Validate method parameters only after version 2_17_0

k-NN introduced validation to prevent index creation api to accept
method paramters that are used by approximate k-NN search
algorithm. For ex: create index api accepts model_id, or,
method params even if knn index setting was not true before 2.17.

However, for index that were created before 2.17 and upgraded to 2.17,
will prevent cluster to parse those previously accepted index mapping.

Hence, k-NN will allow those paramters for indices that were created before 2.17,
but doesn't support those parameters starting 2.17.

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

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

@@ -182,6 +182,8 @@ public static class Builder extends ParametrizedFieldMapper.Builder {
         @Setter
         @Getter
         private OriginalMappingParameters originalParameters;
+        @Setter
+        private boolean isKnnIndex;
 
         public Builder(
             String name,
@@ -195,6 +197,7 @@ public Builder(
             this.indexCreatedVersion = indexCreatedVersion;
             this.knnMethodConfigContext = knnMethodConfigContext;
             this.originalParameters = originalParameters;
+            this.isKnnIndex = true;
         }
 
         @Override
@@ -250,7 +253,7 @@ public KNNVectorFieldMapper build(BuilderContext context) {
                 );
             }
 
-            if (originalParameters.getResolvedKnnMethodContext() == null) {
+            if (originalParameters.getResolvedKnnMethodContext() == null || isKnnIndex == false) {
                 return FlatVectorFieldMapper.createFieldMapper(
                     buildFullName(context),
                     name,
@@ -362,10 +365,14 @@ public Mapper.Builder<?> parse(String name, Map<String, Object> node, ParserCont
                     String.format(Locale.ROOT, "Method and model can not be both specified in the mapping: %s", name)
                 );
             }
-
             // Check for flat configuration
             if (isKNNDisabled(parserContext.getSettings())) {
-                validateFromFlat(builder);
+                builder.setKnnIndex(false);
+                if(parserContext.indexVersionCreated().onOrAfter(Version.V_2_17_0)) {
+                    // on and after 2_17_0 we validate to makes sure that mapping doesn't contain parameters that are
+                    // specific to approximate knn search algorithms
+                    validateFromFlat(builder);
+                }
             } else if (builder.modelId.get() != null) {
                 validateFromModel(builder);
             } else {