses(day))
+        .forEachOrdered(emitters::forEach);
+```
+
+The console output after running the example.
+
+```
+18:21:52.955 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: Warships approaching
+18:21:52.960 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: White walkers sighted
+18:21:52.960 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: Stark sighted
+18:21:52.960 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: Traitor detected
+```
+
 ## Class diagram
 ![alt text](./etc/classes.png "Event Aggregator")
 
@@ -26,9 +160,13 @@ Use the Event Aggregator pattern when
 * Event Aggregator is a good choice when you have lots of objects that are
   potential event sources. Rather than have the observer deal with registering
   with them all, you can centralize the registration logic to the Event
-  Aggregator. As well as simplifying registration, a Event Aggregator also
+  Aggregator. As well as simplifying registration, an Event Aggregator also
   simplifies the memory management issues in using observers.
 
+## Related patterns
+
+* [Observer](https://java-design-patterns.com/patterns/observer/)
+
 ## Credits
 
 * [Martin Fowler - Event Aggregator](http://martinfowler.com/eaaDev/EventAggregator.html)

thread-pool/README.md

@@ -18,11 +18,11 @@ threads and eliminating the latency of creating new threads.
 
 ## Explanation
 
-Real-world example
+Real world example
 
 > We have a large number of relatively short tasks at hand. We need to peel huge amounts of potatoes 
-> and serve a mighty amount of coffee cups. Creating a new thread for each task would be a waste so 
-> we establish a thread pool.       
+> and serve mighty amount of coffee cups. Creating a new thread for each task would be a waste so we 
+> establish a thread pool.       
 
 In plain words
 
@@ -99,7 +99,7 @@ public class PotatoPeelingTask extends Task {
 }
 ```
 
-Next, we present a runnable `Worker` class that the thread pool will utilize to handle all the potato 
+Next we present a runnable `Worker` class that the thread pool will utilize to handle all the potato 
 peeling and coffee making.
 
 ```java

throttling/README.md

@@ -16,12 +16,10 @@ Ensure that a given client is not able to access service resources more than the
 
 ## Explanation
 
-Real-world example
+Real world example
 
-> A young human and an old dwarf walk into a bar. They start ordering beers from the bartender.
-> The bartender immediately sees that the young human shouldn't consume too many drinks too fast
-> and refuses to serve if enough time has not passed. For the old dwarf, the serving rate can
-> be higher.
+> A large multinational corporation offers API to its customers. The API is rate-limited and each 
+> customer can only make certain amount of calls per second.      
 
 In plain words
 
@@ -35,18 +33,15 @@ In plain words
 
 **Programmatic Example**
 
-`BarCustomer` class presents the clients of the `Bartender` API. `CallsCount` tracks the number of 
-calls per `BarCustomer`.
+Tenant class presents the clients of the API. CallsCount tracks the number of API calls per tenant.
 
 ```java
-public class BarCustomer {
+public class Tenant {
 
-    @Getter
   private final String name;
-    @Getter
   private final int allowedCallsPerSecond;
 
-    public BarCustomer(String name, int allowedCallsPerSecond, CallsCount callsCount) {
+  public Tenant(String name, int allowedCallsPerSecond, CallsCount callsCount) {
     if (allowedCallsPerSecond < 0) {
       throw new InvalidParameterException("Number of calls less than 0 not allowed");
     }
@@ -54,6 +49,14 @@ public class BarCustomer {
     this.allowedCallsPerSecond = allowedCallsPerSecond;
     callsCount.addTenant(name);
   }
+
+  public String getName() {
+    return name;
+  }
+
+  public int getAllowedCallsPerSecond() {
+    return allowedCallsPerSecond;
+  }
 }
 
 @Slf4j
@@ -73,14 +76,14 @@ public final class CallsCount {
   }
 
   public void reset() {
+    LOGGER.debug("Resetting the map.");
     tenantCallsCount.replaceAll((k, v) -> new AtomicLong(0));
-    LOGGER.info("reset counters");
   }
 }
 ```
 
-Next, the service that the tenants are calling is introduced. To track the call count, a throttler 
-timer is used.
+Next we introduce the service that the tenants are calling. To track the call count we use the 
+throttler timer.
 
 ```java
 public interface Throttler {
@@ -108,31 +111,26 @@ public class ThrottleTimerImpl implements Throttler {
     }, 0, throttlePeriod);
   }
 }
-```
 
-`Bartender` offers the `orderDrink` service to the `BarCustomer`s. The customers probably don't
-know that the beer serving rate is limited by their appearances.
+class B2BService {
 
-```java
-class Bartender {
-
-    private static final Logger LOGGER = LoggerFactory.getLogger(Bartender.class);
+  private static final Logger LOGGER = LoggerFactory.getLogger(B2BService.class);
   private final CallsCount callsCount;
 
-    public Bartender(Throttler timer, CallsCount callsCount) {
+  public B2BService(Throttler timer, CallsCount callsCount) {
     this.callsCount = callsCount;
     timer.start();
   }
 
-    public int orderDrink(BarCustomer barCustomer) {
-        var tenantName = barCustomer.getName();
+  public int dummyCustomerApi(Tenant tenant) {
+    var tenantName = tenant.getName();
     var count = callsCount.getCount(tenantName);
-        if (count >= barCustomer.getAllowedCallsPerSecond()) {
-            LOGGER.error("I'm sorry {}, you've had enough for today!", tenantName);
+    LOGGER.debug("Counter for {} : {} ", tenant.getName(), count);
+    if (count >= tenant.getAllowedCallsPerSecond()) {
+      LOGGER.error("API access per second limit reached for: {}", tenantName);
       return -1;
     }
     callsCount.incrementCount(tenantName);
-        LOGGER.debug("Serving beer to {} : [{} consumed] ", barCustomer.getName(), count+1);
     return getRandomCustomerId();
   }
 
@@ -142,69 +140,42 @@ class Bartender {
 }
 ```
 
-Now it is possible to see the full example in action. `BarCustomer` young human is rate-limited to 2 
-calls per second and the old dwarf to 4.
+Now we are ready to see the full example in action. Tenant Adidas is rate-limited to 5 calls per 
+second and Nike to 6.
 
 ```java
-public static void main(String[] args) {
+  public static void main(String[] args) {
     var callsCount = new CallsCount();
-    var human = new BarCustomer("young human", 2, callsCount);
-    var dwarf = new BarCustomer("dwarf soldier", 4, callsCount);
+    var adidas = new Tenant("Adidas", 5, callsCount);
+    var nike = new Tenant("Nike", 6, callsCount);
 
     var executorService = Executors.newFixedThreadPool(2);
-
-    executorService.execute(() -> makeServiceCalls(human, callsCount));
-    executorService.execute(() -> makeServiceCalls(dwarf, callsCount));
-
+    executorService.execute(() -> makeServiceCalls(adidas, callsCount));
+    executorService.execute(() -> makeServiceCalls(nike, callsCount));
     executorService.shutdown();
+    
     try {
       executorService.awaitTermination(10, TimeUnit.SECONDS);
     } catch (InterruptedException e) {
-        LOGGER.error("Executor service terminated: {}", e.getMessage());
+      LOGGER.error("Executor Service terminated: {}", e.getMessage());
+    }
   }
-}
 
-private static void makeServiceCalls(BarCustomer barCustomer, CallsCount callsCount) {
-    var timer = new ThrottleTimerImpl(1000, callsCount);
-    var service = new Bartender(timer, callsCount);
+  private static void makeServiceCalls(Tenant tenant, CallsCount callsCount) {
+    var timer = new ThrottleTimerImpl(10, callsCount);
+    var service = new B2BService(timer, callsCount);
     // Sleep is introduced to keep the output in check and easy to view and analyze the results.
-    IntStream.range(0, 50).forEach(i -> {
-        service.orderDrink(barCustomer);
+    IntStream.range(0, 20).forEach(i -> {
+      service.dummyCustomerApi(tenant);
       try {
-            Thread.sleep(100);
+        Thread.sleep(1);
       } catch (InterruptedException e) {
         LOGGER.error("Thread interrupted: {}", e.getMessage());
       }
     });
-}
+  }
 ```
 
-An excerpt from the example's console output:
-
-```
-18:46:36.218 [Timer-0] INFO com.iluwatar.throttling.CallsCount - reset counters
-18:46:36.218 [Timer-1] INFO com.iluwatar.throttling.CallsCount - reset counters
-18:46:36.242 [pool-1-thread-2] DEBUG com.iluwatar.throttling.Bartender - Serving beer to dwarf soldier : [1 consumed] 
-18:46:36.242 [pool-1-thread-1] DEBUG com.iluwatar.throttling.Bartender - Serving beer to young human : [1 consumed] 
-18:46:36.342 [pool-1-thread-2] DEBUG com.iluwatar.throttling.Bartender - Serving beer to dwarf soldier : [2 consumed] 
-18:46:36.342 [pool-1-thread-1] DEBUG com.iluwatar.throttling.Bartender - Serving beer to young human : [2 consumed] 
-18:46:36.443 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
-18:46:36.443 [pool-1-thread-2] DEBUG com.iluwatar.throttling.Bartender - Serving beer to dwarf soldier : [3 consumed] 
-18:46:36.544 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
-18:46:36.544 [pool-1-thread-2] DEBUG com.iluwatar.throttling.Bartender - Serving beer to dwarf soldier : [4 consumed] 
-18:46:36.645 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
-18:46:36.645 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
-18:46:36.745 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
-18:46:36.745 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
-18:46:36.846 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
-18:46:36.846 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
-18:46:36.947 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
-18:46:36.947 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
-18:46:37.048 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
-18:46:37.048 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
-18:46:37.148 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
-18:46:37.148 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
-```
 
 ## Class diagram
 
@@ -214,7 +185,7 @@ An excerpt from the example's console output:
 
 The Throttling pattern should be used:
 
-* When service access needs to be restricted not to have high impact on the performance of the service.
+* When a service access needs to be restricted to not have high impacts on the performance of the service.
 * When multiple clients are consuming the same service resources and restriction has to be made according to the usage per client.
 
 ## Credits

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

@@ -34,11 +34,11 @@ import lombok.extern.slf4j.Slf4j;
  * complete service by users or a particular tenant. This can allow systems to continue to function
  * and meet service level agreements, even when an increase in demand places load on resources.
  * <p>
- * In this example there is a {@link Bartender} serving beer to {@link BarCustomer}s. This is a time
+ * In this example we have ({@link App}) as the initiating point of the service. This is a time
  * based throttling, i.e. only a certain number of calls are allowed per second.
  * </p>
- * ({@link BarCustomer}) is the service tenant class having a name and the number of calls allowed.
- * ({@link Bartender}) is the service which is consumed by the tenants and is throttled.
+ * ({@link Tenant}) is the Tenant POJO class with which many tenants can be created ({@link
+ * B2BService}) is the service which is consumed by the tenants and is throttled.
  */
 @Slf4j
 public class App {
@@ -50,35 +50,33 @@ public class App {
    */
   public static void main(String[] args) {
     var callsCount = new CallsCount();
-    var human = new BarCustomer("young human", 2, callsCount);
-    var dwarf = new BarCustomer("dwarf soldier", 4, callsCount);
+    var adidas = new Tenant("Adidas", 5, callsCount);
+    var nike = new Tenant("Nike", 6, callsCount);
 
     var executorService = Executors.newFixedThreadPool(2);
 
-    executorService.execute(() -> makeServiceCalls(human, callsCount));
-    executorService.execute(() -> makeServiceCalls(dwarf, callsCount));
+    executorService.execute(() -> makeServiceCalls(adidas, callsCount));
+    executorService.execute(() -> makeServiceCalls(nike, callsCount));
 
     executorService.shutdown();
     try {
-      if (!executorService.awaitTermination(10, TimeUnit.SECONDS)) {
-        executorService.shutdownNow();
-      }
+      executorService.awaitTermination(10, TimeUnit.SECONDS);
     } catch (InterruptedException e) {
-      executorService.shutdownNow();
+      LOGGER.error("Executor Service terminated: {}", e.getMessage());
     }
   }
 
   /**
-   * Make calls to the bartender.
+   * Make calls to the B2BService dummy API.
    */
-  private static void makeServiceCalls(BarCustomer barCustomer, CallsCount callsCount) {
-    var timer = new ThrottleTimerImpl(1000, callsCount);
-    var service = new Bartender(timer, callsCount);
+  private static void makeServiceCalls(Tenant tenant, CallsCount callsCount) {
+    var timer = new ThrottleTimerImpl(10, callsCount);
+    var service = new B2BService(timer, callsCount);
     // Sleep is introduced to keep the output in check and easy to view and analyze the results.
-    IntStream.range(0, 50).forEach(i -> {
-      service.orderDrink(barCustomer);
+    IntStream.range(0, 20).forEach(i -> {
+      service.dummyCustomerApi(tenant);
       try {
-        Thread.sleep(100);
+        Thread.sleep(1);
       } catch (InterruptedException e) {
         LOGGER.error("Thread interrupted: {}", e.getMessage());
       }

throttling/src/main/java/com/iluwatar/throttling/B2BService.java

@@ -29,32 +29,33 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 /**
- * Bartender is a service which accepts a BarCustomer (tenant) and throttles
- * the resource based on the time given to the tenant.
+ * A service which accepts a tenant and throttles the resource based on the time given to the
+ * tenant.
  */
-class Bartender {
+class B2BService {
 
-  private static final Logger LOGGER = LoggerFactory.getLogger(Bartender.class);
+  private static final Logger LOGGER = LoggerFactory.getLogger(B2BService.class);
   private final CallsCount callsCount;
 
-  public Bartender(Throttler timer, CallsCount callsCount) {
+  public B2BService(Throttler timer, CallsCount callsCount) {
     this.callsCount = callsCount;
     timer.start();
   }
 
   /**
-   * Orders a drink from the bartender.
+   * Calls dummy customer api.
+   *
    * @return customer id which is randomly generated
    */
-  public int orderDrink(BarCustomer barCustomer) {
-    var tenantName = barCustomer.getName();
+  public int dummyCustomerApi(Tenant tenant) {
+    var tenantName = tenant.getName();
     var count = callsCount.getCount(tenantName);
-    if (count >= barCustomer.getAllowedCallsPerSecond()) {
-      LOGGER.error("I'm sorry {}, you've had enough for today!", tenantName);
+    LOGGER.debug("Counter for {} : {} ", tenant.getName(), count);
+    if (count >= tenant.getAllowedCallsPerSecond()) {
+      LOGGER.error("API access per second limit reached for: {}", tenantName);
       return -1;
     }
     callsCount.incrementCount(tenantName);
-    LOGGER.debug("Serving beer to {} : [{} consumed] ", barCustomer.getName(), count + 1);
     return getRandomCustomerId();
   }
 

throttling/src/main/java/com/iluwatar/throttling/CallsCount.java

@@ -69,7 +69,7 @@ public final class CallsCount {
    * Resets the count of all the tenants in the map.
    */
   public void reset() {
+    LOGGER.debug("Resetting the map.");
     tenantCallsCount.replaceAll((k, v) -> new AtomicLong(0));
-    LOGGER.info("reset counters");
   }
 }

throttling/src/main/java/com/iluwatar/throttling/Tenant.java

@@ -25,26 +25,22 @@ package com.iluwatar.throttling;
 
 import java.security.InvalidParameterException;
 
-import lombok.Getter;
-
 /**
- * BarCustomer is a tenant with a name and a number of allowed calls per second.
+ * A Pojo class to create a basic Tenant with the allowed calls per second.
  */
-public class BarCustomer {
+public class Tenant {
 
-  @Getter
   private final String name;
-  @Getter
   private final int allowedCallsPerSecond;
 
   /**
    * Constructor.
    *
-   * @param name Name of the BarCustomer
-   * @param allowedCallsPerSecond The number of calls allowed for this particular tenant.
+   * @param name                  Name of the tenant
+   * @param allowedCallsPerSecond The number of calls allowed for a particular tenant.
    * @throws InvalidParameterException If number of calls is less than 0, throws exception.
    */
-  public BarCustomer(String name, int allowedCallsPerSecond, CallsCount callsCount) {
+  public Tenant(String name, int allowedCallsPerSecond, CallsCount callsCount) {
     if (allowedCallsPerSecond < 0) {
       throw new InvalidParameterException("Number of calls less than 0 not allowed");
     }
@@ -52,4 +48,12 @@ public class BarCustomer {
     this.allowedCallsPerSecond = allowedCallsPerSecond;
     callsCount.addTenant(name);
   }
+
+  public String getName() {
+    return name;
+  }
+
+  public int getAllowedCallsPerSecond() {
+    return allowedCallsPerSecond;
+  }
 }

throttling/src/test/java/com/iluwatar/throttling/B2BServiceTest.java

@@ -32,18 +32,19 @@ import org.junit.jupiter.api.Test;
 /**
  * B2BServiceTest class to test the B2BService
  */
-public class BartenderTest {
+public class B2BServiceTest {
 
   private final CallsCount callsCount = new CallsCount();
 
   @Test
   void dummyCustomerApiTest() {
-    var tenant = new BarCustomer("pirate", 2, callsCount);
+    var tenant = new Tenant("testTenant", 2, callsCount);
     // In order to assure that throttling limits will not be reset, we use an empty throttling implementation
-    var timer = (Throttler) () -> {};
-    var service = new Bartender(timer, callsCount);
+    var timer = (Throttler) () -> {
+    };
+    var service = new B2BService(timer, callsCount);
 
-    IntStream.range(0, 5).mapToObj(i -> tenant).forEach(service::orderDrink);
+    IntStream.range(0, 5).mapToObj(i -> tenant).forEach(service::dummyCustomerApi);
     var counter = callsCount.getCount(tenant.getName());
     assertEquals(2, counter, "Counter limit must be reached");
   }

throttling/src/test/java/com/iluwatar/throttling/TenantTest.java

@@ -23,21 +23,20 @@
 
 package com.iluwatar.throttling;
 
-import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.security.InvalidParameterException;
-
-import static org.junit.jupiter.api.Assertions.assertThrows;
+import org.junit.jupiter.api.Test;
 
 /**
  * TenantTest to test the creation of Tenant with valid parameters.
  */
-public class BarCustomerTest {
+public class TenantTest {
 
   @Test
   void constructorTest() {
     assertThrows(InvalidParameterException.class, () -> {
-      new BarCustomer("sirBrave", -1, new CallsCount());
+      new Tenant("FailTenant", -1, new CallsCount());
     });
   }
 }