feat: Add headers to understand who responded

dotcom-rendering/src/server/lib/header-middleware.ts

@@ -0,0 +1,40 @@
+import type { NextFunction, Request, RequestHandler, Response } from 'express';
+
+const getTargetGroupHeaderValue = () => {
+	if (process.env.NODE_ENV === 'production') {
+		// See https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ecs-environment-variables.html
+		return process.env.AWS_EXECUTION_ENV?.startsWith('AWS_ECS_') === true
+			? 'ecs'
+			: 'ec2';
+	}
+
+	return 'dev';
+};
+
+const getHeaders: () => Record<string, string> = () => {
+	const backendApp = process.env.GU_APP ?? 'rendering';
+	const targetGroup = getTargetGroupHeaderValue();
+
+	return {
+		'X-Gu-Backend-App': backendApp,
+		'X-Gu-Backend-App-Target-Group': targetGroup,
+	};
+};
+
+/**
+ * Middleware to add response headers useful for debugging.
+ *
+ * @see https://expressjs.com/en/guide/using-middleware
+ */
+export const responseHeaderMiddleware: RequestHandler = (
+	req: Request,
+	res: Response,
+	next: NextFunction,
+) => {
+	const headers = getHeaders();
+
+	for (const [key, value] of Object.entries(headers)) {
+		res.setHeader(key, value);
+	}
+	next();
+};

dotcom-rendering/src/server/server.dev.ts

@@ -28,6 +28,7 @@ import {
 import { handleAppsThrasher } from './handler.thrasher.apps';
 import { getABTestsFromQueryParams } from './lib/get-abtests-from-query-params';
 import { getContentFromURLMiddleware } from './lib/get-content-from-url';
+import { responseHeaderMiddleware } from './lib/header-middleware';
 import { requestLoggerMiddleware } from './lib/logging-middleware';
 import { recordError } from './lib/logging-store';
 
@@ -105,6 +106,8 @@ const renderer = Router();
 renderer.use(getContentFromURLMiddleware);
 renderer.use(getABTestsFromQueryParams);
 renderer.use(requestLoggerMiddleware);
+renderer.use(responseHeaderMiddleware);
+
 renderer.get('/Article/*url', handleArticle);
 renderer.get('/Interactive/*url', handleInteractive);
 renderer.get('/Blocks/*url', handleBlocks);

dotcom-rendering/src/server/server.prod.ts

@@ -28,6 +28,7 @@ import {
 } from './handler.sportDataPage';
 import { handleAppsThrasher } from './handler.thrasher.apps';
 import { recordBaselineCloudWatchMetrics } from './lib/aws/metrics-baseline';
+import { responseHeaderMiddleware } from './lib/header-middleware';
 import { logger } from './lib/logging';
 import { requestLoggerMiddleware } from './lib/logging-middleware';
 import { recordError } from './lib/logging-store';
@@ -40,6 +41,7 @@ export const prodServer = (): void => {
 	app.use(express.json({ limit: '50mb' }));
 	app.use(requestLoggerMiddleware);
 	app.use(compression());
+	app.use(responseHeaderMiddleware);
 
 	app.get('/_healthcheck', (req: Request, res: Response) => {
 		res.status(200).send('OKAY');