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June 22, 2024 02:32
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Example of event sourcing in java
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| import java.util.ArrayList; | |
| import java.util.HashMap; | |
| import java.util.HashSet; | |
| import java.util.List; | |
| import java.util.Map; | |
| import java.util.Set; | |
| /** | |
| * Main entry point | |
| */ | |
| class EventSource { | |
| public static void | |
| main(String args[]) { | |
| Service.startRepeatingEvent(); | |
| EventStore eventStore = new EventStore(); | |
| Client clientA = new Client(eventStore); | |
| System.out.println("\nAccumlating events..."); | |
| try { Thread.sleep(3000L); } catch (Exception e) { } | |
| System.out.println("\nFinal state: "+ clientA.state); | |
| System.out.println("\nEvent store contents..."); | |
| for (Object event : eventStore.events) { | |
| System.out.println((Map) event); | |
| } | |
| EventStore newStore = new EventStore(); | |
| Client clientB = new Client(newStore); | |
| System.out.println("\nReplaying events on new instance..."); | |
| eventStore.replay(clientA, clientB); | |
| System.out.println("\nFinal state: "+ clientB.state); | |
| } | |
| } | |
| /** | |
| * Handles events | |
| */ | |
| interface EventHandler { | |
| void handle(Object event, Object data); | |
| } | |
| /** | |
| * Can be subscribed to | |
| */ | |
| interface Subscribable { | |
| void subscribe(EventHandler subscriber, Object publisher, Object event); | |
| } | |
| /** | |
| * Example of a client object | |
| */ | |
| class Client implements EventHandler { | |
| EventStore eventStore; | |
| String state = ""; | |
| Client(EventStore es) { | |
| eventStore = es; | |
| eventStore.subscribe(this, new Service(), Service.Event.TIMER); | |
| } | |
| @Override public void | |
| handle(Object event, Object data) { | |
| state = event +": "+ data; | |
| System.out.println(state); | |
| } | |
| } | |
| /** | |
| * All (state changing) events routed through an event store instance | |
| */ | |
| class EventStore implements EventHandler, Subscribable { | |
| List events = new ArrayList(); | |
| Map subscriptions = new HashMap(); | |
| void | |
| addSubscriber(EventHandler subscriber, Object event) { | |
| Set subscribers = (Set) subscriptions.getOrDefault(event, new HashSet()); | |
| subscribers.add(subscriber); | |
| subscriptions.put(event, subscribers); | |
| } | |
| @Override public void | |
| subscribe(EventHandler subscriber, | |
| Object publisher, | |
| Object event) | |
| { | |
| addSubscriber(subscriber, event); | |
| ((Subscribable) publisher).subscribe(this, publisher, event); | |
| System.out.println("\nSubscribe to class: "+ publisher); | |
| } | |
| @Override public void | |
| handle(Object event, Object data) { | |
| /* Save event */ | |
| Map map = new HashMap(); | |
| map.put("event", event); | |
| map.put("data", data); | |
| events.add(map); | |
| /* Notify subscribers */ | |
| Set subscribers = (Set) subscriptions.getOrDefault(event, new HashSet()); | |
| for (Object subscriber : subscribers) { | |
| ((EventHandler) subscriber).handle(event, data); | |
| } | |
| } | |
| void | |
| replay(EventHandler oldClient, EventHandler newClient) { | |
| Set emptySet = new HashSet(); | |
| for (Object event : events) { | |
| Set subscribers = (Set) subscriptions.getOrDefault( | |
| ((Map) event).get("event"), emptySet); | |
| if (subscribers.contains(oldClient)) { | |
| newClient.handle( | |
| ((Map) event).get("event"), ((Map) event).get("data")); | |
| } | |
| } | |
| } | |
| } | |
| /** | |
| * An example of a service | |
| */ | |
| class Service extends java.util.TimerTask implements Subscribable { | |
| static enum Event { TIMER } | |
| static Set subscribers = new HashSet(); | |
| static void | |
| startRepeatingEvent() { | |
| new java.util.Timer(true).scheduleAtFixedRate(new Service(), 0L, 1000L); | |
| } | |
| /* Publish repeating event */ | |
| @Override public void | |
| run() { | |
| for (Object subscriber : subscribers) { | |
| ((EventHandler) subscriber).handle( | |
| Event.TIMER, java.time.LocalDateTime.now()); | |
| } | |
| } | |
| @Override public void | |
| subscribe(EventHandler subscriber, | |
| Object publisher, | |
| Object event) | |
| { | |
| subscribers.add(subscriber); | |
| } | |
| } |
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