This adds max_concurrent_searches to multi-search-template endpoint.

Closes #20912

modules/lang-mustache/src/main/java/org/elasticsearch/script/mustache/MultiSearchTemplateRequest.java

@@ -34,6 +34,7 @@ import static org.elasticsearch.action.ValidateActions.addValidationError;
 
 public class MultiSearchTemplateRequest extends ActionRequest implements CompositeIndicesRequest {
 
+    private int maxConcurrentSearchRequests = 0;
     private List<SearchTemplateRequest> requests = new ArrayList<>();
 
     private IndicesOptions indicesOptions = IndicesOptions.strictExpandOpenAndForbidClosed();
@@ -56,6 +57,26 @@ public class MultiSearchTemplateRequest extends ActionRequest implements Composi
         return this;
     }
 
+
+    /**
+     * Returns the amount of search requests specified in this multi search requests are allowed to be ran concurrently.
+     */
+    public int maxConcurrentSearchRequests() {
+        return maxConcurrentSearchRequests;
+    }
+
+    /**
+     * Sets how many search requests specified in this multi search requests are allowed to be ran concurrently.
+     */
+    public MultiSearchTemplateRequest maxConcurrentSearchRequests(int maxConcurrentSearchRequests) {
+        if (maxConcurrentSearchRequests < 1) {
+            throw new IllegalArgumentException("maxConcurrentSearchRequests must be positive");
+        }
+
+        this.maxConcurrentSearchRequests = maxConcurrentSearchRequests;
+        return this;
+    }
+
     public List<SearchTemplateRequest> requests() {
         return this.requests;
     }
@@ -90,12 +111,14 @@ public class MultiSearchTemplateRequest extends ActionRequest implements Composi
     @Override
     public void readFrom(StreamInput in) throws IOException {
         super.readFrom(in);
+        maxConcurrentSearchRequests = in.readVInt();
         requests = in.readStreamableList(SearchTemplateRequest::new);
     }
 
     @Override
     public void writeTo(StreamOutput out) throws IOException {
         super.writeTo(out);
+        out.writeVInt(maxConcurrentSearchRequests);
         out.writeStreamableList(requests);
     }
 }

modules/lang-mustache/src/main/java/org/elasticsearch/script/mustache/MultiSearchTemplateRequestBuilder.java

@@ -58,4 +58,12 @@ public class MultiSearchTemplateRequestBuilder
         request().indicesOptions(indicesOptions);
         return this;
     }
+
+    /**
+     * Sets how many search requests specified in this multi search requests are allowed to be ran concurrently.
+     */
+    public MultiSearchTemplateRequestBuilder setMaxConcurrentSearchRequests(int maxConcurrentSearchRequests) {
+        request().maxConcurrentSearchRequests(maxConcurrentSearchRequests);
+        return this;
+    }
 }

modules/lang-mustache/src/main/java/org/elasticsearch/script/mustache/RestMultiSearchTemplateAction.java

@@ -70,6 +70,10 @@ public class RestMultiSearchTemplateAction extends BaseRestHandler {
      */
     public static MultiSearchTemplateRequest parseRequest(RestRequest restRequest, boolean allowExplicitIndex) throws IOException {
         MultiSearchTemplateRequest multiRequest = new MultiSearchTemplateRequest();
+        if (restRequest.hasParam("max_concurrent_searches")) {
+            multiRequest.maxConcurrentSearchRequests(restRequest.paramAsInt("max_concurrent_searches", 0));
+        }
+
         RestMultiSearchAction.parseMultiLineRequest(restRequest, multiRequest.indicesOptions(), allowExplicitIndex,
                 (searchRequest, bytes) -> {
                     try {

modules/lang-mustache/src/test/java/org/elasticsearch/script/mustache/MultiSearchTemplateRequestTests.java

@@ -19,6 +19,7 @@
 
 package org.elasticsearch.script.mustache;
 
+import org.elasticsearch.action.search.MultiSearchRequest;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.xcontent.XContentType;
 import org.elasticsearch.rest.RestRequest;
@@ -90,4 +91,12 @@ public class MultiSearchTemplateRequestTests extends ESTestCase {
         assertEquals("{\"query\":{\"match_{{template}}\":{}}}", request.requests().get(0).getScript());
         assertEquals(1, request.requests().get(0).getScriptParams().size());
     }
+
+    public void testMaxConcurrentSearchRequests() {
+        MultiSearchTemplateRequest request = new MultiSearchTemplateRequest();
+        request.maxConcurrentSearchRequests(randomIntBetween(1, Integer.MAX_VALUE));
+        expectThrows(IllegalArgumentException.class, () ->
+                request.maxConcurrentSearchRequests(randomIntBetween(Integer.MIN_VALUE, 0)));
+    }
+
 }

rest-api-spec/src/main/resources/rest-api-spec/api/msearch_template.json

@@ -24,6 +24,10 @@
         "typed_keys": {
           "type" : "boolean",
           "description" : "Specify whether aggregation and suggester names should be prefixed by their respective types in the response"
+        },
+        "max_concurrent_searches" : {
+          "type" : "number",
+          "description" : "Controls the maximum number of concurrent searches the multi search api will execute"
         }
       }
     },