[Backport 2.x] Remove minimum constraint on OpenSearch hosts to allow empty host and refactor status API to not include OpenSearch status when host is empty

src/core/server/opensearch/client/cluster_client.ts

@@ -101,7 +101,7 @@ export class ClusterClient implements ICustomClusterClient {
       return;
     }
     this.isClosed = true;
-    await Promise.all([this.asInternalUser.close(), this.rootScopedClient.close()]);
+    await Promise.all([this.asInternalUser.close(noop), this.rootScopedClient.close(noop)]);
   }
 
   private getScopedHeaders(request: ScopeableRequest): Headers {

src/core/server/opensearch/opensearch_config.ts

@@ -53,7 +53,7 @@ export const configSchema = schema.object({
     defaultValue: false,
   }),
   sniffOnConnectionFault: schema.boolean({ defaultValue: false }),
-  hosts: schema.oneOf([hostURISchema, schema.arrayOf(hostURISchema, { minSize: 1 })], {
+  hosts: schema.oneOf([hostURISchema, schema.arrayOf(hostURISchema)], {
     defaultValue: 'http://localhost:9200',
   }),
   username: schema.maybe(

src/core/server/opensearch/opensearch_service.test.ts

@@ -266,6 +266,30 @@ describe('#setup', () => {
       expect(mockedClient.nodes.info).toHaveBeenCalledTimes(1);
     });
   });
+
+  it('opensearchNodeVersionCompatibility$ avoid polling when opensearch hosts is empty', async () => {
+    const mockedClient = mockClusterClientInstance.asInternalUser;
+    configService.atPath.mockReturnValueOnce(
+      new BehaviorSubject({
+        hosts: [],
+        healthCheck: {
+          delay: duration(2000),
+        },
+        ssl: {
+          verificationMode: 'none',
+        },
+      } as any)
+    );
+    opensearchService = new OpenSearchService(coreContext);
+    const setupContract = await opensearchService.setup(setupDeps);
+
+    // reset all mocks called during setup phase
+    MockLegacyClusterClient.mockClear();
+
+    setupContract.opensearchNodesCompatibility$.subscribe(async () => {
+      expect(mockedClient.nodes.info).toHaveBeenCalledTimes(0);
+    });
+  });
 });
 
 describe('#start', () => {

src/core/server/opensearch/opensearch_service.ts

@@ -28,7 +28,7 @@
  * under the License.
  */
 
-import { Observable, Subject } from 'rxjs';
+import { Observable, Subject, of } from 'rxjs';
 import { first, map, shareReplay, takeUntil } from 'rxjs/operators';
 import { merge } from '@osd/std';
 
@@ -91,14 +91,26 @@ export class OpenSearchService
     this.legacyClient = this.createLegacyClusterClient('data', config);
     this.client = this.createClusterClient('data', config);
 
-    const opensearchNodesCompatibility$ = pollOpenSearchNodesVersion({
-      internalClient: this.client.asInternalUser,
-      optimizedHealthcheck: config.optimizedHealthcheck,
-      log: this.log,
-      ignoreVersionMismatch: config.ignoreVersionMismatch,
-      opensearchVersionCheckInterval: config.healthCheckDelay.asMilliseconds(),
-      opensearchDashboardsVersion: this.opensearchDashboardsVersion,
-    }).pipe(takeUntil(this.stop$), shareReplay({ refCount: true, bufferSize: 1 }));
+    let opensearchNodesCompatibility$;
+    if (config.hosts.length > 0) {
+      opensearchNodesCompatibility$ = pollOpenSearchNodesVersion({
+        internalClient: this.client.asInternalUser,
+        optimizedHealthcheck: config.optimizedHealthcheck,
+        log: this.log,
+        ignoreVersionMismatch: config.ignoreVersionMismatch,
+        opensearchVersionCheckInterval: config.healthCheckDelay.asMilliseconds(),
+        opensearchDashboardsVersion: this.opensearchDashboardsVersion,
+      }).pipe(takeUntil(this.stop$), shareReplay({ refCount: true, bufferSize: 1 }));
+    } else {
+      this.log.debug(`Opensearch is not configured.`);
+      opensearchNodesCompatibility$ = of({
+        isCompatible: true,
+        message: 'Opensearch is not configured',
+        incompatibleNodes: [],
+        warningNodes: [],
+        opensearchDashboardsVersion: this.opensearchDashboardsVersion,
+      });
+    }
 
     this.createLegacyCustomClient = (type, clientConfig = {}) => {
       const finalConfig = merge({}, config, clientConfig);

src/core/server/opensearch/status.ts

@@ -77,7 +77,7 @@ export const calculateStatus$ = (
 
           return {
             level: ServiceStatusLevels.available,
-            summary: `OpenSearch is available`,
+            summary: (message ?? `OpenSearch is available`) || `Unknown`,
             meta: {
               warningNodes: [],
               incompatibleNodes: [],