From 64e95e8dc8e7f3e19227dcebd2f44aa05096a575 Mon Sep 17 00:00:00 2001
From: Michael Schnell <michael@fuin.org>
Date: Sat, 21 Mar 2020 11:57:50 +0100
Subject: [PATCH] Create ES projection based on even type names.

---
 .../PersonListEventChunkHandler.java          | 22 +++++++-
 .../views/personlist/PersonListProjector.java | 50 +++++++++++--------
 2 files changed, 51 insertions(+), 21 deletions(-)

diff --git a/spring-boot/query/src/main/java/org/fuin/cqrs4j/example/spring/query/views/personlist/PersonListEventChunkHandler.java b/spring-boot/query/src/main/java/org/fuin/cqrs4j/example/spring/query/views/personlist/PersonListEventChunkHandler.java
index 60a8877..1652a5b 100644
--- a/spring-boot/query/src/main/java/org/fuin/cqrs4j/example/spring/query/views/personlist/PersonListEventChunkHandler.java
+++ b/spring-boot/query/src/main/java/org/fuin/cqrs4j/example/spring/query/views/personlist/PersonListEventChunkHandler.java
@@ -1,8 +1,12 @@
 package org.fuin.cqrs4j.example.spring.query.views.personlist;
 
+import java.util.Set;
+
 import javax.annotation.concurrent.NotThreadSafe;
 
 import org.fuin.cqrs4j.ProjectionService;
+import org.fuin.cqrs4j.example.shared.SharedUtils;
+import org.fuin.ddd4j.ddd.EventType;
 import org.fuin.esc.api.ProjectionStreamId;
 import org.fuin.esc.api.StreamEventsSlice;
 import org.slf4j.Logger;
@@ -30,7 +34,23 @@ public class PersonListEventChunkHandler {
 	@Autowired
 	private ProjectionService projectionService;
 
-	/**
+    private ProjectionStreamId streamId;
+
+    /**
+     * Returns the name of the event store projection that is used by this handler.
+     * 
+     * @return Unique projection stream name.
+     */
+    public ProjectionStreamId getProjectionStreamId() {
+        if (streamId == null) {
+            final Set<EventType> eventTypes = dispatcher.getAllTypes();
+            final String name = "spring-qry-person-" + SharedUtils.calculateChecksum(eventTypes);
+            streamId = new ProjectionStreamId(name);
+        }
+        return streamId;
+    }
+
+    /**
 	 * Returns the next event position to read.
 	 * 
 	 * @return Number of the next event to read.
diff --git a/spring-boot/query/src/main/java/org/fuin/cqrs4j/example/spring/query/views/personlist/PersonListProjector.java b/spring-boot/query/src/main/java/org/fuin/cqrs4j/example/spring/query/views/personlist/PersonListProjector.java
index 2acfb31..967e008 100644
--- a/spring-boot/query/src/main/java/org/fuin/cqrs4j/example/spring/query/views/personlist/PersonListProjector.java
+++ b/spring-boot/query/src/main/java/org/fuin/cqrs4j/example/spring/query/views/personlist/PersonListProjector.java
@@ -1,7 +1,6 @@
 package org.fuin.cqrs4j.example.spring.query.views.personlist;
 
 import static org.fuin.cqrs4j.Cqrs4JUtils.tryLocked;
-import static org.fuin.cqrs4j.example.spring.query.views.personlist.PersonListEventChunkHandler.PROJECTION_STREAM_ID;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -44,7 +43,7 @@ public class PersonListProjector {
 	// Above LOCK prevents multithreaded access
 
 	@Autowired
-	private IESHttpEventStore eventStore;
+	private IESHttpEventStore eventstore;
 
 	@Autowired
 	private PersonListEventChunkHandler chunkHandler;
@@ -85,26 +84,37 @@ public class PersonListProjector {
 		LOG.info("Application stopped");
     }
 	
-	private void readStreamEvents() {
+    private void readStreamEvents() {
 
-		// TODO Make sure a projection with the correct events exists
-		// We must update the projection if new events are defined or some are removed!
-		if (!eventStore.projectionExists(PROJECTION_STREAM_ID)) {
-			final Set<EventType> eventTypes = dispatcher.getAllTypes();
-			final List<TypeName> typeNames = new ArrayList<>();
-			for (final EventType eventType : eventTypes) {
-				typeNames.add(new TypeName(eventType.asBaseType()));
-			}
-			LOG.info("Create projection '{}' with events: {}", PROJECTION_STREAM_ID, typeNames);
-			eventStore.createProjection(PROJECTION_STREAM_ID, true, typeNames);
-		}
+        // Create a projection if it does not exist.
+        // Multiple projections (even in different microservice versions)
+        // might share the same projection based on the event name hash.
+        if (!eventstore.projectionExists(chunkHandler.getProjectionStreamId())) {
+            final List<TypeName> typeNames = getEventTypeNames();
+            LOG.info("Create projection '{}' with events: {}", chunkHandler.getProjectionStreamId(), typeNames);
+            eventstore.createProjection(chunkHandler.getProjectionStreamId(), true, typeNames);
+        }
 
-		// Read and dispatch events
-		final Long nextEventNumber = chunkHandler.readNextEventNumber();
-		eventStore.readAllEventsForward(PROJECTION_STREAM_ID, nextEventNumber, 100, (currentSlice) -> {
-			chunkHandler.handleChunk(currentSlice);
-		});
+        // Read and dispatch events
+        final Long nextEventNumber = chunkHandler.readNextEventNumber();
+        eventstore.readAllEventsForward(chunkHandler.getProjectionStreamId(), nextEventNumber, 100, (currentSlice) -> {
+            chunkHandler.handleChunk(currentSlice);
+        });
 
-	}
+    }
+
+    /**
+     * Returns a list of all event type names used for this projection.
+     * 
+     * @return List of event names.
+     */
+    public List<TypeName> getEventTypeNames() {
+        final List<TypeName> typeNames = new ArrayList<>();
+        final Set<EventType> eventTypes = dispatcher.getAllTypes();
+        for (final EventType eventType : eventTypes) {
+            typeNames.add(new TypeName(eventType.asBaseType()));
+        }
+        return typeNames;
+    }
 
 }