Ensure preference is optional and can be provided as string or number.

src/plugins/data/server/search/routes/msearch.ts

@@ -27,7 +27,7 @@ import { getDefaultSearchParams } from '..';
 
 interface MsearchHeaders {
   index: string;
-  preference: number;
+  preference?: number | string;
 }
 
 interface MsearchRequest {
@@ -81,7 +81,7 @@ export function registerMsearchRoute(router: IRouter, deps: SearchRouteDependenc
               header: schema.object(
                 {
                   index: schema.string(),
-                  preference: schema.number(),
+                  preference: schema.maybe(schema.oneOf([schema.number(), schema.string()])),
                 },
                 { unknowns: 'allow' }
               ),

test/api_integration/apis/index.js

@@ -28,6 +28,7 @@ export default function ({ loadTestFile }) {
     loadTestFile(require.resolve('./saved_objects_management'));
     loadTestFile(require.resolve('./saved_objects'));
     loadTestFile(require.resolve('./scripts'));
+    loadTestFile(require.resolve('./search'));
     loadTestFile(require.resolve('./shorten'));
     loadTestFile(require.resolve('./suggestions'));
     loadTestFile(require.resolve('./status'));

test/api_integration/apis/search/index.ts

@@ -0,0 +1,24 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+export default function ({ loadTestFile }) {
+  describe('search', () => {
+    loadTestFile(require.resolve('./search'));
+  });
+}

test/api_integration/apis/search/search.ts

@@ -0,0 +1,90 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import expect from '@kbn/expect';
+import { Response } from 'supertest';
+import { FtrProviderContext } from '../../ftr_provider_context';
+
+export default function ({ getService }: FtrProviderContext) {
+  const supertest = getService('supertest');
+
+  describe('msearch', () => {
+    describe('post', () => {
+      it('should return 200 when correctly formatted searches are provided', async () =>
+        await supertest
+          .post(`/internal/_msearch`)
+          .send({
+            searches: [
+              {
+                header: { index: 'foo' },
+                body: {
+                  query: {
+                    match_all: {},
+                  },
+                },
+              },
+            ],
+          })
+          .expect(200));
+
+      it('should return 400 if you provide malformed content', async () =>
+        await supertest
+          .post(`/internal/_msearch`)
+          .send({
+            foo: false,
+          })
+          .expect(400));
+
+      it('should require you to provide an index for each request', async () =>
+        await supertest
+          .post(`/internal/_msearch`)
+          .send({
+            searches: [
+              { header: { index: 'foo' }, body: {} },
+              { header: {}, body: {} },
+            ],
+          })
+          .expect(400));
+
+      it('should not require optional params', async () =>
+        await supertest
+          .post(`/internal/_msearch`)
+          .send({
+            searches: [{ header: { index: 'foo' }, body: {} }],
+          })
+          .expect(200));
+
+      it('should allow passing preference as a string', async () =>
+        await supertest
+          .post(`/internal/_msearch`)
+          .send({
+            searches: [{ header: { index: 'foo', preference: '_custom' }, body: {} }],
+          })
+          .expect(200));
+
+      it('should allow passing preference as a number', async () =>
+        await supertest
+          .post(`/internal/_msearch`)
+          .send({
+            searches: [{ header: { index: 'foo', preference: 123 }, body: {} }],
+          })
+          .expect(200));
+    });
+  });
+}