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F R O M U S E C A S E A R C H I T E C T U R E T O E V E N T S O U R C I N G T H R O U G H C Q R S W I T H S O M E D D D I N I T. T H E B U Z Z W O R D C O N F E R E N C E

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@ L I L O B A S E – L G O . G R O U P Arnaud LEMAIRE | LGO

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VS DATA-DRIVEN DOMAIN-DRIVEN WHAT DATA ? WHAT BEHAVIOR ?

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VS UPDATE ADDRESS MISTAKE CORRECTION RELOCATION CRUD ARCHITECTURE TASK BASED UI STAND FOR USER INTERACTION USER INTENT IS LOST :-(

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SO, HOW DO WE START ? SubscriptionController UserController DashboardController SessionController MCD CONTROLLERS

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MoveHub AddMaintenanceOperation TestLighting CommissionHub ENTITIES USE CASES Hub MaintenanceOperation TestPlan Commissioning YOU DON’T NEED TO HAVE THE FULL CORRECT LISTS, JUST ENOUGH TO GET STARTED HOW DO WE START ?

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Line Invoice PRODUCT Line Line PRODUCT PRODUCT INVENTORY CATEGORY PRODUCT CATEGORY CATEGORY PRODUCT PRODUCT

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Line Invoice PRODUCT Line Line PRODUCT PRODUCT INVENTORY CATEGORY CATEGORY CATEGORY

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IL Y A PLUSIEURS METIERS DANS UNE APPLICATION ET VOUS NE DEVEZ PAS LES COUPLER

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U S E C A S E & A G G R E G AT E R O O T A N I N T R O D U C T I O N

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ENTITIES & VALUE OBJECTS DateTime Money BillingInterval ENTITIES VALUE OBJECTS Hub Maintenance Operation Session THE DOMAIN IS EXPRESS THROUGH PLAIN OLD PHP OBJECTS CATEGORIZED BETWEEN ENTITIES & VALUE OBJECTS EQUALITY ON IDENTITY, HAS LIFECYCLE EQUALITY ON VALUES, HAS NO LIFECYCLE

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AGGREGATE ROOT HUB MAINTENANCE OPERATIONS PARTS LOCATION ID OPERATIONS CAN ONLY BE PERFORMED THROUGH THE ROOT ENFORCE INTEGRITY REFERENCE TO COLLABORATOR WITH ≠ LIFECYCLE AGGREGATE OBJECT’S SHARE A COMMON LIFECYCLE: THEY CAN BE SAVED & FETCHED AS A WHOLE

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A G G R E G AT E R O O T A R E B U S I N E S S O B J E C T public class Hub implements AggregateRootWithUUID { static public Hub create(
 String productReference, UUID locationId ) { Hub instance = new Hub(UUID.randomUUID()); instance.locationId = locationId; instance.productReference = productReference; return instance; } private Hub(UUID id){ this.id = id; //We control the ID generation } @Override public UUID id() { return id; } }

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T H AT C O N TA I N C O L L A B O R AT O R S H U B M A I N T E N A N C E O P E R AT I O N M A I N T E N A N C E O P E R AT I O N M A I N T E N A N C E O P E R AT I O N public class MaintenanceOperation { MaintenanceOperation( String description, Type operationType, LocalDateTime dateTime ) { this.description = description; this.operationType = operationType; this.dateTime = dateTime; } }

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T H AT P R O T E C T C O N S I S T E N C Y public class Hub implements AggregateRootWithUUID { //… public void addMaintenanceOperation(
 String description, 
 MaintenanceOperation.Type type
 ) { maintenanceOperations =
 maintenanceOperations.append( new MaintenanceOperation( description, type, LocalDateTime.now())); } }

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A N D C A N H AV E R E F E R E N C E T O O T H E R A G G R E G AT E public class Hub implements AggregateRootWithUUID { static public Hub create(
 String productReference, UUID locationId ) { final Hub instance = new Hub(UUID.randomUUID()); instance.locationId = locationId; instance.productReference = productReference; return instance; } private UUID locationId; }

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U S E C A S E A R E D T O public class NewMaintenanceOperationUseCase implements UseCase { public NewMaintenanceOperationUseCase( String switchId, String description, String type ) { this.switchId = switchId; this.description = description; this.type = type; } public String switchId; public String description; public String type; }

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W I T H A D E D I C AT E D H A N D L E R public class NewMaintenanceOperationUseCaseHandler implements UseCaseHandler { public NewMaintenanceOperationUseCaseHandler( SwitchRepository repository ) { this.repository = repository; } @Override public void handle(NewMaintenanceOperationUseCase command) { Hub hub = repository.get(UUID.fromString(command.switchId)); Type maintenanceOperationType = Type.valueOf(command.type); hub.addMaintenanceOperation(
 command.description, 
 maintenanceOperationType ); repository.add(aSwitch); } private final SwitchRepository repository; }

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C Q R S C O M M A N D Q U E RY R E S P O N S I B I L I T Y S E G R E G AT I O N

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C H A N G E S O M E T H I N G E V E RY T H I N G S TA R T S W I T H U S E C A S E S U S E R I want to … G E T I N F O A B O U T S O M E T H I N G (read) (mutation) C O M M A N D Q U E RY « I need information to make a decision » « I made a decision »

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C O M M A N D S & Q U E R I E S A R E D T O class RelocateHubCommand implements Command {
 
 public RelocateHubCommand(
 String hubId, String newLocationId ) {
 this.hubId = hubId;
 this.newLocationId = newLocationId;
 }
 
 @NotEmpty
 public String hubId;
 
 @NotEmpty
 public String newLocationId;
 
 } A command returns no business data to the client (can return a status & an id)

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S E N T I N A B U S @RestController
 class HubResource {
 
 @Autowired
 public HubResource(CommandBus commandBus) {
 this.commandBus = commandBus;
 }
 
 @PutMapping("/api/hub/location")
 public Void relocate(Request request) {
 Map requestBody = Serializer.deserialize(request.body()); 
 return this.commandBus.dispatch( new RelocateHubCommand( requestBody.get("hubId"), requestBody.get("newLocationId") ));
 }
 }

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S E N T I N A B U S @RestController
 class HubResource {
 
 @Autowired
 public HubResource(CommandBus commandBus) {
 this.commandBus = commandBus;
 }
 
 @PutMapping("/api/hub/location")
 public Void relocate(Request request) {
 Map requestBody = Serializer.deserialize(request.body()); 
 return this.commandBus.dispatch( new RelocateHubCommand( requestBody.get("hubId"), requestBody.get("newLocationId") ));
 }
 } Almost no stickiness to a framework

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D I S PAT C H E D T O A S P E C I F I C H A N D L E R @Component
 public class CommandBus {
 
 @Autowired
 public CommandBus(List extends Handler> handlers) {
 this.handlers = handlers.toMap( h -> Tuple.of(h.listenTo(), h) );
 }
 
 public > R dispatch(C command) {
 return handlers .get(command.getClass()) .handle(command);
 }
 
 private final Map handlers;
 }

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D I S PAT C H E D T O A S P E C I F I C H A N D L E R @Component
 public class CommandBus {
 
 @Autowired
 public CommandBus(List extends Handler> handlers) {
 this.handlers = handlers.toMap( h -> Tuple.of(h.listenTo(), h) );
 }
 
 public > R dispatch(C command) {
 return handlers .get(command.getClass()) .handle(command);
 }
 
 private final Map handlers;
 }

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T H E F U L L P I P E L I N E U S E R C O M M A N D Q U E RY Q U E RY H A N D L E R C O M M A N D H A N D L E R one per query one per command Q U E RY B U S C O M M A N D B U S

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M I D D L E WA R E @Component public class BusValidationMiddleware implements BusMiddleware> { @Autowired
 public BusValidationMiddleware( BusMiddleware next, Validator validator ) {
 this.next = next;
 this.validator = validator;
 }
 
 public T handle(Message message) {
 Violation violations = validator.validate(message);
 if(violations.size() > 1)
 throw new ArgumentException(violations.toArray());
 
 return next.handle(message);
 }
 }

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C O M P O S E D I N T H E B U S U S E R C O M M A N D Q U E RY new BusValidationMiddleware(
 new BusTransactionMiddleware(
 new BusDispatcher( Q U E RY H A N D L E R C O M M A N D H A N D L E R Q U E RY B U S C O M M A N D B U S VA L I D AT I O N C A C H E VA L I D AT I O N U N I T O F W O R K

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C Q R S T H E C O M M A N D

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C O M M A N D H A N D L I N G @Component
 public class RelocateHubCommandHandler implements CommandHandler {
 @Autowired
 public RelocateHubCommandHandler(HubRepository repo) {
 this.repository = repo;
 }
 
 @Override
 public Void handle(RelocateHubCommand command) {
 Hub hub = repository.getSiteInformation();
 hub.relocateTo(command.newLocationId);
 }
 
 public Class listenTo() {
 return RelocateHubCommand.class;
 }
 }

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C O M M A N D H A N D L I N G @Component
 public class RelocateHubCommandHandler implements CommandHandler {
 @Autowired
 public RelocateHubCommandHandler(HubRepository repo) {
 this.repository = repo;
 }
 
 @Override
 public Void handle(RelocateHubCommand command) {
 Hub hub = repository.get(command.hubId);
 hub.relocateTo(command.newLocationId);
 }
 
 public Class listenTo() {
 return RelocateHubCommand.class;
 }
 }

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H A N D L E R S W O R K O N A G G R E G AT E public class Hub {
 public Hub(UUID id) { this.id = id; }
 
 public Hub() { this.id = UUID.randomUUID(); }
 
 public UUID id() { return id; } 
 //...
 
 public void relocateTo(String newLocationId) {
 locationId = UUID.fromString(newLocationId);
 }
 
 //...
 
 private UUID locationId;
 private UUID id;
 } We keep under control the aggregate id creation

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P E R S I S T E N C E A B S T R A C T I O N public interface Repository {
 TRoot get(TId id);
 
 void add(TRoot root);
 
 void delete(TRoot root);
 
 List getAll();
 } The persistence mechanism save snapshot of the application state

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P E R S I S T E N C E A B S T R A C T I O N ( P S E U D O M A P I N T E R FA C E ) public class HubRepository implements Repository {
 
 public HubRepository(Store store)
 this.store = store;
 
 public Hub get(UUID uuid)
 return store.findById(uuid);
 
 public void add(Hub hub)
 store.upsert(hub);
 
 public void delete(Hub hub)
 store.remove(hub);
 
 public Seq getAll()
 return store.findAll();
 } We fetch and save aggregate as a whole

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H A N D L E R I N T E R N A L S S T O R E D B A G G R E G AT E S T O R E D B fetched flushed by the unit of work middleware domain operations handler(state, command) = state’ saved

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T E S T I N G @Test
 public void it_relocates_a_hub() {
 store = new InMemoryStore();
 command = new RelocateHubCommand("hubId", "newLocationId");
 handler = new RelocateHubCommandHander( new HubRepository(store) );
 
 handler.handle(command); 
 expectedHub = store.get("hubId");
 assertThat(expectedHub.location()).equals("newLocationId");
 }

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C Q R S T H E Q U E RY

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C O M M A N D S & Q U E R I E S A R E D T O class FindHubByLocationPathQuery implements Query {
 
 public FindHubByLocationPath(
 String locationPath, ) {
 this.locationPath = locationPath;
 }
 
 @NotEmpty
 public String locationPath;
 
 }

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Q U E RY H A N D L I N G @Component
 public class FindHubByLocationPathQueryHander extends SqlHandler {
 
 @Autowired
 public FindHubByLocationPathQueryHander(SqlConnection sql) {
 this.sql = sql;
 }
 
 @Override
 public Installation handle(FetchAllLocationsQuery query) {
 Record hub = sql.prepare( "SELECT * FROM hub JOIN location ON hub.locationId = location.id WHERE location.path = %s" ).execute(query.locationPath);
 return Serializer.deserialize(hub);
 }
 } We receive a direct connection to the datastore

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Q U E RY H A N D L I N G @Component
 public class FindHubByLocationPathQueryHander extends SqlHandler {
 
 @Autowired
 public FindHubByLocationPathQueryHander(SqlConnection sql) {
 this.sql = sql;
 }
 
 @Override
 public Installation handle(FetchAllLocationsQuery query) {
 Record hub = sql.prepare( "SELECT * FROM hub JOIN location ON hub.locationId = location.id WHERE location.path = %s" ).execute(query.locationPath);
 return Serializer.deserialize(hub);
 }
 }

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C Q R S W R A P P I N G U P

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T H E S A M E D ATA S T O R E C A N B E S H A R E D U S E R C O M M A N D Q U E RY Q U E RY H A N D L E R C O M M A N D H A N D L E R P E R S I S T E N C E while maintaining a complete segregation through the whole application Q U E RY B U S C O M M A N D B U S

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TA K E A WAY • Massive simplification of persistence mechanisms • Unit testing in a breeze • Enable business use case discovery • Drive task based UI instead of CRUD thinking

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D O M A I N E V E N T S T I L L N O T E V E N T S O U R C I N G

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E V E N T ≠ C O M M A N D C O M M A N D E V E N T Intent : « do something » Event : « something happened »

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E V E N T A R E A L S O D T O class HubMovedEvent implements Event {
 
 public HubMovedEvent(
 String hubId, String oldLocationId, String newLocationId ) {
 this.hubId = hubId; this.oldLocationId = oldLocationId;
 this.newLocationId = newLocationId;
 }
 
 public String hubId;
 
 public String newLocationId; public String oldLocationId;
 }

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A N D A L S O S E N T I N A B U S eventBus.dispatch( new HubMoved( "hubId", "oldLocationId", "newLocationId" ));

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B U T W I T H M U LT I P L E H A N D L E R public EventBus(List extends EventHandler> handlers) {
 this.handlers = handlers.groupBy( EventHandler::listenTo()
 ));
 }
 
 public Void dispatch(C event) {
 handlers.get(event.getClass())
 .forEach(h -> h.handle(event));
 }

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A N E V E N T I S T H E R E S U LT O F A B U S I N E S S O P E R AT I O N public class Hub {
 
 public Event relocateTo(String newLocationId) {
 HubMovedEvent event = new HubMovedEvent(
 this.locationId.toString(),
 newLocationId
 );
 
 this.locationId = UUID.fromString(newLocationId);
 
 return event;
 }
 UUID locationId;
 private UUID id;
 }

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R E T U R N E D B Y C O M M A N D H A N D L E R S @Component
 public class RelocateHubCommandHandler implements CommandHandler {
 @Autowired
 public RelocateHubCommandHandler(HubRepository repo) {
 this.repository = repo;
 }
 
 @Override
 public Void handle(RelocateHubCommand command) {
 Hub hub = repository.get(command.hubId);
 hub.relocateTo(command.newLocationId);
 }
 
 public Class listenTo() {
 return RelocateHubCommand.class;
 }
 }

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R E T U R N E D B Y C O M M A N D H A N D L E R S @Component
 public class RelocateHubCommandHandler implements CommandHandler {
 @Autowired
 public RelocateHubCommandHandler(HubRepository repo) {
 this.repository = repo;
 }
 
 @Override
 public Tuple2 handle(RHC command) {
 Hub hub = repository.get(command.hubId);
 return Tuple.of(
 void,
 List.of(hub.relocateTo(command.newLocationId)
 );
 }
 
 public Class listenTo() {
 return RelocateHubCommand.class;
 }
 }

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D I S PAT C H E D B Y T H E C O M M A N D B U S @Component
 public class CommandBus {
 
 @Autowired
 public CommandBus(List extends Handler> handlers) {
 this.handlers = handlers.toMap( h -> Tuple.of(h.listenTo(), h) );
 }
 
 public > R dispatch(C command) {
 return handlers .get(command.getClass()) .handle(command);
 }
 
 private final Map handlers;
 }

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D I S PAT C H E D B Y T H E C O M M A N D B U S @Component public class CommandBus { @Autowired public CommandBus(
 Collection extends CommandHandler> handlers, EventBus eventBus) { //… this.eventBus = eventBus; } public > Try dispatch(C command) { return handlers.get(command.getClass()) .map(h -> execute(h, command)) .getOrElse(() -> Try.failure( new HandlerNotFoundException(command))); } private > Try execute(
 CommandHandler h, C command) { return Try.of(() -> { Tuple2> result = h.handle(command); result._2.forEach(eventBus::publish); return result._1; }); } }

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D O M A I N E V E N T P R O J E C T I O N

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L E T ’ S G O B A C K I N T H E C O M M A N D H A N D L E R U S E R C O M M A N D Q U E RY Q U E RY H A N D L E R C O M M A N D H A N D L E R Q U E RY B U S C O M M A N D B U S we are here…

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O U R C O M M A N D H A N D L E R A G G R E G AT E E V E N T S A G G R E G AT E domain operations handler(state, command) = (state’, [event]) domain event

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A N D R E T U R N S T H E M public List handle(RelocateHubCommand command) {
 Hub hub = repository.getHub(command.hubId);
 events.add(hub.relocateTo(command.newLocationId));
 
 return events;
 }
 
 private List events = List.empty();

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T O B E D I S PAT C H E D B Y A M I D D L E WA R E C O M M A N D C O M M A N D H A N D L E R C O M M A N D B U S VA L I D AT I O N U N I T O F W O R K E V E N T D I S PAT C H E R S E N D E M A I L L A U N C H C O M M A N D

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W E C A N L I S T E N E V E N T T O U P D AT E O T H E R D ATA S T O R E public class HubLocationsUpdater implements EventHandler {
 
 public HubLocationsUpdater(Connection sql) {
 this.sql = sql;
 }
 
 public void handle(HubMovedEvent event) {
 sql.createStatement().executeUpdate(
 "UPDATE hub_location SET location_id = ? WHERE hub_id = ?"
 ).execute(event.newLocationId, event.hubId);
 }
 
 public Class extends Event> listenTo() {
 return HubMovedEvent.class;
 }
 }

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W E C A N C R E AT E A R E A D M O D E L I N T H E S A M E D ATA S T O R E C O M M A N D Q U E RY Q U E RY H A N D L E R C O M M A N D H A N D L E R P R O J E C T I O N U P D AT E R E V E N T D I S PAT C H E R P E R S I S T E N C E W R I T E M O D E L R E A D M O D E L

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O R I N D I F F E R E N T O N E C O M M A N D Q U E RY Q U E RY H A N D L E R C O M M A N D H A N D L E R P R O J E C T I O N U P D AT E R E V E N T D I S PAT C H E R W R I T E M O D E L R E A D M O D E L This is not event sourcing !

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O P E N I N G A W O R L D O F P O S S I B L E G E O Q U E RY S E A R C H U P D AT E R D O M A I N E V E N T G I S D B G I S U P D AT E R S Q L U P D AT E R E L A S T I C S E A R C H P O S T G R E S Q L S E A R C H Q U E RY R E L AT I O N A L Q U E RY You can adapt your datastore to your queries need plus you can even add more later…

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TA K E A WAY • Enable reactive business system • Adapt the datastore to your queries need • Suited even for legacy system

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E V E N T S O U R C I N G AT L E A S T …

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W E A D D S O M E M E TA D ATA T O E V E N T S public abstract class Event> {
 
 public Event(ENTITY entity) {
 aggregateId = entity.id();
 aggregateType = entity.class;
 }
 
 public ID aggregateId;
 public Class aggregateType; public LocalDateTime eventDateTime = LocalDateTime.now();
 }

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W E A D D S O M E M E TA D ATA T O E V E N T S public class HubMovedEvent extends Event> {
 
 public HubMovedEvent(
 Hub hub,
 String oldLocationId,
 String newLocationId
 ) {
 super(hub);
 this.hubId = hub.id().toString();
 this.oldLocationId = oldLocationId;
 this.newLocationId = newLocationId; 
 } 
 public String hubId;
 public String newLocationId;
 public String oldLocationId;
 
 }

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D O M A I N E V E N T A S S O U R C E O F S TAT E public class HubEventApplier implements EventApplier {
 
 public static Hub apply(Hub root, HubMovedEvent event) {
 Hub hub = new Hub(root);
 hub.locationId = UUID.fromString(event.newLocationId);
 
 return hub;
 }
 
 } look like a reducer, no ?

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F R O M B U S I N E S S O P E R AT I O N A S M U TAT I O N public Event relocateTo(String newLocationId) {
 HubMovedEvent event = new HubMovedEvent(
 this,
 this.locationId.toString(),
 newLocationId
 );
 
 this.locationId = UUID.fromString(newLocationId);
 
 return event;
 }

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T O E V E N T S O U R C E D B U S I N E S S O P E R AT I O N S public Tuple2 relocateTo(String newLocationId) {
 HubMovedEvent event = new HubMovedEvent(
 this,
 this.locationId.toString(),
 newLocationId
 );
 
 return Tuple.of(
 HubEventApplier.apply(this, event),
 event
 );
 }

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R E A L E V E N T A P P L I E R public class HubEventApplier implements EventApplier {
 
 static Hub applyHubMovedEvent(Hub h, HubMovedEvent event) {
 Hub hub = new Hub(h);
 hub.locationId = UUID.fromString(event.newLocationId);
 return hub;
 }
 
 public static Hub apply(Hub hub, Event event) {
 return appliers .get(event.getClass()) .apply(hub, event);
 }
 
 static Map appliers = HashMap.of(
 HubMovedEvent.class, (hub, event) -> applyHubMovedEvent(hub, event)
 );
 }

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A N D S AV E D I N E V E N T S T O R E C O M M A N D C O M M A N D H A N D L E R E V E N T S T O R E this is event sourcing E V E N T P E R S I S T E N C E C O M M A N D C O M M A N D H A N D L E R D B U N I T O F W O R K flush serialize

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W H AT I S A N E V E N T S T O R E ?

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W H AT I S A N E V E N T S T O R E ?

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E V E N T S O U R C I N G R E B U I L D I N G S TAT E

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W E G E T E V E N T S F O R A G I V E N A G G R E G AT E public class eventStore {
 public eventStore(Connection sql) {
 this.sql = sql;
 }
 
 public List get(String type, String id) {
 Seq records = sql.createStatement(
 "
 SELECT * FROM events 
 WHERE aggr_type = ? AND aggr_id = ? 
 ORDER BY TIMESTAMP "
 ).execute(aggregateType, aggregateId);
 
 return records.map(records -> Serializer.deserialize( record.getValue("payload"), Event.class
 ));
 }
 }

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A N D W E A P P LY T H E M public HubRepository(EventStore store) {
 
 this.store = store;
 }
 
 public Hub get(UUID uuid) {
 List events = store.get("HUB", uuid.toString());
 return events.foldLeft(
 new Hub(),
 (hub, event) -> HubEventApplier.apply(hub, event)
 );
 }

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TA K E A WAY • No more business memory loss • Pure data persistence (no more plop) • You can build new projection from past events

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F O L D E R H I E R A R C H Y

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THE BIG PICTURE COMMAND HANDLER BUS ACK/NACK EVENT DISPATCHER REPOSITORY PROJECTOR PROJECTOR QUERY HANDLER BUS VIEWMODEL EVENT STORE EVENTS AS A SOURCE OF TRUTH WE CAN CREATE NEW PROJECTION ON DEMAND

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H O W W E H A N D L E C O M M A N D • classic : 
 handler(state, command) = state’ • with domain event : 
 handler(state, command = (state’, [event]) • with event sourcing :
 handler([event], command) = [event]’

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Q & A • How do we snapshot ? • How do we manage version ? • How to use multiple aggregate in a handler ?

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T O G O F U R T H E R • CQRS Documents by Greg Young • The Value of Value by Rich Hickey • What if the user was a function by Andre Staltz

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@ L I L O B A S E | L G O . G R O U P Thanks !