[Synonym] Return Empty RuleSet (#131032)

* Adding filter agg to include empty synonym sets

* Adding featurespecification for synonyms

* linting

* Update docs/changelog/131032.yaml

* update changelog

* Adding synonym feature into module-info

* sorted the expected response

---------

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

docs/changelog/131032.yaml

@@ -0,0 +1,5 @@
+pr: 131032
+summary: "Fix: `GET _synonyms` returns synonyms with empty rules"
+area: Relevance
+type: bug
+issues: []

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/synonyms/40_synonyms_sets_get.yml

@@ -157,3 +157,33 @@ setup:
 
   - match:
       count: 12
+
+---
+"Return empty rule set":
+  - requires:
+      cluster_features: [ synonyms_set.get.return_empty_synonym_sets ]
+      reason: "synonyms_set get api return empty synonym sets"
+
+  - do:
+      synonyms.put_synonym:
+        id: empty-synonyms
+        body:
+          synonyms_set: []
+
+  - do:
+      synonyms.get_synonyms_sets: {}
+
+  - match:
+      count: 4
+
+  - match:
+      results:
+        - synonyms_set: "empty-synonyms"
+          count: 0
+        - synonyms_set: "test-synonyms-1"
+          count: 3
+        - synonyms_set: "test-synonyms-2"
+          count: 1
+        - synonyms_set: "test-synonyms-3"
+          count: 2
+

server/src/main/java/module-info.java

@@ -429,6 +429,7 @@ module org.elasticsearch.server {
             org.elasticsearch.index.mapper.MapperFeatures,
             org.elasticsearch.index.IndexFeatures,
             org.elasticsearch.search.SearchFeatures,
+            org.elasticsearch.synonyms.SynonymFeatures,
             org.elasticsearch.script.ScriptFeatures,
             org.elasticsearch.search.retriever.RetrieversFeatures,
             org.elasticsearch.action.admin.cluster.stats.ClusterStatsFeatures,

server/src/main/java/org/elasticsearch/synonyms/SynonymFeatures.java

@@ -0,0 +1,24 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.synonyms;
+
+import org.elasticsearch.features.FeatureSpecification;
+import org.elasticsearch.features.NodeFeature;
+
+import java.util.Set;
+
+public class SynonymFeatures implements FeatureSpecification {
+    private static final NodeFeature RETURN_EMPTY_SYNONYM_SETS = new NodeFeature("synonyms_set.get.return_empty_synonym_sets");
+
+    @Override
+    public Set<NodeFeature> getTestFeatures() {
+        return Set.of(RETURN_EMPTY_SYNONYM_SETS);
+    }
+}

server/src/main/java/org/elasticsearch/synonyms/SynonymsManagementAPIService.java

@@ -50,6 +50,9 @@ import org.elasticsearch.indices.IndexCreationException;
 import org.elasticsearch.indices.SystemIndexDescriptor;
 import org.elasticsearch.rest.RestStatus;
 import org.elasticsearch.search.aggregations.BucketOrder;
+import org.elasticsearch.search.aggregations.bucket.filter.Filters;
+import org.elasticsearch.search.aggregations.bucket.filter.FiltersAggregationBuilder;
+import org.elasticsearch.search.aggregations.bucket.filter.FiltersAggregator;
 import org.elasticsearch.search.aggregations.bucket.terms.Terms;
 import org.elasticsearch.search.aggregations.bucket.terms.TermsAggregationBuilder;
 import org.elasticsearch.search.builder.SearchSourceBuilder;
@@ -94,6 +97,8 @@ public class SynonymsManagementAPIService {
     private static final int MAX_SYNONYMS_SETS = 10_000;
     private static final String SYNONYM_RULE_ID_FIELD = SynonymRule.ID_FIELD.getPreferredName();
     private static final String SYNONYM_SETS_AGG_NAME = "synonym_sets_aggr";
+    private static final String RULE_COUNT_AGG_NAME = "rule_count";
+    private static final String RULE_COUNT_FILTER_KEY = "synonym_rules";
     private static final int SYNONYMS_INDEX_MAPPINGS_VERSION = 1;
     public static final int INDEX_SEARCHABLE_TIMEOUT_SECONDS = 30;
     private final int maxSynonymsSets;
@@ -185,15 +190,33 @@ public class SynonymsManagementAPIService {
         }
     }
 
+    /**
+     * Returns all synonym sets with their rule counts, including empty synonym sets.
+     * @param from The index of the first synonym set to return
+     * @param size The number of synonym sets to return
+     * @param listener The listener to return the synonym sets to
+     */
     public void getSynonymsSets(int from, int size, ActionListener<PagedResult<SynonymSetSummary>> listener) {
+        BoolQueryBuilder synonymSetQuery = QueryBuilders.boolQuery()
+            .should(QueryBuilders.termQuery(OBJECT_TYPE_FIELD, SYNONYM_SET_OBJECT_TYPE))
+            .should(QueryBuilders.termQuery(OBJECT_TYPE_FIELD, SYNONYM_RULE_OBJECT_TYPE))
+            .minimumShouldMatch(1);
+
+        // Aggregation query to count only synonym rules (excluding synonym set objects)
+        FiltersAggregationBuilder ruleCountAggregation = new FiltersAggregationBuilder(
+            RULE_COUNT_AGG_NAME,
+            new FiltersAggregator.KeyedFilter(RULE_COUNT_FILTER_KEY, QueryBuilders.termQuery(OBJECT_TYPE_FIELD, SYNONYM_RULE_OBJECT_TYPE))
+        );
+
         client.prepareSearch(SYNONYMS_ALIAS_NAME)
             .setSize(0)
             // Retrieves aggregated synonym rules for each synonym set, excluding the synonym set object type
-            .setQuery(QueryBuilders.termQuery(OBJECT_TYPE_FIELD, SYNONYM_RULE_OBJECT_TYPE))
+            .setQuery(synonymSetQuery)
             .addAggregation(
                 new TermsAggregationBuilder(SYNONYM_SETS_AGG_NAME).field(SYNONYMS_SET_FIELD)
                     .order(BucketOrder.key(true))
                     .size(maxSynonymsSets)
+                    .subAggregation(ruleCountAggregation)
             )
             .setPreference(Preference.LOCAL.type())
             .execute(new ActionListener<>() {
@@ -201,11 +224,11 @@ public class SynonymsManagementAPIService {
                 public void onResponse(SearchResponse searchResponse) {
                     Terms termsAggregation = searchResponse.getAggregations().get(SYNONYM_SETS_AGG_NAME);
                     List<? extends Terms.Bucket> buckets = termsAggregation.getBuckets();
-                    SynonymSetSummary[] synonymSetSummaries = buckets.stream()
-                        .skip(from)
-                        .limit(size)
-                        .map(bucket -> new SynonymSetSummary(bucket.getDocCount(), bucket.getKeyAsString()))
-                        .toArray(SynonymSetSummary[]::new);
+                    SynonymSetSummary[] synonymSetSummaries = buckets.stream().skip(from).limit(size).map(bucket -> {
+                        Filters ruleCountFilters = bucket.getAggregations().get(RULE_COUNT_AGG_NAME);
+                        Filters.Bucket ruleCountBucket = ruleCountFilters.getBucketByKey(RULE_COUNT_FILTER_KEY);
+                        return new SynonymSetSummary(ruleCountBucket.getDocCount(), bucket.getKeyAsString());
+                    }).toArray(SynonymSetSummary[]::new);
 
                     listener.onResponse(new PagedResult<>(buckets.size(), synonymSetSummaries));
                 }

server/src/main/resources/META-INF/services/org.elasticsearch.features.FeatureSpecification

@@ -14,6 +14,7 @@ org.elasticsearch.rest.action.admin.cluster.GetSnapshotsFeatures
 org.elasticsearch.index.IndexFeatures
 org.elasticsearch.index.mapper.MapperFeatures
 org.elasticsearch.search.SearchFeatures
+org.elasticsearch.synonyms.SynonymFeatures
 org.elasticsearch.search.retriever.RetrieversFeatures
 org.elasticsearch.script.ScriptFeatures
 org.elasticsearch.cluster.routing.RoutingFeatures