Adding support for additional mapping to simulate ingest API (#114742) (#115284)

docs/changelog/114742.yaml

@@ -0,0 +1,5 @@
+pr: 114742
+summary: Adding support for additional mapping to simulate ingest API
+area: Ingest Node
+type: enhancement
+issues: []

docs/reference/ingest/apis/simulate-ingest.asciidoc

@@ -108,6 +108,14 @@ POST /_ingest/_simulate
       "index_patterns": ["my-index-*"],
       "composed_of": ["component_template_1", "component_template_2"]
     }
+  },
+  "mapping_addition": { <4>
+    "dynamic": "strict",
+    "properties": {
+      "foo": {
+        "type": "keyword"
+      }
+    }
   }
 }
 ----
@@ -117,6 +125,7 @@ POST /_ingest/_simulate
 These templates can be used to change the pipeline(s) used, or to modify the mapping that will be used to validate the result.
 <3> This replaces the existing `my-index-template` index template with the contents given here for the duration of this request.
 These templates can be used to change the pipeline(s) used, or to modify the mapping that will be used to validate the result.
+<4> This mapping is merged into the index's final mapping just before validation. It is used only for the duration of this request.
 
 [[simulate-ingest-api-request]]
 ==== {api-request-title}
@@ -246,6 +255,10 @@ include::{es-ref-dir}/indices/put-index-template.asciidoc[tag=request-body]
 
 ====
 
+`mapping_addition`::
+(Optional, <<mapping,mapping object>>)
+Definition of a mapping that will be merged into the index's mapping for validation during the course of this request.
+
 [[simulate-ingest-api-example]]
 ==== {api-examples-title}
 

qa/smoke-test-ingest-with-all-dependencies/src/yamlRestTest/resources/rest-api-spec/test/ingest/80_ingest_simulate.yml

@@ -1216,3 +1216,358 @@ setup:
   - match: { docs.0.doc._source.foo: "FOO" }
   - match: { docs.0.doc.executed_pipelines: ["foo-pipeline-2"] }
   - not_exists: docs.0.doc.error
+
+---
+"Test ingest simulate with mapping addition for data streams":
+  # In this test, we make sure that when the index template is a data stream template, simulate ingest works the same whether the data
+  # stream has been created or not -- either way, we expect it to use the template rather than the data stream / index mappings and settings.
+
+  - skip:
+      features:
+        - headers
+        - allowed_warnings
+
+  - requires:
+      cluster_features: ["simulate.mapping.addition"]
+      reason: "ingest simulate mapping addition added in 8.16"
+
+  - do:
+      headers:
+        Content-Type: application/json
+      ingest.put_pipeline:
+        id: "foo-pipeline"
+        body:  >
+          {
+            "processors": [
+              {
+                "set": {
+                  "field": "foo",
+                  "value": true
+                }
+              }
+            ]
+          }
+  - match: { acknowledged: true }
+
+  - do:
+      cluster.put_component_template:
+        name: mappings_template
+        body:
+          template:
+            mappings:
+              dynamic: strict
+              properties:
+                foo:
+                  type: boolean
+
+  - do:
+      cluster.put_component_template:
+        name: settings_template
+        body:
+          template:
+            settings:
+              index:
+                default_pipeline: "foo-pipeline"
+
+  - do:
+      allowed_warnings:
+        - "index template [test-composable-1] has index patterns [foo*] matching patterns from existing older templates [global] with patterns (global => [*]); this template [test-composable-1] will take precedence during new index creation"
+      indices.put_index_template:
+        name: test-composable-1
+        body:
+          index_patterns:
+            - foo*
+          composed_of:
+            - mappings_template
+            - settings_template
+
+  - do:
+      allowed_warnings:
+        - "index template [my-template-1] has index patterns [simple-data-stream1] matching patterns from existing older templates [global] with patterns (global => [*]); this template [my-template-1] will take precedence during new index creation"
+      indices.put_index_template:
+        name: my-template-1
+        body:
+          index_patterns: [simple-data-stream1]
+          composed_of:
+            - mappings_template
+            - settings_template
+          data_stream: {}
+
+  # Here we replace my-template-1 with a substitute version that uses the settings_template_2 and mappings_template_2 templates defined in
+  # this request, and foo-pipeline-2 defined in this request.
+  - do:
+      headers:
+        Content-Type: application/json
+      simulate.ingest:
+        index: simple-data-stream1
+        body: >
+          {
+            "docs": [
+              {
+                "_id": "asdf",
+                "_source": {
+                  "@timestamp": 1234,
+                  "foo": false
+                }
+              }
+            ],
+            "pipeline_substitutions": {
+              "foo-pipeline-2": {
+                "processors": [
+                  {
+                    "set": {
+                      "field": "foo",
+                      "value": "FOO"
+                    }
+                  }
+                ]
+              }
+            },
+            "component_template_substitutions": {
+              "settings_template_2": {
+                "template": {
+                  "settings": {
+                    "index": {
+                      "default_pipeline": "foo-pipeline-2"
+                    }
+                  }
+                }
+              },
+              "mappings_template_2": {
+                "template": {
+                  "mappings": {
+                    "dynamic": "strict",
+                    "properties": {
+                      "foo": {
+                        "type": "integer"
+                      }
+                    }
+                  }
+                }
+              }
+            },
+            "index_template_substitutions": {
+              "my-template-1": {
+                "index_patterns": ["simple-data-stream1"],
+                "composed_of": ["settings_template_2", "mappings_template_2"],
+                "data_stream": {}
+              }
+            },
+            "mapping_addition": {
+              "dynamic": "strict",
+              "properties": {
+                "foo": {
+                  "type": "keyword"
+                }
+              }
+            }
+          }
+  - length: { docs: 1 }
+  - match: { docs.0.doc._index: "simple-data-stream1" }
+  - match: { docs.0.doc._source.foo: "FOO" }
+  - match: { docs.0.doc.executed_pipelines: ["foo-pipeline-2"] }
+  - not_exists: docs.0.doc.error
+
+  - do:
+      indices.create_data_stream:
+        name: simple-data-stream1
+  - is_true: acknowledged
+
+  - do:
+      cluster.health:
+        wait_for_status: yellow
+
+  # Now that we have created a data stream, run the exact same simulate ingeset request to make sure we still get the same result, and that
+  # the substitutions and additions from the simulate ingest request are used instead of information from the data stream or its backing
+  # index.
+  - do:
+      headers:
+        Content-Type: application/json
+      simulate.ingest:
+        index: simple-data-stream1
+        body: >
+          {
+            "docs": [
+              {
+                "_id": "asdf",
+                "_source": {
+                  "@timestamp": 1234,
+                  "foo": false
+                }
+              }
+            ],
+            "pipeline_substitutions": {
+              "foo-pipeline-2": {
+                "processors": [
+                  {
+                    "set": {
+                      "field": "foo",
+                      "value": "FOO"
+                    }
+                  }
+                ]
+              }
+            },
+            "component_template_substitutions": {
+              "settings_template_2": {
+                "template": {
+                  "settings": {
+                    "index": {
+                      "default_pipeline": "foo-pipeline-2"
+                    }
+                  }
+                }
+              },
+              "mappings_template_2": {
+                "template": {
+                  "mappings": {
+                    "dynamic": "strict",
+                    "properties": {
+                      "foo": {
+                        "type": "integer"
+                      }
+                    }
+                  }
+                }
+              }
+            },
+            "index_template_substitutions": {
+              "my-template-1": {
+                "index_patterns": ["simple-data-stream1"],
+                "composed_of": ["settings_template_2", "mappings_template_2"],
+                "data_stream": {}
+              }
+            },
+            "mapping_addition": {
+              "dynamic": "strict",
+              "properties": {
+                "foo": {
+                  "type": "keyword"
+                }
+              }
+            }
+          }
+  - length: { docs: 1 }
+  - match: { docs.0.doc._index: "simple-data-stream1" }
+  - match: { docs.0.doc._source.foo: "FOO" }
+  - match: { docs.0.doc.executed_pipelines: ["foo-pipeline-2"] }
+  - not_exists: docs.0.doc.error
+
+---
+"Test mapping addition works with legacy templates":
+  # In this test, we make sure that when the index template is a data stream template, simulate ingest works the same whether the data
+  # stream has been created or not -- either way, we expect it to use the template rather than the data stream / index mappings and settings.
+
+  - skip:
+      features:
+        - headers
+        - allowed_warnings
+
+  - requires:
+      cluster_features: ["simulate.mapping.addition"]
+      reason: "ingest simulate mapping addition added in 8.16"
+
+  - do:
+      indices.put_template:
+        name: my-legacy-template
+        body:
+          index_patterns: foo-*
+          settings:
+            number_of_replicas: 0
+          mappings:
+            dynamic: strict
+            properties:
+              foo:
+                type: integer
+              bar:
+                type: boolean
+
+  - do:
+      headers:
+        Content-Type: application/json
+      simulate.ingest:
+        index: foo-1
+        body: >
+          {
+            "docs": [
+              {
+                "_id": "asdf",
+                "_source": {
+                  "foo": 3,
+                  "bar": "not a boolean"
+                }
+              }
+            ]
+          }
+  - length: { docs: 1 }
+  - match: { docs.0.doc._index: "foo-1" }
+  - match: { docs.0.doc._source.foo: 3 }
+  - match: { docs.0.doc._source.bar: "not a boolean" }
+  - match: { docs.0.doc.error.type: "document_parsing_exception" }
+
+  - do:
+      headers:
+        Content-Type: application/json
+      simulate.ingest:
+        index: foo-1
+        body: >
+          {
+            "docs": [
+              {
+                "_id": "asdf",
+                "_source": {
+                  "foo": 3,
+                  "bar": "not a boolean"
+                }
+              }
+            ],
+            "mapping_addition": {
+              "dynamic": "strict",
+              "properties": {
+                "bar": {
+                  "type": "keyword"
+                }
+              }
+            }
+          }
+  - length: { docs: 1 }
+  - match: { docs.0.doc._index: "foo-1" }
+  - match: { docs.0.doc._source.foo: 3 }
+  - match: { docs.0.doc._source.bar: "not a boolean" }
+  - not_exists: docs.0.doc.error
+
+  - do:
+      indices.create:
+        index: foo-1
+  - match: { acknowledged: true }
+
+  - do:
+      headers:
+        Content-Type: application/json
+      simulate.ingest:
+        index: foo-1
+        body: >
+          {
+            "docs": [
+              {
+                "_id": "asdf",
+                "_source": {
+                  "foo": 3,
+                  "bar": "not a boolean"
+                }
+              }
+            ],
+            "mapping_addition": {
+              "dynamic": "strict",
+              "properties": {
+                "bar": {
+                  "type": "keyword"
+                }
+              }
+            }
+          }
+  - length: { docs: 1 }
+  - match: { docs.0.doc._index: "foo-1" }
+  - match: { docs.0.doc._source.foo: 3 }
+  - match: { docs.0.doc._source.bar: "not a boolean" }
+  - not_exists: docs.0.doc.error

server/src/internalClusterTest/java/org/elasticsearch/action/bulk/TransportSimulateBulkActionIT.java

@@ -34,6 +34,7 @@ import java.io.IOException;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.concurrent.TimeUnit;
 
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
@@ -59,7 +60,7 @@ public class TransportSimulateBulkActionIT extends ESIntegTestCase {
             }
             """;
         indicesAdmin().create(new CreateIndexRequest(indexName).mapping(mapping)).actionGet();
-        BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of());
+        BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of(), Map.of());
         bulkRequest.add(new IndexRequest(indexName).source("""
             {
               "foo1": "baz"
@@ -131,10 +132,10 @@ public class TransportSimulateBulkActionIT extends ESIntegTestCase {
 
         String indexName = "my-index-1";
         // First, run before the index is created:
-        assertMappingsUpdatedFromComponentTemplateSubstitutions(indexName, indexTemplateName);
+        assertMappingsUpdatedFromSubstitutions(indexName, indexTemplateName);
         // Now, create the index and make sure the component template substitutions work the same:
         indicesAdmin().create(new CreateIndexRequest(indexName)).actionGet();
-        assertMappingsUpdatedFromComponentTemplateSubstitutions(indexName, indexTemplateName);
+        assertMappingsUpdatedFromSubstitutions(indexName, indexTemplateName);
         // Now make sure nothing was actually changed:
         indicesAdmin().refresh(new RefreshRequest(indexName)).actionGet();
         SearchResponse searchResponse = client().search(new SearchRequest(indexName)).actionGet();
@@ -146,7 +147,7 @@ public class TransportSimulateBulkActionIT extends ESIntegTestCase {
         assertThat(fields.size(), equalTo(1));
     }
 
-    private void assertMappingsUpdatedFromComponentTemplateSubstitutions(String indexName, String indexTemplateName) {
+    private void assertMappingsUpdatedFromSubstitutions(String indexName, String indexTemplateName) {
         IndexRequest indexRequest1 = new IndexRequest(indexName).source("""
             {
               "foo1": "baz"
@@ -159,7 +160,7 @@ public class TransportSimulateBulkActionIT extends ESIntegTestCase {
             """, XContentType.JSON).id(randomUUID());
         {
             // First we use the original component template, and expect a failure in the second document:
-            BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of());
+            BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of(), Map.of());
             bulkRequest.add(indexRequest1);
             bulkRequest.add(indexRequest2);
             BulkResponse response = client().execute(new ActionType<BulkResponse>(SimulateBulkAction.NAME), bulkRequest).actionGet();
@@ -192,6 +193,7 @@ public class TransportSimulateBulkActionIT extends ESIntegTestCase {
                         )
                     )
                 ),
+                Map.of(),
                 Map.of()
             );
             bulkRequest.add(indexRequest1);
@@ -226,7 +228,34 @@ public class TransportSimulateBulkActionIT extends ESIntegTestCase {
                         )
                     )
                 ),
-                Map.of(indexTemplateName, Map.of("index_patterns", List.of(indexName), "composed_of", List.of("test-component-template-2")))
+                Map.of(
+                    indexTemplateName,
+                    Map.of("index_patterns", List.of(indexName), "composed_of", List.of("test-component-template-2"))
+                ),
+                Map.of()
+            );
+            bulkRequest.add(indexRequest1);
+            bulkRequest.add(indexRequest2);
+            BulkResponse response = client().execute(new ActionType<BulkResponse>(SimulateBulkAction.NAME), bulkRequest).actionGet();
+            assertThat(response.getItems().length, equalTo(2));
+            assertThat(response.getItems()[0].getResponse().getResult(), equalTo(DocWriteResponse.Result.CREATED));
+            assertNull(((SimulateIndexResponse) response.getItems()[0].getResponse()).getException());
+            assertThat(response.getItems()[1].getResponse().getResult(), equalTo(DocWriteResponse.Result.CREATED));
+            assertNull(((SimulateIndexResponse) response.getItems()[1].getResponse()).getException());
+        }
+
+        {
+            /*
+             * Now we mapping_addition that defines both fields, so we expect no exception:
+             */
+            BulkRequest bulkRequest = new SimulateBulkRequest(
+                Map.of(),
+                Map.of(),
+                Map.of(),
+                Map.of(
+                    "_doc",
+                    Map.of("dynamic", "strict", "properties", Map.of("foo1", Map.of("type", "text"), "foo3", Map.of("type", "text")))
+                )
             );
             bulkRequest.add(indexRequest1);
             bulkRequest.add(indexRequest2);
@@ -245,7 +274,7 @@ public class TransportSimulateBulkActionIT extends ESIntegTestCase {
          * mapping-less "random-index-template" created by the parent class), so we expect no mapping validation failure.
          */
         String indexName = randomAlphaOfLength(20).toLowerCase(Locale.ROOT);
-        BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of());
+        BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of(), Map.of());
         bulkRequest.add(new IndexRequest(indexName).source("""
             {
               "foo1": "baz"
@@ -292,7 +321,7 @@ public class TransportSimulateBulkActionIT extends ESIntegTestCase {
         request.indexTemplate(composableIndexTemplate);
 
         client().execute(TransportPutComposableIndexTemplateAction.TYPE, request).actionGet();
-        BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of());
+        BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of(), Map.of());
         bulkRequest.add(new IndexRequest(indexName).source("""
             {
               "foo1": "baz"
@@ -324,7 +353,7 @@ public class TransportSimulateBulkActionIT extends ESIntegTestCase {
         indicesAdmin().putTemplate(
             new PutIndexTemplateRequest("test-template").patterns(List.of("my-index-*")).mapping("foo1", "type=integer")
         ).actionGet();
-        BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of());
+        BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of(), Map.of());
         bulkRequest.add(new IndexRequest(indexName).source("""
             {
               "foo1": "baz"
@@ -378,7 +407,7 @@ public class TransportSimulateBulkActionIT extends ESIntegTestCase {
         client().execute(TransportPutComposableIndexTemplateAction.TYPE, request).actionGet();
         {
             // First, try with no @timestamp to make sure we're picking up data-stream-specific templates
-            BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of());
+            BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of(), Map.of());
             bulkRequest.add(new IndexRequest(indexName).source("""
                 {
                   "foo1": "baz"
@@ -389,7 +418,8 @@ public class TransportSimulateBulkActionIT extends ESIntegTestCase {
                   "foo3": "baz"
                 }
                 """, XContentType.JSON).id(randomUUID()));
-            BulkResponse response = client().execute(new ActionType<BulkResponse>(SimulateBulkAction.NAME), bulkRequest).actionGet();
+            BulkResponse response = client().execute(new ActionType<BulkResponse>(SimulateBulkAction.NAME), bulkRequest)
+                .actionGet(5, TimeUnit.SECONDS);
             assertThat(response.getItems().length, equalTo(2));
             assertThat(response.getItems()[0].getResponse().getResult(), equalTo(DocWriteResponse.Result.CREATED));
             assertThat(
@@ -404,7 +434,7 @@ public class TransportSimulateBulkActionIT extends ESIntegTestCase {
         }
         {
             // Now with @timestamp
-            BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of());
+            BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of(), Map.of());
             bulkRequest.add(new IndexRequest(indexName).source("""
                 {
                   "@timestamp": "2024-08-27",

server/src/main/java/org/elasticsearch/TransportVersions.java

@@ -177,6 +177,8 @@ public class TransportVersions {
     public static final TransportVersion REMOVE_MIN_COMPATIBLE_SHARD_NODE = def(8_773_00_0);
     public static final TransportVersion REVERT_REMOVE_MIN_COMPATIBLE_SHARD_NODE = def(8_774_00_0);
     public static final TransportVersion ESQL_FIELD_ATTRIBUTE_PARENT_SIMPLIFIED = def(8_775_00_0);
+    public static final TransportVersion INFERENCE_DONT_PERSIST_ON_READ = def(8_776_00_0);
+    public static final TransportVersion SIMULATE_MAPPING_ADDITION = def(8_777_00_0);
 
     /*
      * STOP! READ THIS FIRST! No, really,

server/src/main/java/org/elasticsearch/action/bulk/BulkFeatures.java

@@ -16,6 +16,7 @@ import java.util.Set;
 
 import static org.elasticsearch.action.bulk.TransportSimulateBulkAction.SIMULATE_COMPONENT_TEMPLATE_SUBSTITUTIONS;
 import static org.elasticsearch.action.bulk.TransportSimulateBulkAction.SIMULATE_INDEX_TEMPLATE_SUBSTITUTIONS;
+import static org.elasticsearch.action.bulk.TransportSimulateBulkAction.SIMULATE_MAPPING_ADDITION;
 import static org.elasticsearch.action.bulk.TransportSimulateBulkAction.SIMULATE_MAPPING_VALIDATION;
 import static org.elasticsearch.action.bulk.TransportSimulateBulkAction.SIMULATE_MAPPING_VALIDATION_TEMPLATES;
 
@@ -25,7 +26,8 @@ public class BulkFeatures implements FeatureSpecification {
             SIMULATE_MAPPING_VALIDATION,
             SIMULATE_MAPPING_VALIDATION_TEMPLATES,
             SIMULATE_COMPONENT_TEMPLATE_SUBSTITUTIONS,
-            SIMULATE_INDEX_TEMPLATE_SUBSTITUTIONS
+            SIMULATE_INDEX_TEMPLATE_SUBSTITUTIONS,
+            SIMULATE_MAPPING_ADDITION
         );
     }
 }

server/src/main/java/org/elasticsearch/action/bulk/SimulateBulkRequest.java

@@ -15,12 +15,12 @@ import org.elasticsearch.cluster.metadata.ComposableIndexTemplate;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.xcontent.XContentHelper;
-import org.elasticsearch.core.Nullable;
 import org.elasticsearch.xcontent.XContentParserConfiguration;
 
 import java.io.IOException;
-import java.util.HashMap;
 import java.util.Map;
+import java.util.Objects;
+import java.util.stream.Collectors;
 
 /**
  * This extends BulkRequest with support for providing substitute pipeline definitions, component template definitions, and index template
@@ -73,7 +73,8 @@ import java.util.Map;
  *           }
  *         }
  *       }
- *     },
+ *     }
+ *   },
  *   "index_template_substitutions": {
  *     "my-index-template-1": {
  *       "template": {
@@ -84,6 +85,13 @@ import java.util.Map;
  *         ]
  *       }
  *     }
+ *   },
+ *   "mapping_addition": {
+ *     "dynamic": "strict",
+ *     "properties": {
+ *       "foo": {
+ *         "type": "keyword"
+ *       }
  *   }
  *
  *   The pipelineSubstitutions Map held by this class is intended to be the result of XContentHelper.convertToMap(). The top-level keys
@@ -94,6 +102,7 @@ public class SimulateBulkRequest extends BulkRequest {
     private final Map<String, Map<String, Object>> pipelineSubstitutions;
     private final Map<String, Map<String, Object>> componentTemplateSubstitutions;
     private final Map<String, Map<String, Object>> indexTemplateSubstitutions;
+    private final Map<String, Object> mappingAddition;
 
     /**
      * @param pipelineSubstitutions The pipeline definitions that are to be used in place of any pre-existing pipeline definitions with
@@ -103,16 +112,23 @@ public class SimulateBulkRequest extends BulkRequest {
      *                                       component template definitions with the same name.
      * @param indexTemplateSubstitutions The index template definitions that are to be used in place of any pre-existing
      *                                       index template definitions with the same name.
+     * @param mappingAddition   A mapping that will be merged into the final index's mapping for mapping validation
      */
     public SimulateBulkRequest(
-        @Nullable Map<String, Map<String, Object>> pipelineSubstitutions,
-        @Nullable Map<String, Map<String, Object>> componentTemplateSubstitutions,
-        @Nullable Map<String, Map<String, Object>> indexTemplateSubstitutions
+        Map<String, Map<String, Object>> pipelineSubstitutions,
+        Map<String, Map<String, Object>> componentTemplateSubstitutions,
+        Map<String, Map<String, Object>> indexTemplateSubstitutions,
+        Map<String, Object> mappingAddition
     ) {
         super();
+        Objects.requireNonNull(pipelineSubstitutions);
+        Objects.requireNonNull(componentTemplateSubstitutions);
+        Objects.requireNonNull(indexTemplateSubstitutions);
+        Objects.requireNonNull(mappingAddition);
         this.pipelineSubstitutions = pipelineSubstitutions;
         this.componentTemplateSubstitutions = componentTemplateSubstitutions;
         this.indexTemplateSubstitutions = indexTemplateSubstitutions;
+        this.mappingAddition = mappingAddition;
     }
 
     @SuppressWarnings("unchecked")
@@ -129,6 +145,11 @@ public class SimulateBulkRequest extends BulkRequest {
         } else {
             indexTemplateSubstitutions = Map.of();
         }
+        if (in.getTransportVersion().onOrAfter(TransportVersions.SIMULATE_MAPPING_ADDITION)) {
+            this.mappingAddition = (Map<String, Object>) in.readGenericValue();
+        } else {
+            mappingAddition = Map.of();
+        }
     }
 
     @Override
@@ -141,6 +162,9 @@ public class SimulateBulkRequest extends BulkRequest {
         if (out.getTransportVersion().onOrAfter(TransportVersions.SIMULATE_INDEX_TEMPLATES_SUBSTITUTIONS)) {
             out.writeGenericValue(indexTemplateSubstitutions);
         }
+        if (out.getTransportVersion().onOrAfter(TransportVersions.SIMULATE_MAPPING_ADDITION)) {
+            out.writeGenericValue(mappingAddition);
+        }
     }
 
     public Map<String, Map<String, Object>> getPipelineSubstitutions() {
@@ -153,41 +177,39 @@ public class SimulateBulkRequest extends BulkRequest {
     }
 
     @Override
-    public Map<String, ComponentTemplate> getComponentTemplateSubstitutions() throws IOException {
-        if (componentTemplateSubstitutions == null) {
-            return Map.of();
-        }
-        Map<String, ComponentTemplate> result = new HashMap<>(componentTemplateSubstitutions.size());
-        for (Map.Entry<String, Map<String, Object>> rawEntry : componentTemplateSubstitutions.entrySet()) {
-            result.put(rawEntry.getKey(), convertRawTemplateToComponentTemplate(rawEntry.getValue()));
-        }
-        return result;
+    public Map<String, ComponentTemplate> getComponentTemplateSubstitutions() {
+        return componentTemplateSubstitutions.entrySet()
+            .stream()
+            .collect(Collectors.toMap(Map.Entry::getKey, entry -> convertRawTemplateToComponentTemplate(entry.getValue())));
     }
 
     @Override
-    public Map<String, ComposableIndexTemplate> getIndexTemplateSubstitutions() throws IOException {
-        if (indexTemplateSubstitutions == null) {
-            return Map.of();
-        }
-        Map<String, ComposableIndexTemplate> result = new HashMap<>(indexTemplateSubstitutions.size());
-        for (Map.Entry<String, Map<String, Object>> rawEntry : indexTemplateSubstitutions.entrySet()) {
-            result.put(rawEntry.getKey(), convertRawTemplateToIndexTemplate(rawEntry.getValue()));
-        }
-        return result;
+    public Map<String, ComposableIndexTemplate> getIndexTemplateSubstitutions() {
+        return indexTemplateSubstitutions.entrySet()
+            .stream()
+            .collect(Collectors.toMap(Map.Entry::getKey, entry -> convertRawTemplateToIndexTemplate(entry.getValue())));
+    }
+
+    public Map<String, Object> getMappingAddition() {
+        return mappingAddition;
     }
 
-    private static ComponentTemplate convertRawTemplateToComponentTemplate(Map<String, Object> rawTemplate) throws IOException {
+    private static ComponentTemplate convertRawTemplateToComponentTemplate(Map<String, Object> rawTemplate) {
         ComponentTemplate componentTemplate;
         try (var parser = XContentHelper.mapToXContentParser(XContentParserConfiguration.EMPTY, rawTemplate)) {
             componentTemplate = ComponentTemplate.parse(parser);
+        } catch (IOException e) {
+            throw new RuntimeException(e);
         }
         return componentTemplate;
     }
 
-    private static ComposableIndexTemplate convertRawTemplateToIndexTemplate(Map<String, Object> rawTemplate) throws IOException {
+    private static ComposableIndexTemplate convertRawTemplateToIndexTemplate(Map<String, Object> rawTemplate) {
         ComposableIndexTemplate indexTemplate;
         try (var parser = XContentHelper.mapToXContentParser(XContentParserConfiguration.EMPTY, rawTemplate)) {
             indexTemplate = ComposableIndexTemplate.parse(parser);
+        } catch (IOException e) {
+            throw new RuntimeException(e);
         }
         return indexTemplate;
     }
@@ -197,7 +219,8 @@ public class SimulateBulkRequest extends BulkRequest {
         BulkRequest bulkRequest = new SimulateBulkRequest(
             pipelineSubstitutions,
             componentTemplateSubstitutions,
-            indexTemplateSubstitutions
+            indexTemplateSubstitutions,
+            mappingAddition
         );
         bulkRequest.setRefreshPolicy(getRefreshPolicy());
         bulkRequest.waitForActiveShards(waitForActiveShards());

server/src/main/java/org/elasticsearch/action/bulk/TransportSimulateBulkAction.java

@@ -26,10 +26,13 @@ import org.elasticsearch.cluster.metadata.Metadata;
 import org.elasticsearch.cluster.metadata.MetadataCreateIndexService;
 import org.elasticsearch.cluster.metadata.Template;
 import org.elasticsearch.cluster.service.ClusterService;
+import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.UUIDs;
 import org.elasticsearch.common.compress.CompressedXContent;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.util.concurrent.AtomicArray;
+import org.elasticsearch.common.xcontent.XContentHelper;
+import org.elasticsearch.core.Nullable;
 import org.elasticsearch.features.NodeFeature;
 import org.elasticsearch.index.IndexSettingProvider;
 import org.elasticsearch.index.IndexSettingProviders;
@@ -37,6 +40,7 @@ import org.elasticsearch.index.IndexVersion;
 import org.elasticsearch.index.IndexingPressure;
 import org.elasticsearch.index.VersionType;
 import org.elasticsearch.index.engine.Engine;
+import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.index.mapper.SourceToParse;
 import org.elasticsearch.index.seqno.SequenceNumbers;
 import org.elasticsearch.index.shard.IndexShard;
@@ -50,6 +54,10 @@ import org.elasticsearch.tasks.Task;
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.transport.TransportService;
 import org.elasticsearch.xcontent.NamedXContentRegistry;
+import org.elasticsearch.xcontent.XContentFactory;
+import org.elasticsearch.xcontent.XContentParser;
+import org.elasticsearch.xcontent.XContentParserConfiguration;
+import org.elasticsearch.xcontent.XContentType;
 
 import java.io.IOException;
 import java.util.HashMap;
@@ -75,6 +83,7 @@ public class TransportSimulateBulkAction extends TransportAbstractBulkAction {
         "simulate.component.template.substitutions"
     );
     public static final NodeFeature SIMULATE_INDEX_TEMPLATE_SUBSTITUTIONS = new NodeFeature("simulate.index.template.substitutions");
+    public static final NodeFeature SIMULATE_MAPPING_ADDITION = new NodeFeature("simulate.mapping.addition");
     private final IndicesService indicesService;
     private final NamedXContentRegistry xContentRegistry;
     private final Set<IndexSettingProvider> indexSettingProviders;
@@ -122,11 +131,17 @@ public class TransportSimulateBulkAction extends TransportAbstractBulkAction {
         final AtomicArray<BulkItemResponse> responses = new AtomicArray<>(bulkRequest.requests.size());
         Map<String, ComponentTemplate> componentTemplateSubstitutions = bulkRequest.getComponentTemplateSubstitutions();
         Map<String, ComposableIndexTemplate> indexTemplateSubstitutions = bulkRequest.getIndexTemplateSubstitutions();
+        Map<String, Object> mappingAddition = ((SimulateBulkRequest) bulkRequest).getMappingAddition();
         for (int i = 0; i < bulkRequest.requests.size(); i++) {
             DocWriteRequest<?> docRequest = bulkRequest.requests.get(i);
             assert docRequest instanceof IndexRequest : "TransportSimulateBulkAction should only ever be called with IndexRequests";
             IndexRequest request = (IndexRequest) docRequest;
-            Exception mappingValidationException = validateMappings(componentTemplateSubstitutions, indexTemplateSubstitutions, request);
+            Exception mappingValidationException = validateMappings(
+                componentTemplateSubstitutions,
+                indexTemplateSubstitutions,
+                mappingAddition,
+                request
+            );
             responses.set(
                 i,
                 BulkItemResponse.success(
@@ -159,6 +174,7 @@ public class TransportSimulateBulkAction extends TransportAbstractBulkAction {
     private Exception validateMappings(
         Map<String, ComponentTemplate> componentTemplateSubstitutions,
         Map<String, ComposableIndexTemplate> indexTemplateSubstitutions,
+        Map<String, Object> mappingAddition,
         IndexRequest request
     ) {
         final SourceToParse sourceToParse = new SourceToParse(
@@ -174,7 +190,10 @@ public class TransportSimulateBulkAction extends TransportAbstractBulkAction {
         Exception mappingValidationException = null;
         IndexAbstraction indexAbstraction = state.metadata().getIndicesLookup().get(request.index());
         try {
-            if (indexAbstraction != null && componentTemplateSubstitutions.isEmpty() && indexTemplateSubstitutions.isEmpty()) {
+            if (indexAbstraction != null
+                && componentTemplateSubstitutions.isEmpty()
+                && indexTemplateSubstitutions.isEmpty()
+                && mappingAddition.isEmpty()) {
                 /*
                  * In this case the index exists and we don't have any component template overrides. So we can just use withTempIndexService
                  * to do the mapping validation, using all the existing logic for validation.
@@ -250,36 +269,8 @@ public class TransportSimulateBulkAction extends TransportAbstractBulkAction {
                         indexSettingProviders
                     );
                     CompressedXContent mappings = template.mappings();
-                    if (mappings != null) {
-                        MappingMetadata mappingMetadata = new MappingMetadata(mappings);
-                        Settings dummySettings = Settings.builder()
-                            .put(IndexMetadata.SETTING_VERSION_CREATED, IndexVersion.current())
-                            .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 1)
-                            .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
-                            .put(IndexMetadata.SETTING_INDEX_UUID, UUIDs.randomBase64UUID())
-                            .build();
-                        final IndexMetadata imd = IndexMetadata.builder(request.index())
-                            .settings(dummySettings)
-                            .putMapping(mappingMetadata)
-                            .build();
-                        indicesService.withTempIndexService(imd, indexService -> {
-                            indexService.mapperService().updateMapping(null, imd);
-                            return IndexShard.prepareIndex(
-                                indexService.mapperService(),
-                                sourceToParse,
-                                SequenceNumbers.UNASSIGNED_SEQ_NO,
-                                -1,
-                                -1,
-                                VersionType.INTERNAL,
-                                Engine.Operation.Origin.PRIMARY,
-                                Long.MIN_VALUE,
-                                false,
-                                request.ifSeqNo(),
-                                request.ifPrimaryTerm(),
-                                0
-                            );
-                        });
-                    }
+                    CompressedXContent mergedMappings = mergeMappings(mappings, mappingAddition);
+                    validateUpdatedMappings(mappings, mergedMappings, request, sourceToParse);
                 } else {
                     List<IndexTemplateMetadata> matchingTemplates = findV1Templates(simulatedState.metadata(), request.index(), false);
                     final Map<String, Object> mappingsMap = MetadataCreateIndexService.parseV1Mappings(
@@ -287,40 +278,8 @@ public class TransportSimulateBulkAction extends TransportAbstractBulkAction {
                         matchingTemplates.stream().map(IndexTemplateMetadata::getMappings).collect(toList()),
                         xContentRegistry
                     );
-                    final CompressedXContent combinedMappings;
-                    if (mappingsMap.isEmpty()) {
-                        combinedMappings = null;
-                    } else {
-                        combinedMappings = new CompressedXContent(mappingsMap);
-                    }
-                    Settings dummySettings = Settings.builder()
-                        .put(IndexMetadata.SETTING_VERSION_CREATED, IndexVersion.current())
-                        .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 1)
-                        .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
-                        .put(IndexMetadata.SETTING_INDEX_UUID, UUIDs.randomBase64UUID())
-                        .build();
-                    MappingMetadata mappingMetadata = combinedMappings == null ? null : new MappingMetadata(combinedMappings);
-                    final IndexMetadata imd = IndexMetadata.builder(request.index())
-                        .putMapping(mappingMetadata)
-                        .settings(dummySettings)
-                        .build();
-                    indicesService.withTempIndexService(imd, indexService -> {
-                        indexService.mapperService().updateMapping(null, imd);
-                        return IndexShard.prepareIndex(
-                            indexService.mapperService(),
-                            sourceToParse,
-                            SequenceNumbers.UNASSIGNED_SEQ_NO,
-                            -1,
-                            -1,
-                            VersionType.INTERNAL,
-                            Engine.Operation.Origin.PRIMARY,
-                            Long.MIN_VALUE,
-                            false,
-                            request.ifSeqNo(),
-                            request.ifPrimaryTerm(),
-                            0
-                        );
-                    });
+                    final CompressedXContent combinedMappings = mergeMappings(new CompressedXContent(mappingsMap), mappingAddition);
+                    validateUpdatedMappings(null, combinedMappings, request, sourceToParse);
                 }
             }
         } catch (Exception e) {
@@ -329,6 +288,66 @@ public class TransportSimulateBulkAction extends TransportAbstractBulkAction {
         return mappingValidationException;
     }
 
+    /*
+     * Validates that when updatedMappings are applied
+     */
+    private void validateUpdatedMappings(
+        @Nullable CompressedXContent originalMappings,
+        @Nullable CompressedXContent updatedMappings,
+        IndexRequest request,
+        SourceToParse sourceToParse
+    ) throws IOException {
+        if (updatedMappings == null) {
+            return; // no validation to do
+        }
+        Settings dummySettings = Settings.builder()
+            .put(IndexMetadata.SETTING_VERSION_CREATED, IndexVersion.current())
+            .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 1)
+            .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
+            .put(IndexMetadata.SETTING_INDEX_UUID, UUIDs.randomBase64UUID())
+            .build();
+        IndexMetadata.Builder originalIndexMetadataBuilder = IndexMetadata.builder(request.index()).settings(dummySettings);
+        if (originalMappings != null) {
+            originalIndexMetadataBuilder.putMapping(new MappingMetadata(originalMappings));
+        }
+        final IndexMetadata originalIndexMetadata = originalIndexMetadataBuilder.build();
+        final IndexMetadata updatedIndexMetadata = IndexMetadata.builder(request.index())
+            .settings(dummySettings)
+            .putMapping(new MappingMetadata(updatedMappings))
+            .build();
+        indicesService.withTempIndexService(originalIndexMetadata, indexService -> {
+            indexService.mapperService().merge(updatedIndexMetadata, MapperService.MergeReason.MAPPING_UPDATE);
+            return IndexShard.prepareIndex(
+                indexService.mapperService(),
+                sourceToParse,
+                SequenceNumbers.UNASSIGNED_SEQ_NO,
+                -1,
+                -1,
+                VersionType.INTERNAL,
+                Engine.Operation.Origin.PRIMARY,
+                Long.MIN_VALUE,
+                false,
+                request.ifSeqNo(),
+                request.ifPrimaryTerm(),
+                0
+            );
+        });
+    }
+
+    private static CompressedXContent mergeMappings(@Nullable CompressedXContent originalMapping, Map<String, Object> mappingAddition)
+        throws IOException {
+        Map<String, Object> combinedMappingMap = new HashMap<>();
+        if (originalMapping != null) {
+            combinedMappingMap.putAll(XContentHelper.convertToMap(originalMapping.uncompressed(), true, XContentType.JSON).v2());
+        }
+        XContentHelper.update(combinedMappingMap, mappingAddition, true);
+        if (combinedMappingMap.isEmpty()) {
+            return null;
+        } else {
+            return convertMappingMapToXContent(combinedMappingMap);
+        }
+    }
+
     /*
      * This overrides TransportSimulateBulkAction's getIngestService to allow us to provide an IngestService that handles pipeline
      * substitutions defined in the request.
@@ -344,4 +363,25 @@ public class TransportSimulateBulkAction extends TransportAbstractBulkAction {
         // A simulate bulk request should not change any persistent state in the system, so we never write to the failure store
         return null;
     }
+
+    private static CompressedXContent convertMappingMapToXContent(Map<String, Object> rawAdditionalMapping) throws IOException {
+        CompressedXContent compressedXContent;
+        if (rawAdditionalMapping == null || rawAdditionalMapping.isEmpty()) {
+            compressedXContent = null;
+        } else {
+            try (var parser = XContentHelper.mapToXContentParser(XContentParserConfiguration.EMPTY, rawAdditionalMapping)) {
+                compressedXContent = mappingFromXContent(parser);
+            }
+        }
+        return compressedXContent;
+    }
+
+    private static CompressedXContent mappingFromXContent(XContentParser parser) throws IOException {
+        XContentParser.Token token = parser.nextToken();
+        if (token == XContentParser.Token.START_OBJECT) {
+            return new CompressedXContent(Strings.toString(XContentFactory.jsonBuilder().map(parser.mapOrdered())));
+        } else {
+            throw new IllegalArgumentException("Unexpected token: " + token);
+        }
+    }
 }

server/src/main/java/org/elasticsearch/rest/action/ingest/RestSimulateIngestAction.java

@@ -74,10 +74,21 @@ public class RestSimulateIngestAction extends BaseRestHandler {
         String defaultPipeline = request.param("pipeline");
         Tuple<XContentType, BytesReference> sourceTuple = request.contentOrSourceParam();
         Map<String, Object> sourceMap = XContentHelper.convertToMap(sourceTuple.v2(), false, sourceTuple.v1()).v2();
+        Map<String, Map<String, Object>> pipelineSubstitutions = (Map<String, Map<String, Object>>) sourceMap.remove(
+            "pipeline_substitutions"
+        );
+        Map<String, Map<String, Object>> componentTemplateSubstitutions = (Map<String, Map<String, Object>>) sourceMap.remove(
+            "component_template_substitutions"
+        );
+        Map<String, Map<String, Object>> indexTemplateSubstitutions = (Map<String, Map<String, Object>>) sourceMap.remove(
+            "index_template_substitutions"
+        );
+        Object mappingAddition = sourceMap.remove("mapping_addition");
         SimulateBulkRequest bulkRequest = new SimulateBulkRequest(
-            (Map<String, Map<String, Object>>) sourceMap.remove("pipeline_substitutions"),
-            (Map<String, Map<String, Object>>) sourceMap.remove("component_template_substitutions"),
-            (Map<String, Map<String, Object>>) sourceMap.remove("index_template_substitutions")
+            pipelineSubstitutions == null ? Map.of() : pipelineSubstitutions,
+            componentTemplateSubstitutions == null ? Map.of() : componentTemplateSubstitutions,
+            indexTemplateSubstitutions == null ? Map.of() : indexTemplateSubstitutions,
+            mappingAddition == null ? Map.of() : Map.of("_doc", mappingAddition)
         );
         BytesReference transformedData = convertToBulkRequestXContentBytes(sourceMap);
         bulkRequest.add(

server/src/test/java/org/elasticsearch/action/bulk/SimulateBulkRequestTests.java

@@ -22,32 +22,74 @@ import java.nio.charset.StandardCharsets;
 import java.util.List;
 import java.util.Map;
 
+import static java.util.Map.entry;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.instanceOf;
 
 public class SimulateBulkRequestTests extends ESTestCase {
 
     public void testSerialization() throws Exception {
-        testSerialization(getTestPipelineSubstitutions(), getTestComponentTemplateSubstitutions(), getTestIndexTemplateSubstitutions());
-        testSerialization(getTestPipelineSubstitutions(), null, null);
-        testSerialization(getTestPipelineSubstitutions(), getTestComponentTemplateSubstitutions(), null);
-        testSerialization(getTestPipelineSubstitutions(), null, getTestIndexTemplateSubstitutions());
-        testSerialization(null, getTestComponentTemplateSubstitutions(), getTestIndexTemplateSubstitutions());
-        testSerialization(null, getTestComponentTemplateSubstitutions(), null);
-        testSerialization(null, null, getTestIndexTemplateSubstitutions());
-        testSerialization(null, null, null);
-        testSerialization(Map.of(), Map.of(), Map.of());
+        testSerialization(
+            getMapOrEmpty(getTestPipelineSubstitutions()),
+            getMapOrEmpty(getTestComponentTemplateSubstitutions()),
+            getMapOrEmpty(getTestIndexTemplateSubstitutions()),
+            getMapOrEmpty(getTestMappingAddition())
+        );
+    }
+
+    private <K, V> Map<K, V> getMapOrEmpty(Map<K, V> map) {
+        if (randomBoolean()) {
+            return map;
+        } else {
+            return Map.of();
+        }
+    }
+
+    public void testNullsNotAllowed() {
+        assertThrows(
+            NullPointerException.class,
+            () -> new SimulateBulkRequest(
+                null,
+                getTestPipelineSubstitutions(),
+                getTestComponentTemplateSubstitutions(),
+                getTestMappingAddition()
+            )
+        );
+        assertThrows(
+            NullPointerException.class,
+            () -> new SimulateBulkRequest(
+                getTestPipelineSubstitutions(),
+                null,
+                getTestComponentTemplateSubstitutions(),
+                getTestMappingAddition()
+            )
+        );
+        assertThrows(
+            NullPointerException.class,
+            () -> new SimulateBulkRequest(getTestPipelineSubstitutions(), getTestPipelineSubstitutions(), null, getTestMappingAddition())
+        );
+        assertThrows(
+            NullPointerException.class,
+            () -> new SimulateBulkRequest(
+                getTestPipelineSubstitutions(),
+                getTestPipelineSubstitutions(),
+                getTestComponentTemplateSubstitutions(),
+                null
+            )
+        );
     }
 
     private void testSerialization(
         Map<String, Map<String, Object>> pipelineSubstitutions,
         Map<String, Map<String, Object>> componentTemplateSubstitutions,
-        Map<String, Map<String, Object>> indexTemplateSubstitutions
+        Map<String, Map<String, Object>> indexTemplateSubstitutions,
+        Map<String, Object> mappingAddition
     ) throws IOException {
         SimulateBulkRequest simulateBulkRequest = new SimulateBulkRequest(
             pipelineSubstitutions,
             componentTemplateSubstitutions,
-            indexTemplateSubstitutions
+            indexTemplateSubstitutions,
+            mappingAddition
         );
         /*
          * Note: SimulateBulkRequest does not implement equals or hashCode, so we can't test serialization in the usual way for a
@@ -59,7 +101,7 @@ public class SimulateBulkRequestTests extends ESTestCase {
 
     @SuppressWarnings({ "unchecked", "rawtypes" })
     public void testGetComponentTemplateSubstitutions() throws IOException {
-        SimulateBulkRequest simulateBulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of());
+        SimulateBulkRequest simulateBulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of(), Map.of());
         assertThat(simulateBulkRequest.getComponentTemplateSubstitutions(), equalTo(Map.of()));
         String substituteComponentTemplatesString = """
               {
@@ -93,7 +135,7 @@ public class SimulateBulkRequestTests extends ESTestCase {
             XContentType.JSON
         ).v2();
         Map<String, Map<String, Object>> substituteComponentTemplates = (Map<String, Map<String, Object>>) tempMap;
-        simulateBulkRequest = new SimulateBulkRequest(Map.of(), substituteComponentTemplates, Map.of());
+        simulateBulkRequest = new SimulateBulkRequest(Map.of(), substituteComponentTemplates, Map.of(), Map.of());
         Map<String, ComponentTemplate> componentTemplateSubstitutions = simulateBulkRequest.getComponentTemplateSubstitutions();
         assertThat(componentTemplateSubstitutions.size(), equalTo(2));
         assertThat(
@@ -118,7 +160,7 @@ public class SimulateBulkRequestTests extends ESTestCase {
     }
 
     public void testGetIndexTemplateSubstitutions() throws IOException {
-        SimulateBulkRequest simulateBulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of());
+        SimulateBulkRequest simulateBulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of(), Map.of());
         assertThat(simulateBulkRequest.getIndexTemplateSubstitutions(), equalTo(Map.of()));
         String substituteIndexTemplatesString = """
               {
@@ -154,7 +196,7 @@ public class SimulateBulkRequestTests extends ESTestCase {
             randomBoolean(),
             XContentType.JSON
         ).v2();
-        simulateBulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), substituteIndexTemplates);
+        simulateBulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), substituteIndexTemplates, Map.of());
         Map<String, ComposableIndexTemplate> indexTemplateSubstitutions = simulateBulkRequest.getIndexTemplateSubstitutions();
         assertThat(indexTemplateSubstitutions.size(), equalTo(2));
         assertThat(
@@ -179,7 +221,8 @@ public class SimulateBulkRequestTests extends ESTestCase {
         SimulateBulkRequest simulateBulkRequest = new SimulateBulkRequest(
             getTestPipelineSubstitutions(),
             getTestComponentTemplateSubstitutions(),
-            getTestIndexTemplateSubstitutions()
+            getTestIndexTemplateSubstitutions(),
+            getTestMappingAddition()
         );
         simulateBulkRequest.setRefreshPolicy(randomFrom(WriteRequest.RefreshPolicy.values()));
         simulateBulkRequest.waitForActiveShards(randomIntBetween(1, 10));
@@ -204,7 +247,6 @@ public class SimulateBulkRequestTests extends ESTestCase {
         assertThat(shallowCopy.routing(), equalTo(simulateBulkRequest.routing()));
         assertThat(shallowCopy.requireAlias(), equalTo(simulateBulkRequest.requireAlias()));
         assertThat(shallowCopy.requireDataStream(), equalTo(simulateBulkRequest.requireDataStream()));
-
     }
 
     private static Map<String, Map<String, Object>> getTestPipelineSubstitutions() {
@@ -248,4 +290,22 @@ public class SimulateBulkRequestTests extends ESTestCase {
             Map.of("template", Map.of("index_patterns", List.of("foo*", "bar*"), "mappings", Map.of(), "settings", Map.of()))
         );
     }
+
+    private static Map<String, Object> getTestMappingAddition() {
+        return Map.ofEntries(
+            entry(
+                "_doc",
+                Map.ofEntries(
+                    entry("dynamic", "strict"),
+                    entry(
+                        "properties",
+                        Map.ofEntries(
+                            entry("foo", Map.ofEntries(entry("type", "keyword"))),
+                            entry("bar", Map.ofEntries(entry("type", "boolean")))
+                        )
+                    )
+                )
+            )
+        );
+    }
 }

server/src/test/java/org/elasticsearch/action/bulk/TransportSimulateBulkActionTests.java

@@ -135,7 +135,7 @@ public class TransportSimulateBulkActionTests extends ESTestCase {
 
     public void testIndexData() throws IOException {
         Task task = mock(Task.class); // unused
-        BulkRequest bulkRequest = new SimulateBulkRequest(null, null, null);
+        BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of(), Map.of());
         int bulkItemCount = randomIntBetween(0, 200);
         for (int i = 0; i < bulkItemCount; i++) {
             Map<String, ?> source = Map.of(randomAlphaOfLength(10), randomAlphaOfLength(5));
@@ -218,7 +218,11 @@ public class TransportSimulateBulkActionTests extends ESTestCase {
          * (7) An indexing request to a nonexistent index that matches no templates
          */
         Task task = mock(Task.class); // unused
-        BulkRequest bulkRequest = new SimulateBulkRequest(null, null, null);
+        /*
+         * Here we only add a mapping_addition because if there is no mapping at all TransportSimulateBulkAction skips mapping validation
+         * altogether, and we need it to run for this test to pass.
+         */
+        BulkRequest bulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of(), Map.of("_doc", Map.of("dynamic", "strict")));
         int bulkItemCount = randomIntBetween(0, 200);
         Map<String, IndexMetadata> indicesMap = new HashMap<>();
         Map<String, IndexTemplateMetadata> v1Templates = new HashMap<>();

server/src/test/java/org/elasticsearch/ingest/SimulateIngestServiceTests.java

@@ -65,7 +65,7 @@ public class SimulateIngestServiceTests extends ESTestCase {
         ingestService.innerUpdatePipelines(ingestMetadata);
         {
             // First we make sure that if there are no substitutions that we get our original pipeline back:
-            SimulateBulkRequest simulateBulkRequest = new SimulateBulkRequest(null, null, null);
+            SimulateBulkRequest simulateBulkRequest = new SimulateBulkRequest(Map.of(), Map.of(), Map.of(), Map.of());
             SimulateIngestService simulateIngestService = new SimulateIngestService(ingestService, simulateBulkRequest);
             Pipeline pipeline = simulateIngestService.getPipeline("pipeline1");
             assertThat(pipeline.getProcessors(), contains(transformedMatch(Processor::getType, equalTo("processor1"))));
@@ -83,7 +83,7 @@ public class SimulateIngestServiceTests extends ESTestCase {
             );
             pipelineSubstitutions.put("pipeline2", newHashMap("processors", List.of(newHashMap("processor3", Collections.emptyMap()))));
 
-            SimulateBulkRequest simulateBulkRequest = new SimulateBulkRequest(pipelineSubstitutions, null, null);
+            SimulateBulkRequest simulateBulkRequest = new SimulateBulkRequest(pipelineSubstitutions, Map.of(), Map.of(), Map.of());
             SimulateIngestService simulateIngestService = new SimulateIngestService(ingestService, simulateBulkRequest);
             Pipeline pipeline1 = simulateIngestService.getPipeline("pipeline1");
             assertThat(
@@ -103,7 +103,7 @@ public class SimulateIngestServiceTests extends ESTestCase {
              */
             Map<String, Map<String, Object>> pipelineSubstitutions = new HashMap<>();
             pipelineSubstitutions.put("pipeline2", newHashMap("processors", List.of(newHashMap("processor3", Collections.emptyMap()))));
-            SimulateBulkRequest simulateBulkRequest = new SimulateBulkRequest(pipelineSubstitutions, null, null);
+            SimulateBulkRequest simulateBulkRequest = new SimulateBulkRequest(pipelineSubstitutions, Map.of(), Map.of(), Map.of());
             SimulateIngestService simulateIngestService = new SimulateIngestService(ingestService, simulateBulkRequest);
             Pipeline pipeline1 = simulateIngestService.getPipeline("pipeline1");
             assertThat(pipeline1.getProcessors(), contains(transformedMatch(Processor::getType, equalTo("processor1"))));