From 41f186dca043f87b51bf74b41af305887a6e21be Mon Sep 17 00:00:00 2001
From: Keith Massey <keith.massey@elastic.co>
Date: Tue, 27 May 2025 15:42:08 -0500
Subject: [PATCH] Adding prefer_ilm as a whitelisted data stream setting
 (#128375)

---
 .../TransportUpdateDataStreamSettingsAction.java      |  3 ++-
 .../test/data_stream/240_data_stream_settings.yml     | 11 +++++++++--
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/modules/data-streams/src/main/java/org/elasticsearch/datastreams/action/TransportUpdateDataStreamSettingsAction.java b/modules/data-streams/src/main/java/org/elasticsearch/datastreams/action/TransportUpdateDataStreamSettingsAction.java
index dc25e2ce4f4e..9757fca2024b 100644
--- a/modules/data-streams/src/main/java/org/elasticsearch/datastreams/action/TransportUpdateDataStreamSettingsAction.java
+++ b/modules/data-streams/src/main/java/org/elasticsearch/datastreams/action/TransportUpdateDataStreamSettingsAction.java
@@ -33,6 +33,7 @@ import org.elasticsearch.common.settings.SettingsFilter;
 import org.elasticsearch.common.util.concurrent.EsExecutors;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.index.Index;
+import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.indices.SystemIndices;
 import org.elasticsearch.injection.guice.Inject;
 import org.elasticsearch.tasks.Task;
@@ -53,7 +54,7 @@ public class TransportUpdateDataStreamSettingsAction extends TransportMasterNode
     UpdateDataStreamSettingsAction.Request,
     UpdateDataStreamSettingsAction.Response> {
     private static final Logger logger = LogManager.getLogger(TransportUpdateDataStreamSettingsAction.class);
-    private static final Set<String> APPLY_TO_BACKING_INDICES = Set.of("index.lifecycle.name");
+    private static final Set<String> APPLY_TO_BACKING_INDICES = Set.of("index.lifecycle.name", IndexSettings.PREFER_ILM);
     private static final Set<String> APPLY_TO_DATA_STREAM_ONLY = Set.of("index.number_of_shards");
     private final MetadataDataStreamsService metadataDataStreamsService;
     private final MetadataUpdateSettingsService updateSettingsService;
diff --git a/modules/data-streams/src/yamlRestTest/resources/rest-api-spec/test/data_stream/240_data_stream_settings.yml b/modules/data-streams/src/yamlRestTest/resources/rest-api-spec/test/data_stream/240_data_stream_settings.yml
index e642ef008370..8dd15628fa13 100644
--- a/modules/data-streams/src/yamlRestTest/resources/rest-api-spec/test/data_stream/240_data_stream_settings.yml
+++ b/modules/data-streams/src/yamlRestTest/resources/rest-api-spec/test/data_stream/240_data_stream_settings.yml
@@ -51,16 +51,20 @@ setup:
         body:
           index:
             number_of_shards: 2
-            lifecycle.name: my-new-policy
+            lifecycle:
+              name: my-new-policy
+              prefer_ilm: true
   - match: { data_streams.0.name: my-data-stream-1 }
   - match: { data_streams.0.applied_to_data_stream: true }
   - match: { data_streams.0.index_settings_results.applied_to_data_stream_only: [index.number_of_shards]}
-  - match: { data_streams.0.index_settings_results.applied_to_data_stream_and_backing_indices: [index.lifecycle.name] }
+  - length: { data_streams.0.index_settings_results.applied_to_data_stream_and_backing_indices: 2  }
   - match: { data_streams.0.settings.index.number_of_shards: "2" }
   - match: { data_streams.0.settings.index.lifecycle.name: "my-new-policy" }
+  - match: { data_streams.0.settings.index.lifecycle.prefer_ilm: "true" }
   - match: { data_streams.0.effective_settings.index.number_of_shards: "2" }
   - match: { data_streams.0.effective_settings.index.number_of_replicas: "0" }
   - match: { data_streams.0.effective_settings.index.lifecycle.name: "my-new-policy" }
+  - match: { data_streams.0.effective_settings.index.lifecycle.prefer_ilm: "true" }
 
   - do:
       indices.rollover:
@@ -79,6 +83,7 @@ setup:
   - match: { data_streams.0.effective_settings.index.number_of_shards: "2" }
   - match: { data_streams.0.effective_settings.index.number_of_replicas: "0" }
   - match: { data_streams.0.effective_settings.index.lifecycle.name: "my-new-policy" }
+  - match: { data_streams.0.effective_settings.index.lifecycle.prefer_ilm: "true" }
 
   - do:
       indices.get_data_stream:
@@ -86,6 +91,7 @@ setup:
   - match: { data_streams.0.name: my-data-stream-1 }
   - match: { data_streams.0.settings.index.number_of_shards: "2" }
   - match: { data_streams.0.settings.index.lifecycle.name: "my-new-policy" }
+  - match: { data_streams.0.settings.index.lifecycle.prefer_ilm: "true" }
   - match: { data_streams.0.effective_settings: null }
 
   - do:
@@ -101,6 +107,7 @@ setup:
   - match: { .$idx0name.settings.index.lifecycle.name: "my-new-policy" }
   - match: { .$idx1name.settings.index.number_of_shards: "2" }
   - match: { .$idx1name.settings.index.lifecycle.name: "my-new-policy" }
+  - match: { .$idx1name.settings.index.lifecycle.prefer_ilm: "true" }
 
 ---
 "Test multiple data streams":