feat: constant keyword field (opensearch-project#12285)

Constant keyword fields behave similarly to regular keyword fields, except that they are 
defined only in the index mapping, and all documents in the index appear to have the same 
value for the constant keyword field.

---------

Signed-off-by: Mohammad Hasnain <hasnain2808@gmail.com>

CHANGELOG.md

@@ -103,6 +103,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ## [Unreleased 2.x]
 ### Added
+- Constant Keyword Field ([#12285](https://github.com/opensearch-project/OpenSearch/pull/12285))
 - Convert ingest processor supports ip type ([#12818](https://github.com/opensearch-project/OpenSearch/pull/12818))
 - Add a counter to node stat api to track shard going from idle to non-idle ([#12768](https://github.com/opensearch-project/OpenSearch/pull/12768))
 - Allow setting KEYSTORE_PASSWORD through env variable ([#12865](https://github.com/opensearch-project/OpenSearch/pull/12865))

server/src/main/java/org/opensearch/index/mapper/ConstantKeywordFieldMapper.java

@@ -0,0 +1,191 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.index.mapper;
+
+import org.apache.lucene.search.MatchAllDocsQuery;
+import org.apache.lucene.search.Query;
+import org.opensearch.OpenSearchParseException;
+import org.opensearch.common.annotation.PublicApi;
+import org.opensearch.common.regex.Regex;
+import org.opensearch.index.fielddata.IndexFieldData;
+import org.opensearch.index.fielddata.plain.ConstantIndexFieldData;
+import org.opensearch.index.query.QueryShardContext;
+import org.opensearch.search.aggregations.support.CoreValuesSourceType;
+import org.opensearch.search.lookup.SearchLookup;
+
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Supplier;
+
+/**
+ * Index specific field mapper
+ *
+ * @opensearch.api
+ */
+@PublicApi(since = "2.14.0")
+public class ConstantKeywordFieldMapper extends ParametrizedFieldMapper {
+
+    public static final String CONTENT_TYPE = "constant_keyword";
+
+    private static final String valuePropertyName = "value";
+
+    /**
+     * A {@link Mapper.TypeParser} for the constant keyword field.
+     *
+     * @opensearch.internal
+     */
+    public static class TypeParser implements Mapper.TypeParser {
+        @Override
+        public Mapper.Builder parse(String name, Map<String, Object> node, ParserContext parserContext) throws MapperParsingException {
+            if (!node.containsKey(valuePropertyName)) {
+                throw new OpenSearchParseException("Field [" + name + "] is missing required parameter [value]");
+            }
+            Object value = node.remove(valuePropertyName);
+            if (!(value instanceof String)) {
+                throw new OpenSearchParseException("Field [" + name + "] is expected to be a string value");
+            }
+            return new Builder(name, (String) value);
+        }
+    }
+
+    private static ConstantKeywordFieldMapper toType(FieldMapper in) {
+        return (ConstantKeywordFieldMapper) in;
+    }
+
+    /**
+     * Builder for the binary field mapper
+     *
+     * @opensearch.internal
+     */
+    public static class Builder extends ParametrizedFieldMapper.Builder {
+
+        private final Parameter<String> value;
+
+        public Builder(String name, String value) {
+            super(name);
+            this.value = Parameter.stringParam(valuePropertyName, false, m -> toType(m).value, value);
+        }
+
+        @Override
+        public List<Parameter<?>> getParameters() {
+            return Arrays.asList(value);
+        }
+
+        @Override
+        public ConstantKeywordFieldMapper build(BuilderContext context) {
+            return new ConstantKeywordFieldMapper(
+                name,
+                new ConstantKeywordFieldMapper.ConstantKeywordFieldType(buildFullName(context), value.getValue()),
+                multiFieldsBuilder.build(this, context),
+                copyTo.build(),
+                this
+            );
+        }
+    }
+
+    /**
+     * Field type for Index field mapper
+     *
+     * @opensearch.internal
+     */
+    @PublicApi(since = "2.14.0")
+    protected static final class ConstantKeywordFieldType extends ConstantFieldType {
+
+        protected final String value;
+
+        public ConstantKeywordFieldType(String name, String value) {
+            super(name, Collections.emptyMap());
+            this.value = value;
+        }
+
+        @Override
+        public String typeName() {
+            return CONTENT_TYPE;
+        }
+
+        @Override
+        protected boolean matches(String pattern, boolean caseInsensitive, QueryShardContext context) {
+            return Regex.simpleMatch(pattern, value, caseInsensitive);
+        }
+
+        @Override
+        public Query existsQuery(QueryShardContext context) {
+            return new MatchAllDocsQuery();
+        }
+
+        @Override
+        public IndexFieldData.Builder fielddataBuilder(String fullyQualifiedIndexName, Supplier<SearchLookup> searchLookup) {
+            return new ConstantIndexFieldData.Builder(fullyQualifiedIndexName, name(), CoreValuesSourceType.BYTES);
+        }
+
+        @Override
+        public ValueFetcher valueFetcher(QueryShardContext context, SearchLookup searchLookup, String format) {
+            if (format != null) {
+                throw new IllegalArgumentException("Field [" + name() + "] of type [" + typeName() + "] doesn't " + "support formats.");
+            }
+
+            return new SourceValueFetcher(name(), context) {
+                @Override
+                protected Object parseSourceValue(Object value) {
+                    String keywordValue = value.toString();
+                    return Collections.singletonList(keywordValue);
+                }
+            };
+        }
+    }
+
+    private final String value;
+
+    protected ConstantKeywordFieldMapper(
+        String simpleName,
+        MappedFieldType mappedFieldType,
+        MultiFields multiFields,
+        CopyTo copyTo,
+        ConstantKeywordFieldMapper.Builder builder
+    ) {
+        super(simpleName, mappedFieldType, multiFields, copyTo);
+        this.value = builder.value.getValue();
+    }
+
+    public ParametrizedFieldMapper.Builder getMergeBuilder() {
+        return new ConstantKeywordFieldMapper.Builder(simpleName(), this.value).init(this);
+    }
+
+    @Override
+    protected void parseCreateField(ParseContext context) throws IOException {
+
+        final String value;
+        if (context.externalValueSet()) {
+            value = context.externalValue().toString();
+        } else {
+            value = context.parser().textOrNull();
+        }
+        if (value == null) {
+            throw new IllegalArgumentException("constant keyword field [" + name() + "] must have a value");
+        }
+
+        if (!value.equals(fieldType().value)) {
+            throw new IllegalArgumentException("constant keyword field [" + name() + "] must have a value of [" + this.value + "]");
+        }
+
+    }
+
+    @Override
+    public ConstantKeywordFieldMapper.ConstantKeywordFieldType fieldType() {
+        return (ConstantKeywordFieldMapper.ConstantKeywordFieldType) super.fieldType();
+    }
+
+    @Override
+    protected String contentType() {
+        return CONTENT_TYPE;
+    }
+}

server/src/main/java/org/opensearch/indices/IndicesModule.java

@@ -46,6 +46,7 @@
 import org.opensearch.index.mapper.BinaryFieldMapper;
 import org.opensearch.index.mapper.BooleanFieldMapper;
 import org.opensearch.index.mapper.CompletionFieldMapper;
+import org.opensearch.index.mapper.ConstantKeywordFieldMapper;
 import org.opensearch.index.mapper.DataStreamFieldMapper;
 import org.opensearch.index.mapper.DateFieldMapper;
 import org.opensearch.index.mapper.DocCountFieldMapper;
@@ -168,6 +169,7 @@ public static Map<String, Mapper.TypeParser> getMappers(List<MapperPlugin> mappe
         mappers.put(FieldAliasMapper.CONTENT_TYPE, new FieldAliasMapper.TypeParser());
         mappers.put(GeoPointFieldMapper.CONTENT_TYPE, new GeoPointFieldMapper.TypeParser());
         mappers.put(FlatObjectFieldMapper.CONTENT_TYPE, FlatObjectFieldMapper.PARSER);
+        mappers.put(ConstantKeywordFieldMapper.CONTENT_TYPE, new ConstantKeywordFieldMapper.TypeParser());
 
         for (MapperPlugin mapperPlugin : mapperPlugins) {
             for (Map.Entry<String, Mapper.TypeParser> entry : mapperPlugin.getMappers().entrySet()) {

server/src/test/java/org/opensearch/index/mapper/ConstantKeywordFieldMapperTests.java

@@ -0,0 +1,114 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.index.mapper;
+
+import org.apache.lucene.index.IndexableField;
+import org.opensearch.OpenSearchParseException;
+import org.opensearch.common.CheckedConsumer;
+import org.opensearch.common.compress.CompressedXContent;
+import org.opensearch.common.xcontent.XContentFactory;
+import org.opensearch.common.xcontent.json.JsonXContent;
+import org.opensearch.core.common.bytes.BytesReference;
+import org.opensearch.core.xcontent.MediaTypeRegistry;
+import org.opensearch.core.xcontent.XContentBuilder;
+import org.opensearch.index.IndexService;
+import org.opensearch.plugins.Plugin;
+import org.opensearch.test.InternalSettingsPlugin;
+import org.opensearch.test.OpenSearchSingleNodeTestCase;
+import org.junit.Before;
+
+import java.io.IOException;
+import java.util.Collection;
+
+import static org.hamcrest.Matchers.containsString;
+
+public class ConstantKeywordFieldMapperTests extends OpenSearchSingleNodeTestCase {
+
+    private IndexService indexService;
+    private DocumentMapperParser parser;
+
+    @Override
+    protected Collection<Class<? extends Plugin>> getPlugins() {
+        return pluginList(InternalSettingsPlugin.class);
+    }
+
+    @Before
+    public void setup() {
+        indexService = createIndex("test");
+        parser = indexService.mapperService().documentMapperParser();
+    }
+
+    public void testDefaultDisabledIndexMapper() throws Exception {
+
+        XContentBuilder mapping = XContentFactory.jsonBuilder()
+            .startObject()
+            .startObject("type")
+            .startObject("properties")
+            .startObject("field")
+            .field("type", "constant_keyword")
+            .field("value", "default_value")
+            .endObject()
+            .startObject("field2")
+            .field("type", "keyword")
+            .endObject();
+        mapping = mapping.endObject().endObject().endObject();
+        DocumentMapper mapper = parser.parse("type", new CompressedXContent(mapping.toString()));
+
+        MapperParsingException e = expectThrows(MapperParsingException.class, () -> mapper.parse(source(b -> {
+            b.field("field", "sdf");
+            b.field("field2", "szdfvsddf");
+        })));
+        assertThat(
+            e.getMessage(),
+            containsString(
+                "failed to parse field [field] of type [constant_keyword] in document with id '1'. Preview of field's value: 'sdf'"
+            )
+        );
+
+        final ParsedDocument doc = mapper.parse(source(b -> {
+            b.field("field", "default_value");
+            b.field("field2", "field_2_value");
+        }));
+
+        final IndexableField field = doc.rootDoc().getField("field");
+
+        // constantKeywordField should not be stored
+        assertNull(field);
+    }
+
+    public void testMissingDefaultIndexMapper() throws Exception {
+
+        final XContentBuilder mapping = XContentFactory.jsonBuilder()
+            .startObject()
+            .startObject("type")
+            .startObject("properties")
+            .startObject("field")
+            .field("type", "constant_keyword")
+            .endObject()
+            .startObject("field2")
+            .field("type", "keyword")
+            .endObject()
+            .endObject()
+            .endObject()
+            .endObject();
+
+        OpenSearchParseException e = expectThrows(
+            OpenSearchParseException.class,
+            () -> parser.parse("type", new CompressedXContent(mapping.toString()))
+        );
+        assertThat(e.getMessage(), containsString("Field [field] is missing required parameter [value]"));
+    }
+
+    private final SourceToParse source(CheckedConsumer<XContentBuilder, IOException> build) throws IOException {
+        XContentBuilder builder = JsonXContent.contentBuilder().startObject();
+        build.accept(builder);
+        builder.endObject();
+        return new SourceToParse("test", "1", BytesReference.bytes(builder), MediaTypeRegistry.JSON);
+    }
+}