Use a settings instead of a feature flag for LTR.

test/test-clusters/src/main/java/org/elasticsearch/test/cluster/FeatureFlag.java

@@ -16,7 +16,6 @@
  */
 public enum FeatureFlag {
     TIME_SERIES_MODE("es.index_mode_feature_flag_registered=true", Version.fromString("8.0.0"), null),
-    LEARNING_TO_RANK("es.learning_to_rank_feature_flag_enabled=true", Version.fromString("8.12.0"), null),
     FAILURE_STORE_ENABLED("es.failure_store_feature_flag_enabled=true", Version.fromString("8.12.0"), null);
 
     public final String systemProperty;

x-pack/plugin/ml/qa/basic-multi-node/build.gradle

@@ -1,4 +1,3 @@
-import org.elasticsearch.gradle.Version
 import org.elasticsearch.gradle.internal.info.BuildParams
 
 apply plugin: 'elasticsearch.legacy-java-rest-test'
@@ -17,7 +16,7 @@ testClusters.configureEach {
   setting 'xpack.license.self_generated.type', 'trial'
   setting 'indices.lifecycle.history_index_enabled', 'false'
   setting 'slm.history_index_enabled', 'false'
-  requiresFeature 'es.learning_to_rank_feature_flag_enabled', Version.fromString("8.12.0")
+  setting 'xpack.ml.learning_to_rank.enabled', 'true'
 }
 
 if (BuildParams.inFipsJvm){

x-pack/plugin/ml/qa/ml-with-security/build.gradle

@@ -1,4 +1,3 @@
-import org.elasticsearch.gradle.Version
 apply plugin: 'elasticsearch.legacy-yaml-rest-test'
 
 dependencies {
@@ -258,5 +257,5 @@ testClusters.configureEach {
   user username: "no_ml", password: "x-pack-test-password", role: "minimal"
   setting 'xpack.license.self_generated.type', 'trial'
   setting 'xpack.security.enabled', 'true'
-  requiresFeature 'es.learning_to_rank_feature_flag_enabled', Version.fromString("8.12.0")
+  setting 'xpack.ml.learning_to_rank.enabled', 'true'
 }

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/MachineLearning.java

@@ -327,7 +327,6 @@
 import org.elasticsearch.xpack.ml.inference.ingest.InferenceProcessor;
 import org.elasticsearch.xpack.ml.inference.loadingservice.ModelLoadingService;
 import org.elasticsearch.xpack.ml.inference.ltr.LearningToRankRescorerBuilder;
-import org.elasticsearch.xpack.ml.inference.ltr.LearningToRankRescorerFeature;
 import org.elasticsearch.xpack.ml.inference.ltr.LearningToRankService;
 import org.elasticsearch.xpack.ml.inference.modelsize.MlModelSizeNamedXContentProvider;
 import org.elasticsearch.xpack.ml.inference.persistence.TrainedModelProvider;
@@ -890,7 +889,7 @@ private static void reportClashingNodeAttribute(String attrName) {
 
     @Override
     public List<RescorerSpec<?>> getRescorers() {
-        if (enabled && LearningToRankRescorerFeature.isEnabled()) {
+        if (enabled && learningToRankService.get().isEnabled()) {
             return List.of(
                 new RescorerSpec<>(
                     LearningToRankRescorerBuilder.NAME,
@@ -1125,7 +1124,7 @@ public Collection<?> createComponents(PluginServices services) {
         this.modelLoadingService.set(modelLoadingService);
 
         this.learningToRankService.set(
-            new LearningToRankService(modelLoadingService, trainedModelProvider, services.scriptService(), services.xContentRegistry())
+            new LearningToRankService(modelLoadingService, trainedModelProvider, services.scriptService(), services.xContentRegistry(), settings)
         );
 
         this.deploymentManager.set(
@@ -1801,7 +1800,7 @@ public List<NamedXContentRegistry.Entry> getNamedXContent() {
         );
         namedXContent.addAll(new CorrelationNamedContentProvider().getNamedXContentParsers());
         // LTR Combine with Inference named content provider when feature flag is removed
-        if (LearningToRankRescorerFeature.isEnabled()) {
+        if (learningToRankService.get().isEnabled()) {
             namedXContent.addAll(new MlLTRNamedXContentProvider().getNamedXContentParsers());
         }
         return namedXContent;
@@ -1889,7 +1888,7 @@ public List<NamedWriteableRegistry.Entry> getNamedWriteables() {
         namedWriteables.addAll(new CorrelationNamedContentProvider().getNamedWriteables());
         namedWriteables.addAll(new ChangePointNamedContentProvider().getNamedWriteables());
         // LTR Combine with Inference named content provider when feature flag is removed
-        if (LearningToRankRescorerFeature.isEnabled()) {
+        if (learningToRankService.get().isEnabled()) {
             namedWriteables.addAll(new MlLTRNamedXContentProvider().getNamedWriteables());
         }
         return namedWriteables;

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/ltr/LearningToRankService.java

@@ -7,8 +7,11 @@
 
 package org.elasticsearch.xpack.ml.inference.ltr;
 
+import org.elasticsearch.Build;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.common.bytes.BytesReference;
+import org.elasticsearch.common.settings.Setting;
+import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.index.query.MatchAllQueryBuilder;
 import org.elasticsearch.index.query.MatchNoneQueryBuilder;
 import org.elasticsearch.index.query.QueryBuilder;
@@ -52,33 +55,60 @@
 import static org.elasticsearch.xpack.core.ml.job.messages.Messages.INFERENCE_CONFIG_QUERY_BAD_FORMAT;
 
 public class LearningToRankService {
+    /**
+     * This setting behave like a feature flag for the learning to rank feature but can be set by an operator on self-managed environment.
+     * The feature is enabled by default only for snapshots builds.
+     *
+     * Before enabling by default, ensure transport serialization is all corrected for future BWC.
+     *
+     * See {@link LearningToRankRescorerBuilder}
+     */
+    public static Setting<Boolean> LEARNING_TO_RANK_ENABLED = Setting.boolSetting("xpack.ml.learning_to_rank.enabled",
+        Build.current().isSnapshot(),
+        Setting.Property.OperatorDynamic,
+        Setting.Property.NodeScope
+    );
+
     private static final Map<String, String> SCRIPT_OPTIONS = Map.ofEntries(
         entry(MustacheScriptEngine.DETECT_MISSING_PARAMS_OPTION, Boolean.TRUE.toString())
     );
     private final ModelLoadingService modelLoadingService;
     private final TrainedModelProvider trainedModelProvider;
     private final ScriptService scriptService;
     private final XContentParserConfiguration parserConfiguration;
+    private final boolean enabled;
 
     public LearningToRankService(
         ModelLoadingService modelLoadingService,
         TrainedModelProvider trainedModelProvider,
         ScriptService scriptService,
-        NamedXContentRegistry xContentRegistry
+        NamedXContentRegistry xContentRegistry,
+        Settings settings
     ) {
-        this(modelLoadingService, trainedModelProvider, scriptService, XContentParserConfiguration.EMPTY.withRegistry(xContentRegistry));
+        this(modelLoadingService, trainedModelProvider, scriptService, XContentParserConfiguration.EMPTY.withRegistry(xContentRegistry), settings);
     }
 
     LearningToRankService(
         ModelLoadingService modelLoadingService,
         TrainedModelProvider trainedModelProvider,
         ScriptService scriptService,
-        XContentParserConfiguration parserConfiguration
+        XContentParserConfiguration parserConfiguration,
+        Settings settings
     ) {
         this.modelLoadingService = modelLoadingService;
         this.scriptService = scriptService;
         this.trainedModelProvider = trainedModelProvider;
         this.parserConfiguration = parserConfiguration;
+        this.enabled = LEARNING_TO_RANK_ENABLED.get(settings);
+    }
+
+    /**
+     * Check if the learning to rank is enabled or not.
+     *
+     * @return Whether the LTR feature is enabled or not.
+     */
+    public boolean isEnabled() {
+        return enabled;
     }
 
     /**

x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/inference/ltr/LearningToRankServiceTests.java

@@ -238,7 +238,7 @@ private LearningToRankService getTestLearningToRankService(LearningToRankConfig
     }
 
     private LearningToRankService getTestLearningToRankService(TrainedModelProvider trainedModelProvider) {
-        return new LearningToRankService(mockModelLoadingService(), trainedModelProvider, getTestScriptService(), xContentRegistry());
+        return new LearningToRankService(mockModelLoadingService(), trainedModelProvider, getTestScriptService(), xContentRegistry(), Settings.EMPTY);
     }
 
     private ScriptService getTestScriptService() {

x-pack/plugin/src/yamlRestTest/java/org/elasticsearch/xpack/test/rest/XPackRestIT.java

@@ -43,7 +43,7 @@ public class XPackRestIT extends AbstractXPackRestTest {
         .setting("xpack.searchable.snapshot.shared_cache.region_size", "256KB")
         .user("x_pack_rest_user", "x-pack-test-password")
         .feature(FeatureFlag.TIME_SERIES_MODE)
-        .feature(FeatureFlag.LEARNING_TO_RANK)
+        .setting("xpack.ml.learning_to_rank.enabled", "true")
         .configFile("testnode.pem", Resource.fromClasspath("org/elasticsearch/xpack/security/transport/ssl/certs/simple/testnode.pem"))
         .configFile("testnode.crt", Resource.fromClasspath("org/elasticsearch/xpack/security/transport/ssl/certs/simple/testnode.crt"))
         .configFile("service_tokens", Resource.fromClasspath("service_tokens"))