Allow listening to model catalog insertions

This will be used by the Shutdown to track models that were not backed up (instead of dropping the model).
Also metrics can be implemented based on this

Author: FlorentinD

model-catalog-api/src/main/java/org/neo4j/gds/core/model/ModelCatalog.java

@@ -28,6 +28,8 @@
 
 public interface ModelCatalog {
 
+    void registerListener(ModelCatalogListener listener);
+
     void set(Model<?, ?, ?> model);
 
     <D, C extends ModelConfig, I extends ToMapConvertible> Model<D, C, I> get(

model-catalog-api/src/main/java/org/neo4j/gds/core/model/ModelCatalogListener.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) "Neo4j"
+ * Neo4j Sweden AB [http://neo4j.com]
+ *
+ * This file is part of Neo4j.
+ *
+ * Neo4j is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.neo4j.gds.core.model;
+
+public interface ModelCatalogListener {
+
+    void onInsert(Model<?, ?, ?> model);
+}

open-model-catalog/src/main/java/org/neo4j/gds/core/model/OpenModelCatalog.java

@@ -26,6 +26,7 @@
 
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.NoSuchElementException;
@@ -38,8 +39,16 @@ public final class OpenModelCatalog implements ModelCatalog {
 
     private final Map<String, OpenUserCatalog> userCatalogs;
 
+    private final List<ModelCatalogListener> listeners;
+
     public OpenModelCatalog() {
         this.userCatalogs = new ConcurrentHashMap<>();
+        this.listeners = new ArrayList<>();
+    }
+
+    @Override
+    public void registerListener(ModelCatalogListener listener) {
+        listeners.add(listener);
     }
 
     @Override
@@ -51,6 +60,8 @@ public void set(Model<?, ?, ?> model) {
             userCatalog.set(model);
             return userCatalog;
         });
+
+        listeners.forEach(listener -> listener.onInsert(model));
     }
 
     @Override

open-model-catalog/src/test/java/org/neo4j/gds/core/model/OpenModelCatalogTest.java

@@ -19,6 +19,7 @@
  */
 package org.neo4j.gds.core.model;
 
+import org.eclipse.collections.impl.Counter;
 import org.junit.jupiter.api.Test;
 import org.neo4j.gds.annotation.Configuration;
 import org.neo4j.gds.annotation.ValueClass;
@@ -118,6 +119,30 @@ void shouldStoreModelsPerType() {
         );
     }
 
+    @Test
+    void shouldNotifyListeners() {
+        var counter = new Counter();
+        modelCatalog.registerListener(model -> counter.increment());
+
+        var model = Model.of(
+            "testAlgo",
+            GRAPH_SCHEMA,
+            "testTrainData",
+            TestTrainConfig.of(USERNAME, "testModel"),
+            new TestCustomInfo()
+        );
+
+        assertThat(counter.getCount()).isEqualTo(0);
+
+        modelCatalog.set(model);
+
+        assertThat(counter.getCount()).isEqualTo(1);
+
+        assertThatThrownBy(() -> modelCatalog.set(model));
+        // not be called if the set is not successful
+        assertThat(counter.getCount()).isEqualTo(1);
+    }
+
     @Test
     void shouldThrowWhenPublishing() {
         modelCatalog.set(TEST_MODEL);