Added LLD projects

distributed-event-bus/.idea/.gitignore

@@ -0,0 +1,3 @@
+# Default ignored files
+/shelf/
+/workspace.xml

distributed-event-bus/.idea/compiler.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="CompilerConfiguration">
+    <annotationProcessing>
+      <profile name="Maven default annotation processors profile" enabled="true">
+        <sourceOutputDir name="target/generated-sources/annotations" />
+        <sourceTestOutputDir name="target/generated-test-sources/test-annotations" />
+        <outputRelativeToContentRoot value="true" />
+        <module name="distributed-event-bus" />
+      </profile>
+    </annotationProcessing>
+    <bytecodeTargetLevel>
+      <module name="distributed-event-bus" target="11" />
+    </bytecodeTargetLevel>
+  </component>
+</project>

distributed-event-bus/.idea/jarRepositories.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="RemoteRepositoriesConfiguration">
+    <remote-repository>
+      <option name="id" value="central" />
+      <option name="name" value="Central Repository" />
+      <option name="url" value="https://repo.maven.apache.org/maven2" />
+    </remote-repository>
+    <remote-repository>
+      <option name="id" value="central" />
+      <option name="name" value="Maven Central repository" />
+      <option name="url" value="https://repo1.maven.org/maven2" />
+    </remote-repository>
+    <remote-repository>
+      <option name="id" value="jboss.community" />
+      <option name="name" value="JBoss Community repository" />
+      <option name="url" value="https://repository.jboss.org/nexus/content/repositories/public/" />
+    </remote-repository>
+  </component>
+</project>

distributed-event-bus/.idea/libraries/Maven__aopalliance_aopalliance_1_0.xml

@@ -0,0 +1,13 @@
+<component name="libraryTable">
+  <library name="Maven: aopalliance:aopalliance:1.0">
+    <CLASSES>
+      <root url="jar://$MAVEN_REPOSITORY$/aopalliance/aopalliance/1.0/aopalliance-1.0.jar!/" />
+    </CLASSES>
+    <JAVADOC>
+      <root url="jar://$MAVEN_REPOSITORY$/aopalliance/aopalliance/1.0/aopalliance-1.0-javadoc.jar!/" />
+    </JAVADOC>
+    <SOURCES>
+      <root url="jar://$MAVEN_REPOSITORY$/aopalliance/aopalliance/1.0/aopalliance-1.0-sources.jar!/" />
+    </SOURCES>
+  </library>
+</component>

distributed-event-bus/.idea/libraries/Maven__com_google_code_gson_gson_2_8_6.xml

@@ -0,0 +1,13 @@
+<component name="libraryTable">
+  <library name="Maven: com.google.code.gson:gson:2.8.6">
+    <CLASSES>
+      <root url="jar://$MAVEN_REPOSITORY$/com/google/code/gson/gson/2.8.6/gson-2.8.6.jar!/" />
+    </CLASSES>
+    <JAVADOC>
+      <root url="jar://$MAVEN_REPOSITORY$/com/google/code/gson/gson/2.8.6/gson-2.8.6-javadoc.jar!/" />
+    </JAVADOC>
+    <SOURCES>
+      <root url="jar://$MAVEN_REPOSITORY$/com/google/code/gson/gson/2.8.6/gson-2.8.6-sources.jar!/" />
+    </SOURCES>
+  </library>
+</component>

distributed-event-bus/.idea/libraries/Maven__com_google_guava_guava_16_0_1.xml

@@ -0,0 +1,13 @@
+<component name="libraryTable">
+  <library name="Maven: com.google.guava:guava:16.0.1">
+    <CLASSES>
+      <root url="jar://$MAVEN_REPOSITORY$/com/google/guava/guava/16.0.1/guava-16.0.1.jar!/" />
+    </CLASSES>
+    <JAVADOC>
+      <root url="jar://$MAVEN_REPOSITORY$/com/google/guava/guava/16.0.1/guava-16.0.1-javadoc.jar!/" />
+    </JAVADOC>
+    <SOURCES>
+      <root url="jar://$MAVEN_REPOSITORY$/com/google/guava/guava/16.0.1/guava-16.0.1-sources.jar!/" />
+    </SOURCES>
+  </library>
+</component>

distributed-event-bus/.idea/libraries/Maven__com_google_inject_guice_4_0.xml

@@ -0,0 +1,13 @@
+<component name="libraryTable">
+  <library name="Maven: com.google.inject:guice:4.0">
+    <CLASSES>
+      <root url="jar://$MAVEN_REPOSITORY$/com/google/inject/guice/4.0/guice-4.0.jar!/" />
+    </CLASSES>
+    <JAVADOC>
+      <root url="jar://$MAVEN_REPOSITORY$/com/google/inject/guice/4.0/guice-4.0-javadoc.jar!/" />
+    </JAVADOC>
+    <SOURCES>
+      <root url="jar://$MAVEN_REPOSITORY$/com/google/inject/guice/4.0/guice-4.0-sources.jar!/" />
+    </SOURCES>
+  </library>
+</component>

distributed-event-bus/.idea/libraries/Maven__javax_inject_javax_inject_1.xml

@@ -0,0 +1,13 @@
+<component name="libraryTable">
+  <library name="Maven: javax.inject:javax.inject:1">
+    <CLASSES>
+      <root url="jar://$MAVEN_REPOSITORY$/javax/inject/javax.inject/1/javax.inject-1.jar!/" />
+    </CLASSES>
+    <JAVADOC>
+      <root url="jar://$MAVEN_REPOSITORY$/javax/inject/javax.inject/1/javax.inject-1-javadoc.jar!/" />
+    </JAVADOC>
+    <SOURCES>
+      <root url="jar://$MAVEN_REPOSITORY$/javax/inject/javax.inject/1/javax.inject-1-sources.jar!/" />
+    </SOURCES>
+  </library>
+</component>

distributed-event-bus/.idea/libraries/Maven__junit_junit_4_13.xml

@@ -0,0 +1,13 @@
+<component name="libraryTable">
+  <library name="Maven: junit:junit:4.13">
+    <CLASSES>
+      <root url="jar://$MAVEN_REPOSITORY$/junit/junit/4.13/junit-4.13.jar!/" />
+    </CLASSES>
+    <JAVADOC>
+      <root url="jar://$MAVEN_REPOSITORY$/junit/junit/4.13/junit-4.13-javadoc.jar!/" />
+    </JAVADOC>
+    <SOURCES>
+      <root url="jar://$MAVEN_REPOSITORY$/junit/junit/4.13/junit-4.13-sources.jar!/" />
+    </SOURCES>
+  </library>
+</component>

distributed-event-bus/.idea/libraries/Maven__org_hamcrest_hamcrest_core_1_3.xml

@@ -0,0 +1,13 @@
+<component name="libraryTable">
+  <library name="Maven: org.hamcrest:hamcrest-core:1.3">
+    <CLASSES>
+      <root url="jar://$MAVEN_REPOSITORY$/org/hamcrest/hamcrest-core/1.3/hamcrest-core-1.3.jar!/" />
+    </CLASSES>
+    <JAVADOC>
+      <root url="jar://$MAVEN_REPOSITORY$/org/hamcrest/hamcrest-core/1.3/hamcrest-core-1.3-javadoc.jar!/" />
+    </JAVADOC>
+    <SOURCES>
+      <root url="jar://$MAVEN_REPOSITORY$/org/hamcrest/hamcrest-core/1.3/hamcrest-core-1.3-sources.jar!/" />
+    </SOURCES>
+  </library>
+</component>

distributed-event-bus/.idea/misc.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="MavenProjectsManager">
+    <option name="originalFiles">
+      <list>
+        <option value="$PROJECT_DIR$/pom.xml" />
+      </list>
+    </option>
+  </component>
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_11" default="true" project-jdk-name="11" project-jdk-type="JavaSDK">
+    <output url="file://$PROJECT_DIR$/out" />
+  </component>
+</project>

distributed-event-bus/.idea/modules.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/distributed-event-bus.iml" filepath="$PROJECT_DIR$/distributed-event-bus.iml" />
+    </modules>
+  </component>
+</project>

distributed-event-bus/.idea/uiDesigner.xml

@@ -0,0 +1,124 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="Palette2">
+    <group name="Swing">
+      <item class="com.intellij.uiDesigner.HSpacer" tooltip-text="Horizontal Spacer" icon="/com/intellij/uiDesigner/icons/hspacer.png" removable="false" auto-create-binding="false" can-attach-label="false">
+        <default-constraints vsize-policy="1" hsize-policy="6" anchor="0" fill="1" />
+      </item>
+      <item class="com.intellij.uiDesigner.VSpacer" tooltip-text="Vertical Spacer" icon="/com/intellij/uiDesigner/icons/vspacer.png" removable="false" auto-create-binding="false" can-attach-label="false">
+        <default-constraints vsize-policy="6" hsize-policy="1" anchor="0" fill="2" />
+      </item>
+      <item class="javax.swing.JPanel" icon="/com/intellij/uiDesigner/icons/panel.png" removable="false" auto-create-binding="false" can-attach-label="false">
+        <default-constraints vsize-policy="3" hsize-policy="3" anchor="0" fill="3" />
+      </item>
+      <item class="javax.swing.JScrollPane" icon="/com/intellij/uiDesigner/icons/scrollPane.png" removable="false" auto-create-binding="false" can-attach-label="true">
+        <default-constraints vsize-policy="7" hsize-policy="7" anchor="0" fill="3" />
+      </item>
+      <item class="javax.swing.JButton" icon="/com/intellij/uiDesigner/icons/button.png" removable="false" auto-create-binding="true" can-attach-label="false">
+        <default-constraints vsize-policy="0" hsize-policy="3" anchor="0" fill="1" />
+        <initial-values>
+          <property name="text" value="Button" />
+        </initial-values>
+      </item>
+      <item class="javax.swing.JRadioButton" icon="/com/intellij/uiDesigner/icons/radioButton.png" removable="false" auto-create-binding="true" can-attach-label="false">
+        <default-constraints vsize-policy="0" hsize-policy="3" anchor="8" fill="0" />
+        <initial-values>
+          <property name="text" value="RadioButton" />
+        </initial-values>
+      </item>
+      <item class="javax.swing.JCheckBox" icon="/com/intellij/uiDesigner/icons/checkBox.png" removable="false" auto-create-binding="true" can-attach-label="false">
+        <default-constraints vsize-policy="0" hsize-policy="3" anchor="8" fill="0" />
+        <initial-values>
+          <property name="text" value="CheckBox" />
+        </initial-values>
+      </item>
+      <item class="javax.swing.JLabel" icon="/com/intellij/uiDesigner/icons/label.png" removable="false" auto-create-binding="false" can-attach-label="false">
+        <default-constraints vsize-policy="0" hsize-policy="0" anchor="8" fill="0" />
+        <initial-values>
+          <property name="text" value="Label" />
+        </initial-values>
+      </item>
+      <item class="javax.swing.JTextField" icon="/com/intellij/uiDesigner/icons/textField.png" removable="false" auto-create-binding="true" can-attach-label="true">
+        <default-constraints vsize-policy="0" hsize-policy="6" anchor="8" fill="1">
+          <preferred-size width="150" height="-1" />
+        </default-constraints>
+      </item>
+      <item class="javax.swing.JPasswordField" icon="/com/intellij/uiDesigner/icons/passwordField.png" removable="false" auto-create-binding="true" can-attach-label="true">
+        <default-constraints vsize-policy="0" hsize-policy="6" anchor="8" fill="1">
+          <preferred-size width="150" height="-1" />
+        </default-constraints>
+      </item>
+      <item class="javax.swing.JFormattedTextField" icon="/com/intellij/uiDesigner/icons/formattedTextField.png" removable="false" auto-create-binding="true" can-attach-label="true">
+        <default-constraints vsize-policy="0" hsize-policy="6" anchor="8" fill="1">
+          <preferred-size width="150" height="-1" />
+        </default-constraints>
+      </item>
+      <item class="javax.swing.JTextArea" icon="/com/intellij/uiDesigner/icons/textArea.png" removable="false" auto-create-binding="true" can-attach-label="true">
+        <default-constraints vsize-policy="6" hsize-policy="6" anchor="0" fill="3">
+          <preferred-size width="150" height="50" />
+        </default-constraints>
+      </item>
+      <item class="javax.swing.JTextPane" icon="/com/intellij/uiDesigner/icons/textPane.png" removable="false" auto-create-binding="true" can-attach-label="true">
+        <default-constraints vsize-policy="6" hsize-policy="6" anchor="0" fill="3">
+          <preferred-size width="150" height="50" />
+        </default-constraints>
+      </item>
+      <item class="javax.swing.JEditorPane" icon="/com/intellij/uiDesigner/icons/editorPane.png" removable="false" auto-create-binding="true" can-attach-label="true">
+        <default-constraints vsize-policy="6" hsize-policy="6" anchor="0" fill="3">
+          <preferred-size width="150" height="50" />
+        </default-constraints>
+      </item>
+      <item class="javax.swing.JComboBox" icon="/com/intellij/uiDesigner/icons/comboBox.png" removable="false" auto-create-binding="true" can-attach-label="true">
+        <default-constraints vsize-policy="0" hsize-policy="2" anchor="8" fill="1" />
+      </item>
+      <item class="javax.swing.JTable" icon="/com/intellij/uiDesigner/icons/table.png" removable="false" auto-create-binding="true" can-attach-label="false">
+        <default-constraints vsize-policy="6" hsize-policy="6" anchor="0" fill="3">
+          <preferred-size width="150" height="50" />
+        </default-constraints>
+      </item>
+      <item class="javax.swing.JList" icon="/com/intellij/uiDesigner/icons/list.png" removable="false" auto-create-binding="true" can-attach-label="false">
+        <default-constraints vsize-policy="6" hsize-policy="2" anchor="0" fill="3">
+          <preferred-size width="150" height="50" />
+        </default-constraints>
+      </item>
+      <item class="javax.swing.JTree" icon="/com/intellij/uiDesigner/icons/tree.png" removable="false" auto-create-binding="true" can-attach-label="false">
+        <default-constraints vsize-policy="6" hsize-policy="6" anchor="0" fill="3">
+          <preferred-size width="150" height="50" />
+        </default-constraints>
+      </item>
+      <item class="javax.swing.JTabbedPane" icon="/com/intellij/uiDesigner/icons/tabbedPane.png" removable="false" auto-create-binding="true" can-attach-label="false">
+        <default-constraints vsize-policy="3" hsize-policy="3" anchor="0" fill="3">
+          <preferred-size width="200" height="200" />
+        </default-constraints>
+      </item>
+      <item class="javax.swing.JSplitPane" icon="/com/intellij/uiDesigner/icons/splitPane.png" removable="false" auto-create-binding="false" can-attach-label="false">
+        <default-constraints vsize-policy="3" hsize-policy="3" anchor="0" fill="3">
+          <preferred-size width="200" height="200" />
+        </default-constraints>
+      </item>
+      <item class="javax.swing.JSpinner" icon="/com/intellij/uiDesigner/icons/spinner.png" removable="false" auto-create-binding="true" can-attach-label="true">
+        <default-constraints vsize-policy="0" hsize-policy="6" anchor="8" fill="1" />
+      </item>
+      <item class="javax.swing.JSlider" icon="/com/intellij/uiDesigner/icons/slider.png" removable="false" auto-create-binding="true" can-attach-label="false">
+        <default-constraints vsize-policy="0" hsize-policy="6" anchor="8" fill="1" />
+      </item>
+      <item class="javax.swing.JSeparator" icon="/com/intellij/uiDesigner/icons/separator.png" removable="false" auto-create-binding="false" can-attach-label="false">
+        <default-constraints vsize-policy="6" hsize-policy="6" anchor="0" fill="3" />
+      </item>
+      <item class="javax.swing.JProgressBar" icon="/com/intellij/uiDesigner/icons/progressbar.png" removable="false" auto-create-binding="true" can-attach-label="false">
+        <default-constraints vsize-policy="0" hsize-policy="6" anchor="0" fill="1" />
+      </item>
+      <item class="javax.swing.JToolBar" icon="/com/intellij/uiDesigner/icons/toolbar.png" removable="false" auto-create-binding="false" can-attach-label="false">
+        <default-constraints vsize-policy="0" hsize-policy="6" anchor="0" fill="1">
+          <preferred-size width="-1" height="20" />
+        </default-constraints>
+      </item>
+      <item class="javax.swing.JToolBar$Separator" icon="/com/intellij/uiDesigner/icons/toolbarSeparator.png" removable="false" auto-create-binding="false" can-attach-label="false">
+        <default-constraints vsize-policy="0" hsize-policy="0" anchor="0" fill="1" />
+      </item>
+      <item class="javax.swing.JScrollBar" icon="/com/intellij/uiDesigner/icons/scrollbar.png" removable="false" auto-create-binding="true" can-attach-label="false">
+        <default-constraints vsize-policy="6" hsize-policy="0" anchor="0" fill="2" />
+      </item>
+    </group>
+  </component>
+</project>

distributed-event-bus/.idea/vcs.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$/.." vcs="Git" />
+  </component>
+</project>

distributed-event-bus/README.ME

@@ -0,0 +1,8 @@
+1) Multiple publishers and subscribers (Register from any class to eventbus)
+2) Causal ordering of topics
+3) Supports configurable retry attempts.
+4) Have a dead letter queue.
+5) Idempotency on event receiving
+6) Allow both pull and push models
+7) Allow subscribing from a timestamp or offset
+8) Allow preconditions for event subscription

distributed-event-bus/distributed-event-bus.iml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<module org.jetbrains.idea.maven.project.MavenProjectsManager.isMavenModule="true" type="JAVA_MODULE" version="4">
+  <component name="NewModuleRootManager" LANGUAGE_LEVEL="JDK_11">
+    <output url="file://$MODULE_DIR$/target/classes" />
+    <output-test url="file://$MODULE_DIR$/target/test-classes" />
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/src/main/java" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/src/test/java" isTestSource="true" />
+      <sourceFolder url="file://$MODULE_DIR$/src/main/resources" type="java-resource" />
+      <excludeFolder url="file://$MODULE_DIR$/target" />
+    </content>
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+    <orderEntry type="library" scope="TEST" name="Maven: junit:junit:4.13" level="project" />
+    <orderEntry type="library" scope="TEST" name="Maven: org.hamcrest:hamcrest-core:1.3" level="project" />
+    <orderEntry type="library" name="Maven: com.google.inject:guice:4.0" level="project" />
+    <orderEntry type="library" name="Maven: javax.inject:javax.inject:1" level="project" />
+    <orderEntry type="library" name="Maven: aopalliance:aopalliance:1.0" level="project" />
+    <orderEntry type="library" name="Maven: com.google.guava:guava:16.0.1" level="project" />
+    <orderEntry type="library" name="Maven: com.google.code.gson:gson:2.8.6" level="project" />
+  </component>
+</module>

distributed-event-bus/pom.xml

@@ -0,0 +1,42 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>interviewready.io</groupId>
+    <artifactId>distributed-event-bus</artifactId>
+    <version>1.0</version>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>11</source>
+                    <target>11</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <dependencies>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <version>4.13</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.google.inject</groupId>
+            <artifactId>guice</artifactId>
+            <version>4.0</version>
+        </dependency>
+        <dependency>
+            <groupId>com.google.code.gson</groupId>
+            <artifactId>gson</artifactId>
+            <version>2.8.6</version>
+        </dependency>
+    </dependencies>
+
+</project>

distributed-event-bus/src/main/java/EventBus.java

@@ -0,0 +1,203 @@
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+import exceptions.RetryLimitExceededException;
+import exceptions.UnsubscribedPollException;
+import lib.KeyedExecutor;
+import models.Event;
+import models.FailureEvent;
+import models.Subscription;
+import util.Timer;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.*;
+import java.util.function.Function;
+import java.util.function.Predicate;
+
+@Singleton
+public class EventBus {
+    private final Map<String, List<Event>> topics;
+    private final Map<String, Map<String, Integer>> eventIndexes;
+    private final Map<String, ConcurrentSkipListMap<Long, String>> eventTimestamps;
+    private final Map<String, Map<String, Subscription>> pullSubscriptions;
+    private final Map<String, Map<String, Subscription>> pushSubscriptions;
+    private final KeyedExecutor<String> eventExecutor;
+    private final KeyedExecutor<String> broadcastExecutor;
+    private EventBus deadLetterQueue;
+    private final Timer timer;
+
+    @Inject
+    public EventBus(final KeyedExecutor<String> eventExecutor, final KeyedExecutor<String> broadcastExecutor, final Timer timer) {
+        this.topics = new ConcurrentHashMap<>();
+        this.eventIndexes = new ConcurrentHashMap<>();
+        this.eventTimestamps = new ConcurrentHashMap<>();
+        this.pullSubscriptions = new ConcurrentHashMap<>();
+        this.pushSubscriptions = new ConcurrentHashMap<>();
+        this.eventExecutor = eventExecutor;
+        this.broadcastExecutor = broadcastExecutor;
+        this.timer = timer;
+    }
+
+    public void setDeadLetterQueue(final EventBus deadLetterQueue) {
+        this.deadLetterQueue = deadLetterQueue;
+    }
+
+    public CompletionStage<Void> publish(final String topic, final Event event) {
+        return eventExecutor.getThreadFor(topic, publishToBus(topic, event));
+    }
+
+    private CompletionStage<Void> publishToBus(final String topic, final Event event) {
+        if (eventIndexes.containsKey(topic) && eventIndexes.get(topic).containsKey(event.getId())) {
+            return null;
+        }
+        topics.putIfAbsent(topic, new CopyOnWriteArrayList<>());
+        eventIndexes.putIfAbsent(topic, new ConcurrentHashMap<>());
+        eventIndexes.get(topic).put(event.getId(), topics.get(topic).size());
+        eventTimestamps.putIfAbsent(topic, new ConcurrentSkipListMap<>());
+        eventTimestamps.get(topic).put(timer.getCurrentTime(), event.getId());
+        topics.get(topic).add(event);
+        return notifyPushSubscribers(topic, event);
+    }
+
+    private CompletionStage<Void> notifyPushSubscribers(String topic, Event event) {
+        if (!pushSubscriptions.containsKey(topic)) {
+            return CompletableFuture.completedStage(null);
+        }
+        final var subscribersForTopic = pushSubscriptions.get(topic);
+        final var notifications = subscribersForTopic.values()
+                .stream()
+                .filter(subscription -> subscription.getPrecondition().test(event))
+                .map(subscription -> executeEventHandler(event, subscription))
+                .toArray(CompletableFuture[]::new);
+        return CompletableFuture.allOf(notifications);
+    }
+
+    private CompletionStage<Void> executeEventHandler(final Event event, Subscription subscription) {
+        return broadcastExecutor.getThreadFor(subscription.getTopic() + subscription.getSubscriber(),
+                doWithRetry(event, subscription.getEventHandler(),
+                        1, subscription.getNumberOfRetries())
+                        .exceptionally(throwable -> {
+                            if (deadLetterQueue != null) {
+                                deadLetterQueue.publish(subscription.getTopic(), new FailureEvent(event, throwable, timer.getCurrentTime()));
+                            }
+                            return null;
+                        }));
+    }
+
+    private CompletionStage<Void> doWithRetry(final Event event,
+                                              final Function<Event, CompletionStage<Void>> task,
+                                              final int coolDownIntervalInMillis,
+                                              final int remainingTries) {
+        return task.apply(event).handle((__, throwable) -> {
+            if (throwable != null) {
+                if (remainingTries == 1) {
+                    throw new RetryLimitExceededException(throwable);
+                }
+                try {
+                    Thread.sleep(coolDownIntervalInMillis);
+                } catch (InterruptedException e) {
+                    throw new RuntimeException(e);
+                }
+                return doWithRetry(event, task, Math.max(coolDownIntervalInMillis * 2, 10), remainingTries - 1);
+            } else {
+                return CompletableFuture.completedFuture((Void) null);
+            }
+        }).thenCompose(Function.identity());
+    }
+
+
+    public CompletionStage<Event> poll(final String topic, final String subscriber) {
+        return eventExecutor.getThreadFor(topic + subscriber, () -> pollBus(topic, subscriber));
+    }
+
+    private Event pollBus(final String topic, final String subscriber) {
+        var subscription = pullSubscriptions.getOrDefault(topic, new HashMap<>()).get(subscriber);
+        if (subscription == null) {
+            throw new UnsubscribedPollException();
+        }
+        for (var index = subscription.getCurrentIndex(); index.intValue() < topics.get(topic).size(); index.increment()) {
+            var event = topics.get(topic).get(index.intValue());
+            if (subscription.getPrecondition().test(event)) {
+                index.increment();
+                return event;
+            }
+        }
+        return null;
+    }
+
+    public CompletionStage<Void> subscribeToEventsAfter(final String topic, final String subscriber, final long timeStamp) {
+        return eventExecutor.getThreadFor(topic + subscriber, () -> moveIndexAtTimestamp(topic, subscriber, timeStamp));
+    }
+
+    private void moveIndexAtTimestamp(final String topic, final String subscriber, final long timeStamp) {
+        final var closestEventAfter = eventTimestamps.get(topic).higherEntry(timeStamp);
+        if (closestEventAfter == null) {
+            pullSubscriptions.get(topic).get(subscriber).setCurrentIndex(eventIndexes.get(topic).size());
+        } else {
+            final var eventIndex = eventIndexes.get(topic).get(closestEventAfter.getValue());
+            pullSubscriptions.get(topic).get(subscriber).setCurrentIndex(eventIndex);
+        }
+    }
+
+    public CompletionStage<Void> subscribeToEventsAfter(final String topic, final String subscriber, final String eventId) {
+        return eventExecutor.getThreadFor(topic + subscriber, () -> moveIndexAfterEvent(topic, subscriber, eventId));
+    }
+
+    private void moveIndexAfterEvent(final String topic, final String subscriber, final String eventId) {
+        if (eventId == null) {
+            pullSubscriptions.get(topic).get(subscriber).setCurrentIndex(0);
+        } else {
+            final var eventIndex = eventIndexes.get(topic).get(eventId) + 1;
+            pullSubscriptions.get(topic).get(subscriber).setCurrentIndex(eventIndex);
+        }
+    }
+
+    public CompletionStage<Void> subscribeForPush(final String topic,
+                                                  final String subscriber,
+                                                  final Predicate<Event> precondition,
+                                                  final Function<Event, CompletionStage<Void>> handler,
+                                                  final int numberOfRetries) {
+        return eventExecutor.getThreadFor(topic + subscriber,
+                () -> subscribeForPushEvents(topic, subscriber, precondition, handler, numberOfRetries));
+    }
+
+    private void subscribeForPushEvents(final String topic,
+                                        final String subscriber,
+                                        final Predicate<Event> precondition,
+                                        final Function<Event, CompletionStage<Void>> handler,
+                                        final int numberOfRetries) {
+        addSubscriber(pushSubscriptions, subscriber, precondition, topic, handler, numberOfRetries);
+    }
+
+    private void addSubscriber(final Map<String, Map<String, Subscription>> pullSubscriptions,
+                               final String subscriber,
+                               final Predicate<Event> precondition,
+                               final String topic,
+                               final Function<Event, CompletionStage<Void>> handler,
+                               final int numberOfRetries) {
+        pullSubscriptions.putIfAbsent(topic, new ConcurrentHashMap<>());
+        final var subscription = new Subscription(topic, subscriber, precondition, handler, numberOfRetries);
+        subscription.setCurrentIndex(topics.getOrDefault(topic, new ArrayList<>()).size());
+        pullSubscriptions.get(topic).put(subscriber, subscription);
+    }
+
+    public CompletionStage<Void> subscribeForPull(final String topic, final String subscriber, final Predicate<Event> precondition) {
+        return eventExecutor.getThreadFor(topic + subscriber, () -> subscribeForPullEvents(topic, subscriber, precondition));
+    }
+
+    private void subscribeForPullEvents(final String topic, final String subscriber, final Predicate<Event> precondition) {
+        addSubscriber(pullSubscriptions, subscriber, precondition, topic, null, 0);
+    }
+
+    public CompletionStage<Void> unsubscribe(final String topic, final String subscriber) {
+        return eventExecutor.getThreadFor(topic + subscriber, () -> unsubscribeFromTopic(topic, subscriber));
+    }
+
+    private void unsubscribeFromTopic(final String topic, final String subscriber) {
+        pushSubscriptions.getOrDefault(topic, new HashMap<>()).remove(subscriber);
+        pullSubscriptions.getOrDefault(topic, new HashMap<>()).remove(subscriber);
+    }
+}
+

distributed-event-bus/src/main/java/exceptions/RetryLimitExceededException.java

@@ -0,0 +1,7 @@
+package exceptions;
+
+public class RetryLimitExceededException extends RuntimeException {
+    public RetryLimitExceededException(Throwable cause) {
+        super(cause);
+    }
+}

distributed-event-bus/src/main/java/exceptions/UnsubscribedPollException.java

@@ -0,0 +1,4 @@
+package exceptions;
+
+public class UnsubscribedPollException extends RuntimeException {
+}

distributed-event-bus/src/main/java/lib/KeyedExecutor.java

@@ -0,0 +1,31 @@
+package lib;
+
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionStage;
+import java.util.concurrent.Executor;
+import java.util.concurrent.Executors;
+import java.util.function.Function;
+import java.util.function.Supplier;
+
+public class KeyedExecutor<KEY> {
+    private final Executor[] executorPool;
+
+    public KeyedExecutor(final int poolSize) {
+        this.executorPool = new Executor[poolSize];
+        for (int i = 0; i < poolSize; i++) {
+            executorPool[i] = Executors.newSingleThreadExecutor();
+        }
+    }
+
+    public CompletionStage<Void> getThreadFor(KEY key, Runnable task) {
+        return CompletableFuture.runAsync(task, executorPool[Math.abs(key.hashCode() % executorPool.length)]);
+    }
+
+    public <U> CompletionStage<U> getThreadFor(KEY key, Supplier<U> task) {
+        return CompletableFuture.supplyAsync(task, executorPool[Math.abs(key.hashCode() % executorPool.length)]);
+    }
+
+    public <U> CompletionStage<U> getThreadFor(KEY key, CompletionStage<U> task) {
+        return CompletableFuture.supplyAsync(() -> task, executorPool[Math.abs(key.hashCode() % executorPool.length)]).thenCompose(Function.identity());
+    }
+}

distributed-event-bus/src/main/java/models/Event.java

@@ -0,0 +1,43 @@
+package models;
+
+import java.util.Objects;
+import java.util.UUID;
+
+public class Event {
+    private final String id;
+    private final String publisher;
+    private final EventType eventType;
+    private final String description;
+    private final long creationTime;
+
+    public Event(final String publisher,
+                 final EventType eventType,
+                 final String description,
+                 final long creationTime) {
+        this.description = description;
+        this.id = UUID.randomUUID().toString();
+        this.publisher = publisher;
+        this.eventType = eventType;
+        this.creationTime = creationTime;
+    }
+
+    public String getId() {
+        return id;
+    }
+
+    public String getPublisher() {
+        return publisher;
+    }
+
+    public EventType getEventType() {
+        return eventType;
+    }
+
+    public long getCreationTime() {
+        return creationTime;
+    }
+
+    public String getDescription() {
+        return description;
+    }
+}

distributed-event-bus/src/main/java/models/EventType.java

@@ -0,0 +1,5 @@
+package models;
+
+public enum EventType {
+    PRIORITY, LOGGING, ERROR
+}

distributed-event-bus/src/main/java/models/FailureEvent.java

@@ -0,0 +1,20 @@
+package models;
+
+public class FailureEvent extends Event {
+    private final Event event;
+    private final Throwable throwable;
+
+    public FailureEvent(Event event, Throwable throwable, long failureTimestamp) {
+        super("dead-letter-queue", EventType.ERROR, throwable.getMessage(), failureTimestamp);
+        this.event = event;
+        this.throwable = throwable;
+    }
+
+    public Event getEvent() {
+        return event;
+    }
+
+    public Throwable getThrowable() {
+        return throwable;
+    }
+}

distributed-event-bus/src/main/java/models/Subscription.java

@@ -0,0 +1,57 @@
+package models;
+
+import java.util.concurrent.CompletionStage;
+import java.util.concurrent.atomic.LongAdder;
+import java.util.function.Function;
+import java.util.function.Predicate;
+
+public class Subscription {
+    private final String topic;
+    private final String subscriber;
+    private final Predicate<Event> precondition;
+    private final Function<Event, CompletionStage<Void>> eventHandler;
+    private final int numberOfRetries;
+    private final LongAdder currentIndex;
+
+    public Subscription(final String topic,
+                        final String subscriber,
+                        final Predicate<Event> precondition,
+                        final Function<Event, CompletionStage<Void>> eventHandler,
+                        final int numberOfRetries) {
+        this.topic = topic;
+        this.subscriber = subscriber;
+        this.precondition = precondition;
+        this.eventHandler = eventHandler;
+        this.currentIndex = new LongAdder();
+        this.numberOfRetries = numberOfRetries;
+    }
+
+    public String getTopic() {
+        return topic;
+    }
+
+    public String getSubscriber() {
+        return subscriber;
+    }
+
+    public Predicate<Event> getPrecondition() {
+        return precondition;
+    }
+
+    public Function<Event, CompletionStage<Void>> getEventHandler() {
+        return eventHandler;
+    }
+
+    public LongAdder getCurrentIndex() {
+        return currentIndex;
+    }
+
+    public void setCurrentIndex(final int offset) {
+        currentIndex.reset();
+        currentIndex.add(offset);
+    }
+
+    public int getNumberOfRetries() {
+        return numberOfRetries;
+    }
+}

distributed-event-bus/src/main/java/util/Timer.java

@@ -0,0 +1,10 @@
+package util;
+
+import com.google.inject.Singleton;
+
+@Singleton
+public class Timer {
+    public long getCurrentTime() {
+        return System.nanoTime();
+    }
+}

distributed-event-bus/src/test/java/EventBusTest.java

@@ -0,0 +1,340 @@
+import com.google.gson.Gson;
+import exceptions.RetryLimitExceededException;
+import exceptions.UnsubscribedPollException;
+import lib.KeyedExecutor;
+import models.Event;
+import models.EventType;
+import models.FailureEvent;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+import util.Timer;
+
+import java.time.Duration;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.UUID;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.atomic.AtomicLong;
+
+
+// Causal ordering of topics
+
+public class EventBusTest {
+    public static final String TOPIC_1 = "topic-1";
+    public static final String TOPIC_2 = "topic-2";
+    public static final String PUBLISHER_1 = "publisher-1";
+    public static final String SUBSCRIBER_1 = "subscriber-1";
+    public static final String SUBSCRIBER_2 = "subscriber-2";
+    private Timer timer;
+    private KeyedExecutor<String> keyedExecutor;
+    private KeyedExecutor<String> broadcastExecutor;
+
+    @Before
+    public void setUp() {
+        keyedExecutor = new KeyedExecutor<>(16);
+        broadcastExecutor = new KeyedExecutor<>(16);
+        timer = new Timer();
+    }
+
+    private Event constructEvent(EventType priority, String description) {
+        return new Event(PUBLISHER_1, priority, description, timer.getCurrentTime());
+    }
+
+    @Test
+    public void defaultBehavior() {
+        final EventBus eventBus = new EventBus(keyedExecutor, broadcastExecutor, timer);
+        eventBus.publish(TOPIC_1, constructEvent(EventType.LOGGING, "first event"));
+        eventBus.subscribeForPull(TOPIC_1, SUBSCRIBER_1, (event) -> true).toCompletableFuture().join();
+        Assert.assertNull(eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join());
+
+        eventBus.publish(TOPIC_1, constructEvent(EventType.PRIORITY, "second event"));
+        final Event secondEvent = eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+
+        Assert.assertEquals(EventType.PRIORITY, secondEvent.getEventType());
+        Assert.assertEquals("second event", secondEvent.getDescription());
+
+        eventBus.subscribeToEventsAfter(TOPIC_1, SUBSCRIBER_1, null).toCompletableFuture().join();
+        final Event firstEvent = eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+
+        Assert.assertEquals(EventType.LOGGING, firstEvent.getEventType());
+        Assert.assertEquals("first event", firstEvent.getDescription());
+        Assert.assertEquals(PUBLISHER_1, firstEvent.getPublisher());
+
+        final List<Event> eventCollector = new ArrayList<>();
+        eventBus.subscribeForPush(TOPIC_1,
+                SUBSCRIBER_2,
+                (event) -> true,
+                (event) -> CompletableFuture.runAsync(() -> eventCollector.add(event)),
+                0).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, constructEvent(EventType.ERROR, "third event")).toCompletableFuture().join();
+
+        Assert.assertEquals(EventType.ERROR, eventCollector.get(0).getEventType());
+        Assert.assertEquals("third event", eventCollector.get(0).getDescription());
+
+        eventBus.unsubscribe(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, constructEvent(EventType.LOGGING, "fourth event")).toCompletableFuture().join();
+        Assert.assertTrue(eventBus.poll(TOPIC_1, SUBSCRIBER_1)
+                .handle((__, throwable) -> throwable.getCause() instanceof UnsubscribedPollException)
+                .toCompletableFuture().join());
+
+        eventCollector.clear();
+        eventBus.unsubscribe(TOPIC_1, SUBSCRIBER_2).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, constructEvent(EventType.LOGGING, "fifth event")).toCompletableFuture().join();
+        Assert.assertTrue(eventCollector.isEmpty());
+    }
+
+    @Test
+    public void indexMove() {
+        final EventBus eventBus = new EventBus(keyedExecutor, broadcastExecutor, timer);
+        eventBus.subscribeForPull(TOPIC_1, SUBSCRIBER_1, (event) -> true).toCompletableFuture().join();
+        final Event firstEvent = constructEvent(EventType.PRIORITY, "first event");
+        final Event secondEvent = constructEvent(EventType.PRIORITY, "second event");
+        final Event thirdEvent = constructEvent(EventType.PRIORITY, "third event");
+        eventBus.publish(TOPIC_1, firstEvent).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, secondEvent).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, thirdEvent).toCompletableFuture().join();
+
+        eventBus.subscribeToEventsAfter(TOPIC_1, SUBSCRIBER_1, secondEvent.getId()).toCompletableFuture().join();
+        final Event firstPoll = eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+        Assert.assertEquals("third event", firstPoll.getDescription());
+        Assert.assertNull(eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join());
+
+        eventBus.subscribeToEventsAfter(TOPIC_1, SUBSCRIBER_1, null).toCompletableFuture().join();
+        final Event secondPoll = eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+        Assert.assertEquals("first event", secondPoll.getDescription());
+
+        eventBus.subscribeToEventsAfter(TOPIC_1, SUBSCRIBER_1, firstEvent.getId()).toCompletableFuture().join();
+        final Event thirdPoll = eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+        Assert.assertEquals("second event", thirdPoll.getDescription());
+
+        eventBus.subscribeToEventsAfter(TOPIC_1, SUBSCRIBER_1, thirdEvent.getId()).toCompletableFuture().join();
+        Assert.assertNull(eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join());
+    }
+
+    @Test
+    public void timestampMove() {
+        final TestTimer timer = new TestTimer();
+        final EventBus eventBus = new EventBus(keyedExecutor, broadcastExecutor, timer);
+        eventBus.subscribeForPull(TOPIC_1, SUBSCRIBER_1, (event) -> true).toCompletableFuture().join();
+
+        final Event firstEvent = new Event(PUBLISHER_1, EventType.PRIORITY, "first event", timer.getCurrentTime());
+        eventBus.publish(TOPIC_1, firstEvent).toCompletableFuture().join();
+        timer.setCurrentTime(timer.getCurrentTime() + Duration.ofSeconds(10).toNanos());
+
+        final Event secondEvent = new Event(PUBLISHER_1, EventType.PRIORITY, "second event", timer.getCurrentTime());
+        eventBus.publish(TOPIC_1, secondEvent).toCompletableFuture().join();
+        timer.setCurrentTime(timer.getCurrentTime() + Duration.ofSeconds(10).toNanos());
+
+        final Event thirdEvent = new Event(PUBLISHER_1, EventType.PRIORITY, "third event", timer.getCurrentTime());
+        eventBus.publish(TOPIC_1, thirdEvent).toCompletableFuture().join();
+
+        eventBus.subscribeToEventsAfter(TOPIC_1, SUBSCRIBER_1, secondEvent.getCreationTime() + Duration.ofSeconds(5).toNanos()).toCompletableFuture().join();
+        final Event firstPoll = eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+        Assert.assertEquals("third event", firstPoll.getDescription());
+        Assert.assertNull(eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join());
+
+        eventBus.subscribeToEventsAfter(TOPIC_1, SUBSCRIBER_1, 0).toCompletableFuture().join();
+        final Event secondPoll = eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+        Assert.assertEquals("first event", secondPoll.getDescription());
+
+        eventBus.subscribeToEventsAfter(TOPIC_1, SUBSCRIBER_1, firstEvent.getCreationTime() + Duration.ofSeconds(5).toNanos()).toCompletableFuture().join();
+        final Event thirdPoll = eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+        Assert.assertEquals("second event", thirdPoll.getDescription());
+
+        eventBus.subscribeToEventsAfter(TOPIC_1, SUBSCRIBER_1, thirdEvent.getCreationTime() + Duration.ofNanos(1).toNanos()).toCompletableFuture().join();
+        Assert.assertNull(eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join());
+    }
+
+    @Test
+    public void idempotency() {
+        final EventBus eventBus = new EventBus(keyedExecutor, broadcastExecutor, timer);
+        eventBus.subscribeForPull(TOPIC_1, SUBSCRIBER_1, (event) -> true).toCompletableFuture().join();
+        Event event1 = new Gson().fromJson("{\n" +
+                "  \"id\": \"event-5435\",\n" +
+                "  \"publisher\": \"random-publisher-1\",\n" +
+                "  \"eventType\": \"LOGGING\",\n" +
+                "  \"description\": \"random-event-1\",\n" +
+                "  \"creationTime\": 31884739810179363\n" +
+                "}", Event.class);
+        eventBus.publish(TOPIC_1, event1);
+
+        Event event2 = new Gson().fromJson("{\n" +
+                "  \"id\": \"event-5435\",\n" +
+                "  \"publisher\": \"random-publisher-2\",\n" +
+                "  \"eventType\": \"PRIORITY\",\n" +
+                "  \"description\": \"random-event-2\",\n" +
+                "  \"creationTime\": 31824735510179363\n" +
+                "}", Event.class);
+        eventBus.publish(TOPIC_1, event2);
+
+
+        final Event firstEvent = eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+        Assert.assertEquals(EventType.LOGGING, firstEvent.getEventType());
+        Assert.assertEquals("random-event-1", firstEvent.getDescription());
+        Assert.assertNull(eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join());
+    }
+
+    @Test
+    public void unsubscribePushEvents() {
+        final EventBus eventBus = new EventBus(keyedExecutor, broadcastExecutor, timer);
+        final List<Event> topic1 = new ArrayList<>(), topic2 = new ArrayList<>();
+        eventBus.subscribeForPush(TOPIC_1, SUBSCRIBER_1, event -> true, event -> {
+            topic1.add(event);
+            return CompletableFuture.completedStage(null);
+        }, 0).toCompletableFuture().join();
+        eventBus.subscribeForPush(TOPIC_2, SUBSCRIBER_1, event -> true, event -> {
+            topic2.add(event);
+            return CompletableFuture.completedStage(null);
+        }, 0).toCompletableFuture().join();
+
+        for (int i = 0; i < 3; i++) {
+            eventBus.publish(TOPIC_1, constructEvent(EventType.PRIORITY, UUID.randomUUID().toString())).toCompletableFuture().join();
+        }
+        eventBus.publish(TOPIC_2, constructEvent(EventType.PRIORITY, UUID.randomUUID().toString())).toCompletableFuture().join();
+        eventBus.unsubscribe(TOPIC_1, SUBSCRIBER_1);
+        Assert.assertEquals(3, topic1.size());
+        Assert.assertEquals(1, topic2.size());
+
+        for (int i = 0; i < 2; i++) {
+            eventBus.publish(TOPIC_2, constructEvent(EventType.PRIORITY, UUID.randomUUID().toString())).toCompletableFuture().join();
+        }
+        for (int i = 0; i < 3; i++) {
+            eventBus.publish(TOPIC_1, constructEvent(EventType.PRIORITY, UUID.randomUUID().toString())).toCompletableFuture().join();
+        }
+        Assert.assertEquals(3, topic1.size());
+        Assert.assertEquals(3, topic2.size());
+
+        eventBus.subscribeForPush(TOPIC_1, SUBSCRIBER_1, event -> true, event -> {
+            topic1.add(event);
+            return CompletableFuture.completedStage(null);
+        }, 0).toCompletableFuture().join();
+        for (int i = 0; i < 3; i++) {
+            eventBus.publish(TOPIC_1, constructEvent(EventType.PRIORITY, UUID.randomUUID().toString())).toCompletableFuture().join();
+        }
+        Assert.assertEquals(6, topic1.size());
+        Assert.assertEquals(3, topic2.size());
+    }
+
+    @Test
+    public void unsubscribePullEvents() {
+        final EventBus eventBus = new EventBus(keyedExecutor, broadcastExecutor, timer);
+        eventBus.subscribeForPull(TOPIC_1, SUBSCRIBER_1, event -> true).toCompletableFuture().join();
+        eventBus.subscribeForPull(TOPIC_2, SUBSCRIBER_1, event -> true).toCompletableFuture().join();
+        for (int i = 0; i < 3; i++) {
+            eventBus.publish(TOPIC_1, constructEvent(EventType.PRIORITY, UUID.randomUUID().toString())).toCompletableFuture().join();
+        }
+        eventBus.publish(TOPIC_2, constructEvent(EventType.PRIORITY, UUID.randomUUID().toString())).toCompletableFuture().join();
+
+        for (int i = 0; i < 3; i++) {
+            Assert.assertNotNull(eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join());
+        }
+        Assert.assertNotNull(eventBus.poll(TOPIC_2, SUBSCRIBER_1).toCompletableFuture().join());
+
+        eventBus.unsubscribe(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+        for (int i = 0; i < 2; i++) {
+            eventBus.publish(TOPIC_2, constructEvent(EventType.PRIORITY, UUID.randomUUID().toString())).toCompletableFuture().join();
+        }
+        for (int i = 0; i < 3; i++) {
+            eventBus.publish(TOPIC_1, constructEvent(EventType.PRIORITY, UUID.randomUUID().toString())).toCompletableFuture().join();
+        }
+
+        Assert.assertTrue(eventBus.poll(TOPIC_1, SUBSCRIBER_1)
+                .handle((__, throwable) -> throwable.getCause() instanceof UnsubscribedPollException).toCompletableFuture().join());
+        for (int i = 0; i < 2; i++) {
+            Assert.assertNotNull(eventBus.poll(TOPIC_2, SUBSCRIBER_1).toCompletableFuture().join());
+        }
+
+        eventBus.subscribeForPull(TOPIC_1, SUBSCRIBER_1, event -> true).toCompletableFuture().join();
+        for (int i = 0; i < 3; i++) {
+            eventBus.publish(TOPIC_1, constructEvent(EventType.PRIORITY, UUID.randomUUID().toString())).toCompletableFuture().join();
+        }
+
+        for (int i = 0; i < 3; i++) {
+            Assert.assertNotNull(eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join());
+        }
+        Assert.assertNull(eventBus.poll(TOPIC_2, SUBSCRIBER_1).toCompletableFuture().join());
+    }
+
+    @Test
+    public void deadLetterQueue() {
+        final EventBus eventBus = new EventBus(keyedExecutor, broadcastExecutor, timer);
+        final EventBus dlq = new EventBus(new KeyedExecutor<>(3), new KeyedExecutor<>(3), new Timer());
+        eventBus.setDeadLetterQueue(dlq);
+        dlq.subscribeForPull(TOPIC_1, SUBSCRIBER_1, event -> event.getEventType().equals(EventType.ERROR));
+        final AtomicLong attempts = new AtomicLong();
+        final int maxTries = 5;
+        eventBus.subscribeForPush(TOPIC_1, SUBSCRIBER_1, event -> true, event -> {
+            attempts.incrementAndGet();
+            return CompletableFuture.failedStage(new RuntimeException());
+        }, maxTries).toCompletableFuture().join();
+        final Event event = new Event(PUBLISHER_1, EventType.LOGGING, "random", timer.getCurrentTime());
+        eventBus.publish(TOPIC_1, event).toCompletableFuture().join();
+        Assert.assertEquals(5, attempts.intValue());
+        final Event failureEvent = dlq.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+        Assert.assertTrue(failureEvent instanceof FailureEvent);
+        Assert.assertEquals(event.getId(), ((FailureEvent) failureEvent).getEvent().getId());
+        Assert.assertEquals(EventType.ERROR, failureEvent.getEventType());
+        Assert.assertTrue(((FailureEvent) failureEvent).getThrowable().getCause() instanceof RetryLimitExceededException);
+    }
+
+    @Test
+    public void retrySuccess() {
+        final EventBus eventBus = new EventBus(keyedExecutor, broadcastExecutor, timer);
+        final AtomicLong attempts = new AtomicLong();
+        final int maxTries = 5;
+        final List<Event> events = new ArrayList<>();
+        eventBus.subscribeForPush(TOPIC_1, SUBSCRIBER_1, event -> true, event -> {
+            if (attempts.incrementAndGet() == maxTries) {
+                events.add(event);
+                return CompletableFuture.completedStage(null);
+            } else {
+                return CompletableFuture.failedStage(new RuntimeException("TRY no: " + attempts.intValue()));
+            }
+        }, maxTries).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, new Event(PUBLISHER_1, EventType.LOGGING, "random", timer.getCurrentTime())).toCompletableFuture().join();
+
+        Assert.assertEquals(EventType.LOGGING, events.get(0).getEventType());
+        Assert.assertEquals("random", events.get(0).getDescription());
+        Assert.assertEquals(5, attempts.intValue());
+        Assert.assertEquals(1, events.size());
+    }
+
+    @Test
+    public void preconditionCheckForPush() {
+        final EventBus eventBus = new EventBus(keyedExecutor, broadcastExecutor, timer);
+        final List<Event> events = new ArrayList<>();
+        eventBus.subscribeForPush(TOPIC_1, SUBSCRIBER_1, event -> event.getDescription().contains("-1"), event -> {
+            events.add(event);
+            return CompletableFuture.completedStage(null);
+        }, 0).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, new Event(PUBLISHER_1, EventType.LOGGING, "random-event-1", timer.getCurrentTime())).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, new Event(PUBLISHER_1, EventType.LOGGING, "random-event-2", timer.getCurrentTime())).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, new Event(PUBLISHER_1, EventType.LOGGING, "random-event-12", timer.getCurrentTime())).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, new Event(PUBLISHER_1, EventType.LOGGING, "random-event-21", timer.getCurrentTime())).toCompletableFuture().join();
+
+        Assert.assertEquals(events.size(), 2);
+        Assert.assertEquals(EventType.LOGGING, events.get(0).getEventType());
+        Assert.assertEquals("random-event-1", events.get(0).getDescription());
+        Assert.assertEquals(EventType.LOGGING, events.get(1).getEventType());
+        Assert.assertEquals("random-event-12", events.get(1).getDescription());
+    }
+
+    @Test
+    public void preconditionCheckForPull() {
+        final EventBus eventBus = new EventBus(keyedExecutor, broadcastExecutor, timer);
+        eventBus.subscribeForPull(TOPIC_1, SUBSCRIBER_1, event -> event.getDescription().contains("-1")).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, new Event(PUBLISHER_1, EventType.LOGGING, "random-event-1", timer.getCurrentTime())).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, new Event(PUBLISHER_1, EventType.LOGGING, "random-event-2", timer.getCurrentTime())).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, new Event(PUBLISHER_1, EventType.LOGGING, "random-event-12", timer.getCurrentTime())).toCompletableFuture().join();
+        eventBus.publish(TOPIC_1, new Event(PUBLISHER_1, EventType.LOGGING, "random-event-21", timer.getCurrentTime())).toCompletableFuture().join();
+
+        final Event event1 = eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+        Assert.assertEquals(EventType.LOGGING, event1.getEventType());
+        Assert.assertEquals("random-event-1", event1.getDescription());
+        final Event event2 = eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join();
+        Assert.assertEquals(EventType.LOGGING, event2.getEventType());
+        Assert.assertEquals("random-event-12", event2.getDescription());
+        Assert.assertNull(eventBus.poll(TOPIC_1, SUBSCRIBER_1).toCompletableFuture().join());
+    }
+}

distributed-event-bus/src/test/java/TestTimer.java

@@ -0,0 +1,18 @@
+import util.Timer;
+
+public class TestTimer extends Timer {
+    private long currentTime;
+
+    public TestTimer() {
+        this.currentTime = System.nanoTime();
+    }
+
+    @Override
+    public long getCurrentTime() {
+        return currentTime;
+    }
+
+    public void setCurrentTime(final long currentTime) {
+        this.currentTime = currentTime;
+    }
+}