Fixing get data stream API when data stream index mode has been changed to time_series (#137852)

This fixes an edge case introduced by #137407. If a data stream is
created with the standard `index_mode`, and then the template changes
the mode to `time_series` without a `routing_path` and the data stream
is not rolled over, then the get data stream API and the get data stream
mappings API will fail with errors mentioning something like `failed to
apply settings java.lang.IllegalArgumentException:
[index.mode=time_series] requires a non-empty [index.routing_path]`.
Below is an example stack trace:

```
[2025-11-10T12:51:39,973][WARN ][o.e.c.s.IndexScopedSettings] [runTask-0] [.ds-quickstart-2-2025.11.10-000001] failed to apply settings java.lang.IllegalArgumentException: [index.mode=time_series] requires a non-empty [index.routing_path]
        at [email protected]/org.elasticsearch.index.IndexMode$2.validateWithOtherSettings(IndexMode.java:156)
        at [email protected]/org.elasticsearch.index.IndexSettings$3.validate(IndexSettings.java:751)
        at [email protected]/org.elasticsearch.index.IndexSettings$3.validate(IndexSettings.java:745)
        at [email protected]/org.elasticsearch.common.settings.Setting.get(Setting.java:589)
        at [email protected]/org.elasticsearch.common.settings.Setting.get(Setting.java:561)
        at [email protected]/org.elasticsearch.index.mapper.MapperService.lambda$static$0(MapperService.java:134)
        at [email protected]/org.elasticsearch.common.settings.Setting.innerGetRaw(Setting.java:657)
        at [email protected]/org.elasticsearch.common.settings.Setting.getRaw(Setting.java:632)
        at [email protected]/org.elasticsearch.common.settings.Setting$Updater.hasChanged(Setting.java:1317)
        at [email protected]/org.elasticsearch.common.settings.AbstractScopedSettings$SettingUpdater.updater(AbstractScopedSettings.java:683)
        at [email protected]/org.elasticsearch.common.settings.AbstractScopedSettings.applySettings(AbstractScopedSettings.java:168)
        at [email protected]/org.elasticsearch.index.IndexSettings.updateIndexMetadata(IndexSettings.java:1495)
        at [email protected]/org.elasticsearch.cluster.metadata.DataStream.lambda$getEffectiveMappings$1(DataStream.java:520)
        at [email protected]/org.elasticsearch.indices.IndicesService.withTempIndexService(IndicesService.java:773)
        at [email protected]/org.elasticsearch.cluster.metadata.DataStream.getEffectiveMappings(DataStream.java:495)
        at [email protected]/org.elasticsearch.cluster.metadata.MetadataDataStreamsService.getEffectiveSettings(MetadataDataStreamsService.java:612)
        at [email protected]/org.elasticsearch.cluster.metadata.MetadataDataStreamsService.getEffectiveSettings(MetadataDataStreamsService.java:593)
        at [email protected]/org.elasticsearch.datastreams.action.TransportGetDataStreamsAction.innerOperation(TransportGetDataStreamsAction.java:283)
        at [email protected]/org.elasticsearch.datastreams.action.TransportGetDataStreamsAction.localClusterStateOperation(TransportGetDataStreamsAction.java:179)
        at [email protected]/org.elasticsearch.datastreams.action.TransportGetDataStreamsAction.localClusterStateOperation(TransportGetDataStreamsAction.java:72)
        at [email protected]/org.elasticsearch.action.support.local.TransportLocalProjectMetadataAction.localClusterStateOperation(TransportLocalProjectMetadataAction.java:59)
        at [email protected]/org.elasticsearch.action.support.local.TransportLocalClusterStateAction.lambda$innerDoExecute$0(TransportLocalClusterStateAction.java:92)
        at [email protected]/org.elasticsearch.action.ActionRunnable$4.doRun(ActionRunnable.java:101)
        at [email protected]/org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:1076)
        at [email protected]/org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:27)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1090)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:614)
        at java.base/java.lang.Thread.run(Thread.java:1474)
```

Author: masseyke

docs/changelog/137852.yaml

@@ -0,0 +1,6 @@
+pr: 137852
+summary: Fixing get data stream API when data stream index mode has been changed
+  to `time_series`
+area: Data streams
+type: bug
+issues: []

modules/data-streams/src/yamlRestTest/resources/rest-api-spec/test/data_stream/280_data_stream_mappings-timeseries.yml

@@ -322,3 +322,267 @@
   - match: { data_streams.0.effective_mappings: null }
   - set: { data_streams.0.indices.0.index_name: oldIndexName }
   - set: { data_streams.0.indices.1.index_name: newIndexName }
+
+---
+"Test change data stream from standard to time_series with no routing_path":
+  # This test makes sure that if we change the index mode of a data stream from standard to time_series without a
+  # configured routing_path, the get data stream and get data stream mappings APIs do not blow up before or after
+  # rollover.
+  - do:
+      indices.put_index_template:
+        name: my-template
+        body:
+          index_patterns: [ test-data-stream-* ]
+          data_stream: { }
+          template:
+            settings:
+              number_of_replicas: 0
+            mappings:
+              properties:
+                field1:
+                  type: keyword
+
+  - do:
+      indices.create_data_stream:
+        name: test-data-stream-1
+
+  - do:
+      indices.put_index_template:
+        name: my-template
+        body:
+          index_patterns: [ test-data-stream-* ]
+          data_stream: { }
+          template:
+            settings:
+              number_of_replicas: 0
+              mode: time_series
+            mappings:
+              properties:
+                field1:
+                  type: keyword
+                  time_series_dimension: true
+
+  - do:
+      cluster.health:
+        index: "test-data-stream-1"
+        wait_for_status: green
+
+  - do:
+      indices.get_data_stream_mappings:
+        name: test-data-stream-1
+  - match: { data_streams.0.name: test-data-stream-1 }
+  - match: { data_streams.0.mappings: {} }
+  - length: { data_streams.0.effective_mappings.properties: 2 }
+
+  - do:
+      indices.get_data_stream:
+        name: test-data-stream-1
+  - match: { data_streams.0.name: test-data-stream-1 }
+  - match: { data_streams.0.mappings: {} }
+  - match: { data_streams.0.effective_mappings: null }
+
+  - do:
+      indices.rollover:
+        alias: "test-data-stream-1"
+
+  - do:
+      cluster.health:
+        index: "test-data-stream-1"
+        wait_for_status: green
+
+  - do:
+      indices.get_data_stream_mappings:
+        name: test-data-stream-1
+  - match: { data_streams.0.name: test-data-stream-1 }
+  - length: { data_streams.0.effective_mappings.properties: 2 }
+  - match: { data_streams.0.effective_mappings.properties.field1.type: "keyword" }
+
+  - do:
+      indices.get_data_stream:
+        name: test-data-stream-1
+  - match: { data_streams.0.name: test-data-stream-1 }
+  - match: { data_streams.0.mappings: { } }
+  - match: { data_streams.0.effective_mappings: null }
+  - set: { data_streams.0.indices.0.index_name: oldIndexName }
+  - set: { data_streams.0.indices.1.index_name: newIndexName }
+
+---
+"Test change data stream from time_series to standard with no routing_path":
+  # This test makes sure that if we change the index mode of a data stream from time_series without a configured
+  # routing_path to standard, the get data stream and get data stream mappings APIs do not blow up before or after
+  # rollover.
+  - do:
+      indices.put_index_template:
+        name: my-template
+        body:
+          index_patterns: [ my-data-stream-* ]
+          data_stream: { }
+          template:
+            settings:
+              number_of_replicas: 0
+              mode: time_series
+            mappings:
+              properties:
+                field1:
+                  type: keyword
+                  time_series_dimension: true
+
+  - do:
+      indices.create_data_stream:
+        name: my-data-stream-1
+
+  - do:
+      cluster.health:
+        index: "my-data-stream-1"
+        wait_for_status: green
+
+  - do:
+      indices.get_data_stream_mappings:
+        name: my-data-stream-1
+  - match: { data_streams.0.name: my-data-stream-1 }
+  - match: { data_streams.0.mappings: {} }
+  - length: { data_streams.0.effective_mappings.properties: 2 }
+
+  - do:
+      indices.get_data_stream:
+        name: my-data-stream-1
+  - match: { data_streams.0.name: my-data-stream-1 }
+  - match: { data_streams.0.mappings: {} }
+  - match: { data_streams.0.effective_mappings: null }
+
+  - do:
+      indices.put_index_template:
+        name: my-template
+        body:
+          index_patterns: [ my-data-stream-* ]
+          data_stream: { }
+          template:
+            settings:
+              number_of_replicas: 0
+            mappings:
+              properties:
+                field1:
+                  type: keyword
+
+  - do:
+      indices.get_data_stream_mappings:
+        name: my-data-stream-1
+  - match: { data_streams.0.name: my-data-stream-1 }
+  - match: { data_streams.0.mappings: {} }
+  - length: { data_streams.0.effective_mappings.properties: 2 }
+
+  - do:
+      indices.get_data_stream:
+        name: my-data-stream-1
+  - match: { data_streams.0.name: my-data-stream-1 }
+  - match: { data_streams.0.mappings: {} }
+  - match: { data_streams.0.effective_mappings: null }
+
+  - do:
+      indices.rollover:
+        alias: "my-data-stream-1"
+
+  - do:
+      cluster.health:
+        index: "my-data-stream-1"
+        wait_for_status: green
+
+  - do:
+      indices.get_data_stream_mappings:
+        name: my-data-stream-1
+  - match: { data_streams.0.name: my-data-stream-1 }
+  - length: { data_streams.0.effective_mappings.properties: 2 }
+  - match: { data_streams.0.effective_mappings.properties.field1.type: "keyword" }
+
+  - do:
+      indices.get_data_stream:
+        name: my-data-stream-1
+  - match: { data_streams.0.name: my-data-stream-1 }
+  - match: { data_streams.0.mappings: { } }
+  - match: { data_streams.0.effective_mappings: null }
+  - set: { data_streams.0.indices.0.index_name: oldIndexName }
+  - set: { data_streams.0.indices.1.index_name: newIndexName }
+
+---
+"Test change data stream from standard to time_series with routing_path":
+  # This test makes sure that if we change the index mode of a data stream from standard to time_series with a
+  # configured routing_path, the get data stream and get data stream mappings APIs do not blow up before or after
+  # rollover.
+  - do:
+      indices.put_index_template:
+        name: my-template
+        body:
+          index_patterns: [ test-data-stream-* ]
+          data_stream: { }
+          template:
+            settings:
+              number_of_replicas: 0
+            mappings:
+              properties:
+                field1:
+                  type: keyword
+
+  - do:
+      indices.create_data_stream:
+        name: test-data-stream-1
+
+  - do:
+      indices.put_index_template:
+        name: my-template
+        body:
+          index_patterns: [ test-data-stream-* ]
+          data_stream: { }
+          template:
+            settings:
+              number_of_replicas: 0
+              routing_path: field1
+              mode: time_series
+            mappings:
+              properties:
+                field1:
+                  type: keyword
+                  time_series_dimension: true
+
+  - do:
+      cluster.health:
+        index: "test-data-stream-1"
+        wait_for_status: green
+
+  - do:
+      indices.get_data_stream_mappings:
+        name: test-data-stream-1
+  - match: { data_streams.0.name: test-data-stream-1 }
+  - match: { data_streams.0.mappings: {} }
+  - length: { data_streams.0.effective_mappings.properties: 2 }
+
+  - do:
+      indices.get_data_stream:
+        name: test-data-stream-1
+  - match: { data_streams.0.name: test-data-stream-1 }
+  - match: { data_streams.0.mappings: {} }
+  - match: { data_streams.0.effective_mappings: null }
+
+  - do:
+      indices.rollover:
+        alias: "test-data-stream-1"
+
+  - do:
+      cluster.health:
+        index: "test-data-stream-1"
+        wait_for_status: green
+
+  - do:
+      indices.get_data_stream_mappings:
+        name: test-data-stream-1
+  - match: { data_streams.0.name: test-data-stream-1 }
+  - length: { data_streams.0.effective_mappings.properties: 2 }
+  - match: { data_streams.0.effective_mappings.properties.field1.type: "keyword" }
+
+  - do:
+      indices.get_data_stream:
+        name: test-data-stream-1
+  - match: { data_streams.0.name: test-data-stream-1 }
+  - match: { data_streams.0.mappings: { } }
+  - match: { data_streams.0.effective_mappings: null }
+  - set: { data_streams.0.indices.0.index_name: oldIndexName }
+  - set: { data_streams.0.indices.1.index_name: newIndexName }

server/src/main/java/org/elasticsearch/cluster/metadata/DataStream.java

@@ -511,13 +511,23 @@ public static CompressedXContent getEffectiveMappings(
                  * mapping, we make sure to correct the index mode and index routing path here.
                  */
                 IndexMetadata oldIndexMetadata = indexService.getMetadata();
-                Settings.Builder settingsBuilder = Settings.builder().put(oldIndexMetadata.getSettings());
-                settingsBuilder.put(indexModeSettingName, templateSettings.get(indexModeSettingName));
 
+                Settings oldIndexSettings = oldIndexMetadata.getSettings();
                 String indexRoutingPathSettingName = IndexMetadata.INDEX_ROUTING_PATH.getKey();
-                settingsBuilder.put(indexRoutingPathSettingName, templateSettings.get(indexRoutingPathSettingName));
-                IndexMetadata newIndexMetadata = new IndexMetadata.Builder(oldIndexMetadata).settings(settingsBuilder.build()).build();
-                mapperService.getIndexSettings().updateIndexMetadata(newIndexMetadata);
+                if (Objects.equals(
+                    templateSettings.get(indexRoutingPathSettingName),
+                    oldIndexSettings.get(indexRoutingPathSettingName)
+                ) == false) {
+                    /*
+                     * If the routing_path has changed, we need to make sure to update it so that validation does not fail when we merge
+                     * mappings.
+                     */
+                    Settings.Builder settingsBuilder = Settings.builder().put(oldIndexSettings);
+                    settingsBuilder.put(indexModeSettingName, templateSettings.get(indexModeSettingName));
+                    settingsBuilder.put(indexRoutingPathSettingName, templateSettings.get(indexRoutingPathSettingName));
+                    IndexMetadata newIndexMetadata = new IndexMetadata.Builder(oldIndexMetadata).settings(settingsBuilder.build()).build();
+                    mapperService.getIndexSettings().updateIndexMetadata(newIndexMetadata);
+                }
             }
             CompressedXContent mergedMapping = mapperService.merge(
                 MapperService.SINGLE_MAPPING_NAME,