[Inference API] Add "rerank" task type to "elastic" provider

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

@@ -273,6 +273,7 @@ static TransportVersion def(int id) {
     public static final TransportVersion INFERENCE_CUSTOM_SERVICE_ADDED = def(9_084_0_00);
     public static final TransportVersion ESQL_LIMIT_ROW_SIZE = def(9_085_0_00);
     public static final TransportVersion ESQL_REGEX_MATCH_WITH_CASE_INSENSITIVITY = def(9_086_0_00);
+    public static final TransportVersion ML_INFERENCE_ELASTIC_RERANK = def(9_087_0_00);
 
     /*
      * STOP! READ THIS FIRST! No, really,

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/InferenceNamedWriteablesProvider.java

@@ -70,6 +70,7 @@
 import org.elasticsearch.xpack.inference.services.custom.response.TextEmbeddingResponseParser;
 import org.elasticsearch.xpack.inference.services.deepseek.DeepSeekChatCompletionModel;
 import org.elasticsearch.xpack.inference.services.elastic.completion.ElasticInferenceServiceCompletionServiceSettings;
+import org.elasticsearch.xpack.inference.services.elastic.rerank.ElasticInferenceServiceRerankServiceSettings;
 import org.elasticsearch.xpack.inference.services.elastic.sparseembeddings.ElasticInferenceServiceSparseEmbeddingsServiceSettings;
 import org.elasticsearch.xpack.inference.services.elasticsearch.CustomElandInternalServiceSettings;
 import org.elasticsearch.xpack.inference.services.elasticsearch.CustomElandInternalTextEmbeddingServiceSettings;
@@ -165,7 +166,7 @@ public static List<NamedWriteableRegistry.Entry> getNamedWriteables() {
         addAnthropicNamedWritables(namedWriteables);
         addAmazonBedrockNamedWriteables(namedWriteables);
         addAwsNamedWriteables(namedWriteables);
-        addEisNamedWriteables(namedWriteables);
+        addElasticNamedWriteables(namedWriteables);
         addAlibabaCloudSearchNamedWriteables(namedWriteables);
         addJinaAINamedWriteables(namedWriteables);
         addVoyageAINamedWriteables(namedWriteables);
@@ -734,20 +735,32 @@ private static void addVoyageAINamedWriteables(List<NamedWriteableRegistry.Entry
         );
     }
 
-    private static void addEisNamedWriteables(List<NamedWriteableRegistry.Entry> namedWriteables) {
+    private static void addElasticNamedWriteables(List<NamedWriteableRegistry.Entry> namedWriteables) {
+        // Sparse Text Embeddings
         namedWriteables.add(
             new NamedWriteableRegistry.Entry(
                 ServiceSettings.class,
                 ElasticInferenceServiceSparseEmbeddingsServiceSettings.NAME,
                 ElasticInferenceServiceSparseEmbeddingsServiceSettings::new
             )
         );
+
+        // Completion
         namedWriteables.add(
             new NamedWriteableRegistry.Entry(
                 ServiceSettings.class,
                 ElasticInferenceServiceCompletionServiceSettings.NAME,
                 ElasticInferenceServiceCompletionServiceSettings::new
             )
         );
+
+        // Rerank
+        namedWriteables.add(
+            new NamedWriteableRegistry.Entry(
+                ServiceSettings.class,
+                ElasticInferenceServiceRerankServiceSettings.NAME,
+                ElasticInferenceServiceRerankServiceSettings::new
+            )
+        );
     }
 }

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/external/request/elastic/rerank/ElasticInferenceServiceRerankRequest.java

@@ -0,0 +1,94 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.inference.external.request.elastic.rerank;
+
+import org.apache.http.HttpHeaders;
+import org.apache.http.client.methods.HttpPost;
+import org.apache.http.client.methods.HttpRequestBase;
+import org.apache.http.entity.ByteArrayEntity;
+import org.apache.http.message.BasicHeader;
+import org.elasticsearch.common.Strings;
+import org.elasticsearch.xcontent.XContentType;
+import org.elasticsearch.xpack.inference.external.request.Request;
+import org.elasticsearch.xpack.inference.services.elastic.request.ElasticInferenceServiceRequest;
+import org.elasticsearch.xpack.inference.services.elastic.request.ElasticInferenceServiceRequestMetadata;
+import org.elasticsearch.xpack.inference.services.elastic.rerank.ElasticInferenceServiceRerankModel;
+import org.elasticsearch.xpack.inference.telemetry.TraceContext;
+import org.elasticsearch.xpack.inference.telemetry.TraceContextHandler;
+
+import java.net.URI;
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+import java.util.Objects;
+
+public class ElasticInferenceServiceRerankRequest extends ElasticInferenceServiceRequest {
+
+    private final String query;
+    private final List<String> documents;
+    private final Integer topN;
+    private final TraceContextHandler traceContextHandler;
+    private final ElasticInferenceServiceRerankModel model;
+
+    public ElasticInferenceServiceRerankRequest(
+        String query,
+        List<String> documents,
+        Integer topN,
+        ElasticInferenceServiceRerankModel model,
+        TraceContext traceContext,
+        ElasticInferenceServiceRequestMetadata metadata
+    ) {
+        super(metadata);
+        this.query = query;
+        this.documents = documents;
+        this.topN = topN;
+        this.model = Objects.requireNonNull(model);
+        this.traceContextHandler = new TraceContextHandler(traceContext);
+    }
+
+    @Override
+    public HttpRequestBase createHttpRequestBase() {
+        var httpPost = new HttpPost(getURI());
+        var requestEntity = Strings.toString(
+            new ElasticInferenceServiceRerankRequestEntity(query, documents, model.getServiceSettings().modelId(), topN)
+        );
+
+        ByteArrayEntity byteEntity = new ByteArrayEntity(requestEntity.getBytes(StandardCharsets.UTF_8));
+        httpPost.setEntity(byteEntity);
+
+        traceContextHandler.propagateTraceContext(httpPost);
+        httpPost.setHeader(new BasicHeader(HttpHeaders.CONTENT_TYPE, XContentType.JSON.mediaType()));
+
+        return httpPost;
+    }
+
+    public TraceContext getTraceContext() {
+        return traceContextHandler.traceContext();
+    }
+
+    @Override
+    public String getInferenceEntityId() {
+        return model.getInferenceEntityId();
+    }
+
+    @Override
+    public URI getURI() {
+        return model.uri();
+    }
+
+    @Override
+    public Request truncate() {
+        // no truncation
+        return this;
+    }
+
+    @Override
+    public boolean[] getTruncationInfo() {
+        // no truncation
+        return null;
+    }
+}

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/external/request/elastic/rerank/ElasticInferenceServiceRerankRequestEntity.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.inference.external.request.elastic.rerank;
+
+import org.elasticsearch.core.Nullable;
+import org.elasticsearch.xcontent.ToXContentObject;
+import org.elasticsearch.xcontent.XContentBuilder;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.Objects;
+
+public record ElasticInferenceServiceRerankRequestEntity(
+    String query,
+    List<String> documents,
+    String modelId,
+    @Nullable Integer topNDocumentsOnly
+) implements ToXContentObject {
+
+    private static final String QUERY_FIELD = "query";
+    private static final String MODEL_FIELD = "model";
+    private static final String TOP_N_DOCUMENTS_ONLY_FIELD = "top_n";
+    private static final String DOCUMENTS_FIELD = "documents";
+
+    public ElasticInferenceServiceRerankRequestEntity {
+        Objects.requireNonNull(query);
+        Objects.requireNonNull(documents);
+        Objects.requireNonNull(modelId);
+    }
+
+    @Override
+    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+        builder.startObject();
+
+        builder.field(QUERY_FIELD, query);
+
+        builder.field(MODEL_FIELD, modelId);
+
+        if (Objects.nonNull(topNDocumentsOnly)) {
+            builder.field(TOP_N_DOCUMENTS_ONLY_FIELD, topNDocumentsOnly);
+        }
+
+        builder.startArray(DOCUMENTS_FIELD);
+        for (String document : documents) {
+            builder.value(document);
+        }
+
+        builder.endArray();
+
+        builder.endObject();
+
+        return builder;
+    }
+}

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/external/response/elastic/ElasticInferenceServiceRerankResponseEntity.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.inference.external.response.elastic;
+
+import org.elasticsearch.common.xcontent.LoggingDeprecationHandler;
+import org.elasticsearch.inference.InferenceServiceResults;
+import org.elasticsearch.xcontent.ConstructingObjectParser;
+import org.elasticsearch.xcontent.ParseField;
+import org.elasticsearch.xcontent.XContentFactory;
+import org.elasticsearch.xcontent.XContentParser;
+import org.elasticsearch.xcontent.XContentParserConfiguration;
+import org.elasticsearch.xcontent.XContentType;
+import org.elasticsearch.xpack.core.inference.results.RankedDocsResults;
+import org.elasticsearch.xpack.inference.external.http.HttpResult;
+
+import java.io.IOException;
+import java.util.List;
+
+import static org.elasticsearch.xcontent.ConstructingObjectParser.constructorArg;
+
+public class ElasticInferenceServiceRerankResponseEntity {
+
+    record RerankResult(List<RerankResultEntry> entries) {
+
+        @SuppressWarnings("unchecked")
+        public static final ConstructingObjectParser<RerankResult, Void> PARSER = new ConstructingObjectParser<>(
+            RerankResult.class.getSimpleName(),
+            true,
+            args -> new RerankResult((List<RerankResultEntry>) args[0])
+        );
+
+        static {
+            PARSER.declareObjectArray(constructorArg(), RerankResultEntry.PARSER::apply, new ParseField("results"));
+        }
+
+        record RerankResultEntry(Integer index, Float relevanceScore) {
+
+            public static final ConstructingObjectParser<RerankResultEntry, Void> PARSER = new ConstructingObjectParser<>(
+                RerankResultEntry.class.getSimpleName(),
+                args -> new RerankResultEntry((Integer) args[0], (Float) args[1])
+            );
+
+            static {
+                PARSER.declareInt(constructorArg(), new ParseField("index"));
+                PARSER.declareFloat(constructorArg(), new ParseField("relevance_score"));
+            }
+
+            public RankedDocsResults.RankedDoc toRankedDoc() {
+                return new RankedDocsResults.RankedDoc(index, relevanceScore, null);
+            }
+        }
+    }
+
+    public static InferenceServiceResults fromResponse(HttpResult response) throws IOException {
+        var parserConfig = XContentParserConfiguration.EMPTY.withDeprecationHandler(LoggingDeprecationHandler.INSTANCE);
+
+        try (XContentParser jsonParser = XContentFactory.xContent(XContentType.JSON).createParser(parserConfig, response.body())) {
+            var rerankResult = RerankResult.PARSER.apply(jsonParser, null);
+
+            return new RankedDocsResults(rerankResult.entries.stream().map(RerankResult.RerankResultEntry::toRankedDoc).toList());
+        }
+    }
+
+    private ElasticInferenceServiceRerankResponseEntity() {}
+}

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/services/elastic/ElasticInferenceService.java

@@ -51,6 +51,7 @@
 import org.elasticsearch.xpack.inference.services.elastic.authorization.ElasticInferenceServiceAuthorizationRequestHandler;
 import org.elasticsearch.xpack.inference.services.elastic.completion.ElasticInferenceServiceCompletionModel;
 import org.elasticsearch.xpack.inference.services.elastic.completion.ElasticInferenceServiceCompletionServiceSettings;
+import org.elasticsearch.xpack.inference.services.elastic.rerank.ElasticInferenceServiceRerankModel;
 import org.elasticsearch.xpack.inference.services.elastic.sparseembeddings.ElasticInferenceServiceSparseEmbeddingsModel;
 import org.elasticsearch.xpack.inference.services.elastic.sparseembeddings.ElasticInferenceServiceSparseEmbeddingsServiceSettings;
 import org.elasticsearch.xpack.inference.services.settings.RateLimitSettings;
@@ -79,7 +80,11 @@ public class ElasticInferenceService extends SenderService {
     public static final String NAME = "elastic";
     public static final String ELASTIC_INFERENCE_SERVICE_IDENTIFIER = "Elastic Inference Service";
 
-    private static final EnumSet<TaskType> IMPLEMENTED_TASK_TYPES = EnumSet.of(TaskType.SPARSE_EMBEDDING, TaskType.CHAT_COMPLETION);
+    private static final EnumSet<TaskType> IMPLEMENTED_TASK_TYPES = EnumSet.of(
+        TaskType.SPARSE_EMBEDDING,
+        TaskType.CHAT_COMPLETION,
+        TaskType.RERANK
+    );
     private static final String SERVICE_NAME = "Elastic";
 
     // rainbow-sprinkles
@@ -93,7 +98,7 @@ public class ElasticInferenceService extends SenderService {
     /**
      * The task types that the {@link InferenceAction.Request} can accept.
      */
-    private static final EnumSet<TaskType> SUPPORTED_INFERENCE_ACTION_TASK_TYPES = EnumSet.of(TaskType.SPARSE_EMBEDDING);
+    private static final EnumSet<TaskType> SUPPORTED_INFERENCE_ACTION_TASK_TYPES = EnumSet.of(TaskType.SPARSE_EMBEDDING, TaskType.RERANK);
 
     public static String defaultEndpointId(String modelId) {
         return Strings.format(".%s-elastic", modelId);
@@ -163,6 +168,18 @@ public void onNodeStarted() {
         authorizationHandler.init();
     }
 
+    @Override
+    protected void validateRerankParameters(Boolean returnDocuments, Integer topN, ValidationException validationException) {
+        if (returnDocuments != null) {
+            validationException.addValidationError(
+                org.elasticsearch.core.Strings.format(
+                    "Invalid return_documents [%s]. The return_documents option is not supported by this service",
+                    returnDocuments
+                )
+            );
+        }
+    }
+
     /**
      * Only use this in tests.
      *
@@ -335,7 +352,7 @@ private static ElasticInferenceServiceModel createModel(
         Map<String, Object> serviceSettings,
         Map<String, Object> taskSettings,
         @Nullable Map<String, Object> secretSettings,
-        ElasticInferenceServiceComponents eisServiceComponents,
+        ElasticInferenceServiceComponents elasticInferenceServiceComponents,
         String failureMessage,
         ConfigurationParseContext context
     ) {
@@ -347,7 +364,7 @@ private static ElasticInferenceServiceModel createModel(
                 serviceSettings,
                 taskSettings,
                 secretSettings,
-                eisServiceComponents,
+                elasticInferenceServiceComponents,
                 context
             );
             case CHAT_COMPLETION -> new ElasticInferenceServiceCompletionModel(
@@ -357,7 +374,17 @@ private static ElasticInferenceServiceModel createModel(
                 serviceSettings,
                 taskSettings,
                 secretSettings,
-                eisServiceComponents,
+                elasticInferenceServiceComponents,
+                context
+            );
+            case RERANK -> new ElasticInferenceServiceRerankModel(
+                inferenceEntityId,
+                taskType,
+                NAME,
+                serviceSettings,
+                taskSettings,
+                secretSettings,
+                elasticInferenceServiceComponents,
                 context
             );
             default -> throw new ElasticsearchStatusException(failureMessage, RestStatus.BAD_REQUEST);
@@ -462,9 +489,8 @@ private LazyInitializable<InferenceServiceConfiguration, RuntimeException> initC
 
                 configurationMap.put(
                     MODEL_ID,
-                    new SettingsConfiguration.Builder(EnumSet.of(TaskType.SPARSE_EMBEDDING, TaskType.CHAT_COMPLETION)).setDescription(
-                        "The name of the model to use for the inference task."
-                    )
+                    new SettingsConfiguration.Builder(EnumSet.of(TaskType.SPARSE_EMBEDDING, TaskType.CHAT_COMPLETION, TaskType.RERANK))
+                        .setDescription("The name of the model to use for the inference task.")
                         .setLabel("Model ID")
                         .setRequired(true)
                         .setSensitive(false)
@@ -487,7 +513,9 @@ private LazyInitializable<InferenceServiceConfiguration, RuntimeException> initC
                 );
 
                 configurationMap.putAll(
-                    RateLimitSettings.toSettingsConfiguration(EnumSet.of(TaskType.SPARSE_EMBEDDING, TaskType.CHAT_COMPLETION))
+                    RateLimitSettings.toSettingsConfiguration(
+                        EnumSet.of(TaskType.SPARSE_EMBEDDING, TaskType.CHAT_COMPLETION, TaskType.RERANK)
+                    )
                 );
 
                 return new InferenceServiceConfiguration.Builder().setService(NAME)