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diff --git a/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/index/Indexer.java b/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/index/Indexer.java
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--- /dev/null
+++ b/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/index/Indexer.java
@@ -0,0 +1,71 @@
+/*******************************************************************************
+ * Copyright (c) 2004-2008 Gabor Bergmann and Daniel Varro
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-v20.html.
+ * 
+ * SPDX-License-Identifier: EPL-2.0
+ *******************************************************************************/
+
+package tools.refinery.viatra.runtime.rete.index;
+
+import java.util.Collection;
+import java.util.Map;
+
+import tools.refinery.viatra.runtime.matchers.tuple.Tuple;
+import tools.refinery.viatra.runtime.matchers.tuple.TupleMask;
+import tools.refinery.viatra.runtime.matchers.util.timeline.Timeline;
+import tools.refinery.viatra.runtime.rete.network.Node;
+import tools.refinery.viatra.runtime.rete.network.Supplier;
+import tools.refinery.viatra.runtime.rete.network.communication.Timestamp;
+
+/**
+ * A node that indexes incoming Tuples by their signatures as specified by a TupleMask. Notifies listeners about such
+ * update events through the IndexerListener.
+ * 
+ * Signature tuples are created by transforming the update tuples using the mask. Tuples stored with the same signature
+ * are grouped together. The group or a reduction thereof is retrievable.
+ * 
+ * @author Gabor Bergmann
+ */
+public interface Indexer extends Node {
+    /**
+     * @return the mask by which the contents are indexed.
+     */
+    public TupleMask getMask();
+
+    /**
+     * @return the node whose contents are indexed.
+     */
+    public Supplier getParent();
+
+    /**
+     * @return all stored tuples that conform to the specified signature, null if there are none such. CONTRACT: do not
+     *         modify!
+     */
+    public Collection<Tuple> get(Tuple signature);
+    
+    /**
+     * @since 2.4
+     */
+    default public Map<Tuple, Timeline<Timestamp>> getTimeline(Tuple signature) {
+        throw new UnsupportedOperationException();
+    }
+
+    /**
+     * This indexer will be updated whenever a Rete update is sent to the active node (or an equivalent time slot
+     * allotted to it). The active node is typically the indexer itself, but it can be a different node such as its
+     * parent.
+     * 
+     * @return the active node that operates this indexer
+     */
+    public Node getActiveNode();
+
+    
+    public Collection<IndexerListener> getListeners();
+    
+    public void attachListener(IndexerListener listener);
+
+    public void detachListener(IndexerListener listener);
+
+}