Respond with 503 if Kibana is not ready yet.

src/core/server/legacy_compat/legacy_service.ts

@@ -130,7 +130,18 @@ export class LegacyService implements CoreService {
           path: '/{p*}',
           method: '*',
           options: { payload: { output: 'stream', parse: false, timeout: false } },
-          handler: ({ raw: { req, res } }, responseToolkit) => {
+          handler: async ({ raw: { req, res } }, responseToolkit) => {
+            if (this.kbnServer === undefined) {
+              this.log.debug(`Kibana server is not ready yet ${req.method}:${req.url}.`);
+
+              // If legacy server is not ready yet (e.g. it's still in optimization phase),
+              // we should let client know that and ask to retry after 30 seconds.
+              return responseToolkit
+                .response('Kibana server is not ready yet')
+                .code(503)
+                .header('Retry-After', '30');
+            }
+
             this.log.debug(`Request will be handled by proxy ${req.method}:${req.url}.`);
 
             // Forward request and response objects to the legacy platform. This method