Deprecate _source.mode in mappings

[dnhatn]

This change deprecates _source.mode in mappings, replacing it with the index.mapping.source.mode index setting.

[elasticsearchmachine]

Hi @dnhatn, I've created a changelog YAML for you. Note that since this PR is labelled >deprecation, you need to update the changelog YAML to fill out the extended information sections.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[martijnvg]

Thanks Nhat, I left questions are the deprecation logic in MetadataCreateIndexService.

I wonder whether the following is perhaps a better mechanism to emit depreciation warning in case _source.mode is used:

Subject: [PATCH] Adjust SyntheticSourceLicenseService 

Allow gold and platinum license to use synthetic source for a limited time.
If the start time of a license if before a cut off date, then gold and platinum licenses will not fallback to stored source if synthetic source is used.
---
Index: server/src/main/java/org/elasticsearch/index/mapper/MappingParser.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/server/src/main/java/org/elasticsearch/index/mapper/MappingParser.java b/server/src/main/java/org/elasticsearch/index/mapper/MappingParser.java
--- a/server/src/main/java/org/elasticsearch/index/mapper/MappingParser.java	(revision bd091d3d96b33adb4121f980dc4f9f7a2a87b043)
+++ b/server/src/main/java/org/elasticsearch/index/mapper/MappingParser.java	(date 1731573325979)
@@ -10,6 +10,8 @@
 package org.elasticsearch.index.mapper;
 
 import org.elasticsearch.common.compress.CompressedXContent;
+import org.elasticsearch.common.logging.DeprecationCategory;
+import org.elasticsearch.common.logging.DeprecationLogger;
 import org.elasticsearch.common.xcontent.XContentHelper;
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.index.mapper.MapperService.MergeReason;
@@ -27,6 +29,9 @@
  * Parser for {@link Mapping} provided in {@link CompressedXContent} format
  */
 public final class MappingParser {
+
+    private static final DeprecationLogger DEPRECATION_LOGGER = DeprecationLogger.getLogger(MappingParser.class);
+
     private final Supplier<MappingParserContext> mappingParserContextSupplier;
     private final Supplier<Map<Class<? extends MetadataFieldMapper>, MetadataFieldMapper>> metadataMappersSupplier;
     private final Map<String, MetadataFieldMapper.TypeParser> metadataMapperParsers;
@@ -144,6 +149,14 @@
                 }
                 @SuppressWarnings("unchecked")
                 Map<String, Object> fieldNodeMap = (Map<String, Object>) fieldNode;
+
+                // iirc after parsing fieldNodeMap, keys are removed
+                if (reason == MergeReason.MAPPING_UPDATE && SourceFieldMapper.CONTENT_TYPE.equals(fieldName)) {
+                    if (fieldNodeMap.containsKey("mode")) {
+                        DEPRECATION_LOGGER.critical(DeprecationCategory.MAPPINGS, "mapping_source_mode", SourceFieldMapper.DEPRECATION_WARNING);
+                    }
+                }
+
                 MetadataFieldMapper metadataFieldMapper = typeParser.parse(fieldName, fieldNodeMap, mappingParserContext).build();
                 metadataMappers.put(metadataFieldMapper.getClass(), metadataFieldMapper);
                 assert fieldNodeMap.isEmpty();

I think this also returns warning when mapping gets updated, maybe that isn't too bad? But this should only return a warning when _source.mode attribute is really used.

[martijnvg]

I think we need to check whether index.mapping.source.mode index setting has not be defined here? Other we only check whether this setting has been set to synthetic?

[martijnvg]

If we check whether index.mapping.source.mode has not be defined, then I think mapperService.documentMapper().sourceMapper().isSynthetic() can return true if index.mode=logsdb|time_series and no _source.mode has been defined?