feat: Add headers to understand who responded

[akash1810]

What does this change?

This change adds two headers to each response:

• X-Gu-Backend-App which represents which app is running (tag-page-rendering, article-rendering, etc.)
• X-Gu-Backend-App-Target-Group which represents which target group served the request (EC2 or ECS)

One use-case for this is in the "run DCR on ECS" project (see also #16321).

This is similar to guardian/frontend.

X-Gu-Backend-App is already removed for reader requests within Fastly. X-Gu-Backend-App-Target-Group is removed in https://github.com/guardian/fastly-edge-cache/pull/1199.

Why?

As part of @guardian/devx-reliability-and-ops's project to run DCR on AWS, we'll be splitting traffic between the current EC2 target group and the new ECS target group. This header helps to distinguish what infrastructure responded.

How has this change been tested?

After deploying to CODE and following the steps listed in #16377, we can see the new headers:

curl -s "https://www.theguardian.com/tone/minutebyminute.json?dcr=true" | \
  curl -s -X POST https://tag-page-rendering.code.dev-guardianapis.com/TagPage \
    -H "Content-Type: application/json" \
    -d @- -D - -o /dev/null

HTTP/2 200
date: Wed, 15 Jul 2026 10:13:41 GMT
content-type: text/html; charset=utf-8
content-length: 365212
x-gu-backend-app: tag-page-rendering
x-gu-backend-app-target-group: ec2
x-powered-by: Express
link: <https://assets.guim.co.uk/polyfill.io/v3/polyfill.min.js?rum=0&features=es6%2Ces7%2Ces2017%2Ces2018%2Ces2019%2Cdefault-3.6%2CHTMLPictureElement%2CIntersectionObserver%2CIntersectionObserverEntry%2CURLSearchParams%2Cfetch%2CNodeList.prototype.forEach%2Cnavigator.sendBeacon%2Cperformance.now%2CPromise.allSettled&flags=gated&callback=guardianPolyfilled&unknown=polyfill&cacheClear=1>; rel=prefetch,<https://assets-code.guim.co.uk/assets/frameworks.client.web.6b19980dfdb962b2f0d0.js>; rel=prefetch,<https://assets-code.guim.co.uk/assets/index.client.web.b98149db4bc4f45b7f3c.js>; rel=prefetch,<https://assets.guim.co.uk/commercial/454fa0f267b7fb516298/graun.standalone.commercial.js>; rel=prefetch,
etag: W/"5929c-f+HFJg+uAM7mWBRL5skzmVOMj+Q"
vary: Accept-Encoding

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[github-actions]

Deploy build 2204 of dotcom:ab-testing to CODE

All deployment options

• Deploy build 2204 of dotcom:ab-testing to CODE
• Deploy parts of build 2204 to CODE by previewing it first
• What's on CODE right now?

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From guardian/actions-riff-raff.

[github-actions]

Deploy build 29122 of dotcom:rendering-all to CODE

All deployment options

• Deploy build 29122 of dotcom:rendering-all to CODE
• Deploy parts of build 29122 to CODE by previewing it first
• What's on CODE right now?

---

From guardian/actions-riff-raff.

[Copilot]

Pull request overview

Adds a reusable Express middleware to stamp responses with backend-identification headers so downstream tooling can tell which DCR app and which infrastructure target group (EC2 vs ECS) served a request. This supports traffic-splitting/verification work (e.g., ECS rollout) by making the responder visible in each HTTP response.

Changes:

• Introduce responseHeaderMiddleware that sets X-Gu-Backend-App and X-Gu-Backend-App-Target-Group.
• Register the middleware in both production and development servers so headers are applied to responses.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
dotcom-rendering/src/server/server.prod.ts	Registers the new response-header middleware for prod server responses.
dotcom-rendering/src/server/server.dev.ts	Registers the new response-header middleware for dev renderer routes.
dotcom-rendering/src/server/lib/header-middleware.ts	Implements the middleware and environment-derived header values.

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[gu-prout]

Seen on PROD (merged by @akash1810 13 minutes and 24 seconds ago) Please check your changes!