Make factory is only place to get transformer

Signed-off-by: Balasubramanian <[email protected]>

src/main/java/org/opensearch/knn/index/mapper/FlatVectorFieldMapper.java

@@ -110,8 +110,4 @@ protected PerDimensionProcessor getPerDimensionProcessor() {
         return PerDimensionProcessor.NOOP_PROCESSOR;
     }
 
-    @Override
-    protected VectorTransformer getVectorTransformer() {
-        return VectorTransformer.NOOP_VECTOR_TRANSFORMER;
-    }
 }

src/main/java/org/opensearch/knn/index/mapper/KNNVectorFieldMapper.java

@@ -689,11 +689,21 @@ protected void validatePreparse() {
     protected abstract PerDimensionProcessor getPerDimensionProcessor();
 
     /**
-     * Getter for vector transformer after vector parsing and validation
+     * Retrieves the vector transformer for the KNN vector field.
+     * This method provides access to the vector transformer instance that will be used
+     * for processing vectors in the KNN field. The transformer is responsible for any
+     * necessary vector transformations before indexing or searching.
+     * This implementation delegates to the VectorTransformerFactory to obtain
+     * the appropriate transformer instance. The returned transformer is typically
+     * stateless and thread-safe.
+     *
+     * @return VectorTransformer An instance of VectorTransformer that will be used
+     *         for vector transformations in this field
      *
-     * @return VectorTransformer
      */
-    protected abstract VectorTransformer getVectorTransformer();
+    protected VectorTransformer getVectorTransformer() {
+        return VectorTransformerFactory.getVectorTransformer();
+    }
 
     protected void parseCreateField(ParseContext context, int dimension, VectorDataType vectorDataType) throws IOException {
         validatePreparse();

src/main/java/org/opensearch/knn/index/mapper/VectorTransformer.java

@@ -34,10 +34,4 @@ default void transform(final byte[] vector) {
             throw new IllegalArgumentException("Input vector cannot be null");
         }
     }
-
-    /**
-     * A no-operation transformer that returns vector values unchanged.
-     */
-    VectorTransformer NOOP_VECTOR_TRANSFORMER = new VectorTransformer() {
-    };
 }

src/main/java/org/opensearch/knn/index/mapper/VectorTransformerFactory.java

@@ -19,6 +19,12 @@
 @NoArgsConstructor(access = AccessLevel.PRIVATE)
 public final class VectorTransformerFactory {
 
+    /**
+     * A no-operation transformer that returns vector values unchanged.
+     */
+    private final static VectorTransformer NOOP_VECTOR_TRANSFORMER = new VectorTransformer() {
+    };
+
     /**
      * Returns a vector transformer based on the provided KNN method context.
      * For FAISS engine with cosine similarity space type, returns a NormalizeVectorTransformer
@@ -36,6 +42,20 @@ public static VectorTransformer getVectorTransformer(final KNNMethodContext cont
         return getVectorTransformer(context.getKnnEngine(), context.getSpaceType());
     }
 
+    /**
+     * Returns a vector transformer instance for vector transformations.
+     * This method provides access to the default no-operation vector transformer
+     * that performs identity transformation on vectors. The transformer does not
+     * modify the input vectors and returns them as-is.This implementation returns a stateless, thread-safe transformer
+     * instance that can be safely shared across multiple calls
+     *
+     * @return VectorTransformer A singleton instance of the no-operation vector
+     *         transformer (NOOP_VECTOR_TRANSFORMER)
+     */
+    public static VectorTransformer getVectorTransformer() {
+        return NOOP_VECTOR_TRANSFORMER;
+    }
+
     /**
      * Creates a VectorTransformer based on the provided model metadata.
      *
@@ -69,7 +89,7 @@ public static VectorTransformer getVectorTransformer(final ModelMetadata metadat
      * @return VectorTransformer An appropriate vector transformer instance
      */
     private static VectorTransformer getVectorTransformer(final KNNEngine knnEngine, final SpaceType spaceType) {
-        return shouldNormalizeVector(knnEngine, spaceType) ? new NormalizeVectorTransformer() : VectorTransformer.NOOP_VECTOR_TRANSFORMER;
+        return shouldNormalizeVector(knnEngine, spaceType) ? new NormalizeVectorTransformer() : getVectorTransformer();
     }
 
     private static boolean shouldNormalizeVector(final KNNEngine knnEngine, final SpaceType spaceType) {

src/test/java/org/opensearch/knn/index/mapper/VectorTransformerFactoryTests.java

@@ -76,7 +76,7 @@ private static void validateTransformer(SpaceType spaceType, KNNEngine engine, V
         } else {
             assertSame(
                 "Should return NOOP transformer for " + engine + " with COSINESIMIL",
-                VectorTransformer.NOOP_VECTOR_TRANSFORMER,
+                VectorTransformerFactory.getVectorTransformer(),
                 transformer
             );
         }