Add sequenceIdentifier method to the EventHandlerInvoker

A dead lettering EventHandlerInvoker should be able to retrieve the
sequence id for a given event from its delegate. Without this, it will
not be able to correctly store the events in the right order. By added
EventHandlerInvoker#sequenceIdentifier, any specific implementation can
retrieve the seqId for any event from its delegate.

#2021

messaging/src/main/java/org/axonframework/eventhandling/EventHandlerInvoker.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2010-2020. Axon Framework
+ * Copyright (c) 2010-2021. Axon Framework
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -90,4 +90,15 @@ default <R> void performReset(R resetContext) {
             );
         }
     }
+
+    /**
+     * Retrieve the sequence identifier for the given {@code event}, used to handle events in the desired order.
+     * Defaults to {@link EventMessage#getIdentifier()}.
+     *
+     * @param event the {@link EventMessage} to deduce the sequence identifier for
+     * @return the sequence identifier for the given {@code event}
+     */
+    default Object sequenceIdentifier(EventMessage<?> event) {
+        return event.getIdentifier();
+    }
 }

messaging/src/main/java/org/axonframework/eventhandling/SimpleEventHandlerInvoker.java

@@ -122,6 +122,10 @@ public boolean canHandle(EventMessage<?> eventMessage, Segment segment) {
         return hasHandler(eventMessage) && sequencingPolicyMatchesSegment(eventMessage, segment);
     }
 
+    private boolean sequencingPolicyMatchesSegment(EventMessage<?> message, Segment segment) {
+        return segment.matches(Objects.hashCode(sequenceIdentifier(message)));
+    }
+
     @Override
     public boolean canHandleType(Class<?> payloadType) {
         return wrappedEventHandlers.stream().anyMatch(eh -> eh.canHandleType(payloadType));
@@ -146,14 +150,6 @@ public boolean supportsReset() {
         return true;
     }
 
-    private boolean sequencingPolicyMatchesSegment(EventMessage<?> message, Segment segment) {
-        return segment.matches(Objects.hashCode(sequenceIdentifier(message)));
-    }
-
-    protected Object sequenceIdentifier(EventMessage<?> event) {
-        return getOrDefault(sequencingPolicy.getSequenceIdentifierFor(event), event::getIdentifier);
-    }
-
     @Override
     public void performReset() {
         performReset(null);
@@ -166,6 +162,11 @@ public <R> void performReset(R resetContext) {
         }
     }
 
+    @Override
+    public Object sequenceIdentifier(EventMessage<?> event) {
+        return getOrDefault(sequencingPolicy.getSequenceIdentifierFor(event), event::getIdentifier);
+    }
+
     /**
      * Return the {@link ListenerInvocationErrorHandler} as configured for this {@link EventHandlerInvoker}.
      *