Updated missingTrainedModel message to include: you may need to create it (#104155)

* Updated missingTrainedModel message to include: you may need to create it

* Update docs/changelog/104155.yaml

* Added special exception message for ELSER

* Use ExceptionsHelper.unwrapCause instead of instanceof

Co-authored-by: David Kyle <david.kyle@elastic.co>

* Added missing import

---------

Co-authored-by: David Kyle <david.kyle@elastic.co>

docs/changelog/104155.yaml

@@ -0,0 +1,6 @@
+pr: 104155
+summary: "Updated `missingTrainedModel` message to include: you may need to create\
+  \ it"
+area: Machine Learning
+type: enhancement
+issues: []

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/utils/ExceptionsHelper.java

@@ -51,11 +51,11 @@ public class ExceptionsHelper {
     }
 
     public static ResourceNotFoundException missingTrainedModel(String modelId) {
-        return new ResourceNotFoundException("No known trained model with model_id [{}]", modelId);
+        return new ResourceNotFoundException("No known trained model with model_id [{}], you may need to create it", modelId);
     }
 
     public static ResourceNotFoundException missingTrainedModel(String modelId, Exception cause) {
-        return new ResourceNotFoundException("No known trained model with model_id [{}]", cause, modelId);
+        return new ResourceNotFoundException("No known trained model with model_id [{}], you may need to create it", cause, modelId);
     }
 
     public static ElasticsearchException serverError(String msg) {

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/services/elser/ElserMlNodeService.java

@@ -4,12 +4,13 @@
  * 2.0; you may not use this file except in compliance with the Elastic License
  * 2.0.
  *
- * this file was contributed to by a generative AI
+ * this file has been contributed to by a Generative AI
  */
 
 package org.elasticsearch.xpack.inference.services.elser;
 
 import org.elasticsearch.ElasticsearchStatusException;
+import org.elasticsearch.ResourceNotFoundException;
 import org.elasticsearch.TransportVersion;
 import org.elasticsearch.TransportVersions;
 import org.elasticsearch.action.ActionListener;
@@ -24,10 +25,12 @@ import org.elasticsearch.inference.TaskType;
 import org.elasticsearch.rest.RestStatus;
 import org.elasticsearch.xpack.core.ClientHelper;
 import org.elasticsearch.xpack.core.inference.results.SparseEmbeddingResults;
+import org.elasticsearch.xpack.core.ml.action.CreateTrainedModelAssignmentAction;
 import org.elasticsearch.xpack.core.ml.action.InferTrainedModelDeploymentAction;
 import org.elasticsearch.xpack.core.ml.action.StartTrainedModelDeploymentAction;
 import org.elasticsearch.xpack.core.ml.action.StopTrainedModelDeploymentAction;
 import org.elasticsearch.xpack.core.ml.inference.trainedmodel.TextExpansionConfigUpdate;
+import org.elasticsearch.xpack.core.ml.utils.ExceptionsHelper;
 
 import java.io.IOException;
 import java.util.List;
@@ -73,16 +76,8 @@ public class ElserMlNodeService implements InferenceService {
         Map<String, Object> serviceSettingsMap = removeFromMapOrThrowIfNull(config, ModelConfigurations.SERVICE_SETTINGS);
         var serviceSettingsBuilder = ElserMlNodeServiceSettings.fromMap(serviceSettingsMap);
 
-        // choose a default model version based on the cluster architecture
         if (serviceSettingsBuilder.getModelVariant() == null) {
-            boolean homogenous = modelArchitectures.size() == 1;
-            if (homogenous && modelArchitectures.iterator().next().equals("linux-x86_64")) {
-                // Use the hardware optimized model
-                serviceSettingsBuilder.setModelVariant(ELSER_V2_MODEL_LINUX_X86);
-            } else {
-                // default to the platform-agnostic model
-                serviceSettingsBuilder.setModelVariant(ELSER_V2_MODEL);
-            }
+            serviceSettingsBuilder.setModelVariant(selectDefaultModelVersionBasedOnClusterArchitecture(modelArchitectures));
         }
 
         Map<String, Object> taskSettingsMap;
@@ -102,6 +97,18 @@ public class ElserMlNodeService implements InferenceService {
         return new ElserMlNodeModel(modelId, taskType, NAME, serviceSettingsBuilder.build(), taskSettings);
     }
 
+    private static String selectDefaultModelVersionBasedOnClusterArchitecture(Set<String> modelArchitectures) {
+        // choose a default model version based on the cluster architecture
+        boolean homogenous = modelArchitectures.size() == 1;
+        if (homogenous && modelArchitectures.iterator().next().equals("linux-x86_64")) {
+            // Use the hardware optimized model
+            return ELSER_V2_MODEL_LINUX_X86;
+        } else {
+            // default to the platform-agnostic model
+            return ELSER_V2_MODEL;
+        }
+    }
+
     @Override
     public ElserMlNodeModel parsePersistedConfigWithSecrets(
         String modelId,
@@ -157,11 +164,33 @@ public class ElserMlNodeService implements InferenceService {
         startRequest.setThreadsPerAllocation(serviceSettings.getNumThreads());
         startRequest.setWaitForState(STARTED);
 
-        client.execute(
-            StartTrainedModelDeploymentAction.INSTANCE,
-            startRequest,
-            listener.delegateFailureAndWrap((l, r) -> l.onResponse(Boolean.TRUE))
-        );
+        client.execute(StartTrainedModelDeploymentAction.INSTANCE, startRequest, elserNotDownloadedListener(model, listener));
+    }
+
+    private static ActionListener<CreateTrainedModelAssignmentAction.Response> elserNotDownloadedListener(
+        Model model,
+        ActionListener<Boolean> listener
+    ) {
+        return new ActionListener<>() {
+            @Override
+            public void onResponse(CreateTrainedModelAssignmentAction.Response response) {
+                listener.onResponse(Boolean.TRUE);
+            }
+
+            @Override
+            public void onFailure(Exception e) {
+                if (ExceptionsHelper.unwrapCause(e) instanceof ResourceNotFoundException) {
+                    listener.onFailure(
+                        new ResourceNotFoundException(
+                            "Could not start the ELSER service as the ELSER model for this platform cannot be found."
+                                + " ELSER needs to be downloaded before it can be started"
+                        )
+                    );
+                    return;
+                }
+                listener.onFailure(e);
+            }
+        };
     }
 
     @Override