[7.x][ML] Validate classification dependent_variable cardinality is at lea… (#51232) (#51309)

Data frame analytics classification currently only supports 2 classes for the
dependent variable. We were checking that the field's cardinality is not higher
than 2 but we should also check it is not less than that as otherwise the process
fails.

Backport of #51232

Author: dimitris-athanasiou

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/dataframe/analyses/Classification.java

@@ -245,9 +245,9 @@ public List<RequiredField> getRequiredFields() {
     }
 
     @Override
-    public Map<String, Long> getFieldCardinalityLimits() {
+    public List<FieldCardinalityConstraint> getFieldCardinalityConstraints() {
         // This restriction is due to the fact that currently the C++ backend only supports binomial classification.
-        return Collections.singletonMap(dependentVariable, 2L);
+        return Collections.singletonList(FieldCardinalityConstraint.between(dependentVariable, 2, 2));
     }
 
     @SuppressWarnings("unchecked")

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/dataframe/analyses/DataFrameAnalysis.java

@@ -37,9 +37,9 @@ public interface DataFrameAnalysis extends ToXContentObject, NamedWriteable {
     List<RequiredField> getRequiredFields();
 
     /**
-     * @return {@link Map} containing cardinality limits for the selected (analysis-specific) fields
+     * @return {@link List} containing cardinality constraints for the selected (analysis-specific) fields
      */
-    Map<String, Long> getFieldCardinalityLimits();
+    List<FieldCardinalityConstraint> getFieldCardinalityConstraints();
 
     /**
      * Returns fields for which the mappings should be either predefined or copied from source index to destination index.

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/dataframe/analyses/FieldCardinalityConstraint.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+package org.elasticsearch.xpack.core.ml.dataframe.analyses;
+
+import org.elasticsearch.xpack.core.ml.utils.ExceptionsHelper;
+
+import java.util.Objects;
+
+/**
+ * Allows checking a field's cardinality against given lower and upper bounds
+ */
+public class FieldCardinalityConstraint {
+
+    private final String field;
+    private final long lowerBound;
+    private final long upperBound;
+
+    public static FieldCardinalityConstraint between(String field, long lowerBound, long upperBound) {
+        return new FieldCardinalityConstraint(field, lowerBound, upperBound);
+    }
+
+    private FieldCardinalityConstraint(String field, long lowerBound, long upperBound) {
+        this.field = Objects.requireNonNull(field);
+        this.lowerBound = lowerBound;
+        this.upperBound = upperBound;
+    }
+
+    public String getField() {
+        return field;
+    }
+
+    public long getLowerBound() {
+        return lowerBound;
+    }
+
+    public long getUpperBound() {
+        return upperBound;
+    }
+
+    public void check(long fieldCardinality) {
+        if (fieldCardinality < lowerBound) {
+            throw ExceptionsHelper.badRequestException(
+                                    "Field [{}] must have at least [{}] distinct values but there were [{}]",
+                                    field, lowerBound, fieldCardinality);
+        }
+        if (fieldCardinality > upperBound) {
+            throw ExceptionsHelper.badRequestException(
+                "Field [{}] must have at most [{}] distinct values but there were at least [{}]",
+                field, upperBound, fieldCardinality);
+        }
+    }
+}

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/dataframe/analyses/OutlierDetection.java

@@ -225,8 +225,8 @@ public List<RequiredField> getRequiredFields() {
     }
 
     @Override
-    public Map<String, Long> getFieldCardinalityLimits() {
-        return Collections.emptyMap();
+    public List<FieldCardinalityConstraint> getFieldCardinalityConstraints() {
+        return Collections.emptyList();
     }
 
     @Override

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/dataframe/analyses/Regression.java

@@ -182,8 +182,8 @@ public List<RequiredField> getRequiredFields() {
     }
 
     @Override
-    public Map<String, Long> getFieldCardinalityLimits() {
-        return Collections.emptyMap();
+    public List<FieldCardinalityConstraint> getFieldCardinalityConstraints() {
+        return Collections.emptyList();
     }
 
     @Override

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/dataframe/analyses/ClassificationTests.java

@@ -22,6 +22,7 @@
 import java.io.IOException;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
@@ -169,7 +170,13 @@ public void testRequiredFieldsIsNonEmpty() {
     }
 
     public void testFieldCardinalityLimitsIsNonEmpty() {
-        assertThat(createTestInstance().getFieldCardinalityLimits(), is(not(anEmptyMap())));
+        Classification classification = createTestInstance();
+        List<FieldCardinalityConstraint> constraints = classification.getFieldCardinalityConstraints();
+
+        assertThat(constraints.size(), equalTo(1));
+        assertThat(constraints.get(0).getField(), equalTo(classification.getDependentVariable()));
+        assertThat(constraints.get(0).getLowerBound(), equalTo(2L));
+        assertThat(constraints.get(0).getUpperBound(), equalTo(2L));
     }
 
     public void testGetExplicitlyMappedFields() {

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/dataframe/analyses/FieldCardinalityConstraintTests.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+package org.elasticsearch.xpack.core.ml.dataframe.analyses;
+
+import org.elasticsearch.ElasticsearchStatusException;
+import org.elasticsearch.rest.RestStatus;
+import org.elasticsearch.test.ESTestCase;
+
+import static org.hamcrest.Matchers.equalTo;
+
+public class FieldCardinalityConstraintTests extends ESTestCase {
+
+    public void testBetween_GivenWithinLimits() {
+        FieldCardinalityConstraint constraint = FieldCardinalityConstraint.between("foo", 3, 6);
+
+        constraint.check(3);
+        constraint.check(4);
+        constraint.check(5);
+        constraint.check(6);
+    }
+
+    public void testBetween_GivenLessThanLowerBound() {
+        FieldCardinalityConstraint constraint = FieldCardinalityConstraint.between("foo", 3, 6);
+
+        ElasticsearchStatusException e = expectThrows(ElasticsearchStatusException.class, () -> constraint.check(2L));
+        assertThat(e.getMessage(), equalTo("Field [foo] must have at least [3] distinct values but there were [2]"));
+        assertThat(e.status(), equalTo(RestStatus.BAD_REQUEST));
+    }
+
+    public void testBetween_GivenGreaterThanUpperBound() {
+        FieldCardinalityConstraint constraint = FieldCardinalityConstraint.between("foo", 3, 6);
+
+        ElasticsearchStatusException e = expectThrows(ElasticsearchStatusException.class, () -> constraint.check(7L));
+        assertThat(e.getMessage(), equalTo("Field [foo] must have at most [6] distinct values but there were at least [7]"));
+        assertThat(e.status(), equalTo(RestStatus.BAD_REQUEST));
+    }
+}

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/dataframe/analyses/OutlierDetectionTests.java

@@ -89,7 +89,7 @@ public void testRequiredFieldsIsEmpty() {
     }
 
     public void testFieldCardinalityLimitsIsEmpty() {
-        assertThat(createTestInstance().getFieldCardinalityLimits(), is(anEmptyMap()));
+        assertThat(createTestInstance().getFieldCardinalityConstraints(), is(empty()));
     }
 
     public void testGetExplicitlyMappedFields() {

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/dataframe/analyses/RegressionTests.java

@@ -19,7 +19,6 @@
 import java.util.Collections;
 
 import static org.hamcrest.Matchers.allOf;
-import static org.hamcrest.Matchers.anEmptyMap;
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.empty;
 import static org.hamcrest.Matchers.equalTo;
@@ -107,7 +106,7 @@ public void testRequiredFieldsIsNonEmpty() {
     }
 
     public void testFieldCardinalityLimitsIsEmpty() {
-        assertThat(createTestInstance().getFieldCardinalityLimits(), is(anEmptyMap()));
+        assertThat(createTestInstance().getFieldCardinalityConstraints(), is(empty()));
     }
 
     public void testGetExplicitlyMappedFields() {

x-pack/plugin/ml/qa/native-multi-node-tests/src/test/java/org/elasticsearch/xpack/ml/integration/ExplainDataFrameAnalyticsIT.java

@@ -43,17 +43,23 @@ public void testSourceQueryIsApplied() throws IOException {
         String sourceIndex = "test-source-query-is-applied";
 
         client().admin().indices().prepareCreate(sourceIndex)
-            .addMapping("_doc", "numeric_1", "type=double", "numeric_2", "type=float", "categorical", "type=keyword")
+            .addMapping("_doc",
+                "numeric_1", "type=double",
+                "numeric_2", "type=float",
+                "categorical", "type=keyword",
+                "filtered_field", "type=keyword")
             .get();
 
         BulkRequestBuilder bulkRequestBuilder = client().prepareBulk();
         bulkRequestBuilder.setRefreshPolicy(WriteRequest.RefreshPolicy.IMMEDIATE);
 
         for (int i = 0; i < 30; i++) {
             IndexRequest indexRequest = new IndexRequest(sourceIndex);
-
-            // We insert one odd value out of 5 for one feature
-            indexRequest.source("numeric_1", 1.0, "numeric_2", 2.0, "categorical", i == 0 ? "only-one" : "normal");
+            indexRequest.source(
+                "numeric_1", 1.0,
+                "numeric_2", 2.0,
+                "categorical", i % 2 == 0 ? "class_1" : "class_2",
+                "filtered_field", i < 2 ? "bingo" : "rest"); // We tag bingo on the first two docs to ensure we have 2 classes
             bulkRequestBuilder.add(indexRequest);
         }
         BulkResponse bulkResponse = bulkRequestBuilder.get();
@@ -66,7 +72,7 @@ public void testSourceQueryIsApplied() throws IOException {
         DataFrameAnalyticsConfig config = new DataFrameAnalyticsConfig.Builder()
             .setId(id)
             .setSource(new DataFrameAnalyticsSource(new String[] { sourceIndex },
-                QueryProvider.fromParsedQuery(QueryBuilders.termQuery("categorical", "only-one")),
+                QueryProvider.fromParsedQuery(QueryBuilders.termQuery("filtered_field", "bingo")),
                 null))
             .setAnalysis(new Classification("categorical"))
             .buildForExplain();