1. Question and Context
You maintain a support widget embedded by multiple merchant sites. It must remember a visitor session for each merchant without linking the same user across unrelated top-level sites. Browsers may isolate cookies, localStorage, caches, and network state by the top-level site and the third-party origin.
2. What the Interviewer Is Evaluating
- Whether you distinguish a shared third-party origin from a shared storage partition.
- Whether you understand CHIPS scope, required attributes, and isolation semantics.
- Whether you design explicit consent and fallback paths for genuinely unpartitioned state.
- Whether privacy, usability, login experience, and compatibility appear in one decision framework.
3. Clarifying Questions Before You Answer
- What must be shared across top-level sites, and is shared identity truly required?
- Does each site have its own tenant, user, and session boundary?
- Can the user click once to authorize, or must the widget load without interaction?
- Which browsers and embedding modes matter: iframe, popup, or top-level navigation?
4. A 30-Second Answer Framework
Answer with data boundary, default, authorized exception, fallback, and verification.
I would scope the session key to the top-level site and widget origin, then use a secure cookie with the Partitioned attribute so each merchant gets an independent session. For cross-site login, I would use a top-level navigation for explicit authorization and return a short-lived one-time credential; if the browser lacks the capability, I would fall back to an anonymous mode and explain how to sign in. I would verify isolation with multiple top-level sites, site-data clearing, and denied-access tests.
5. Step-by-Step Deep Dive
Step 1: Define the Partition Key and Data Classes
Google Privacy Sandbox describes third-party storage and communication as isolated by partition. MDN describes state partitioning as a browser privacy effort that reduces cross-site tracking. Classify data as merchant-local session state, publicly cacheable assets, or account state that genuinely needs cross-site identity. The first class should not bypass partitioning.
Step 2: Use CHIPS for Per-Site Sessions
CHIPS lets a third-party cookie opt into partitioned storage. The response should use Partitioned; Secure and satisfy the browser’s other cookie security requirements. The same widget origin then receives different cookie jars on merchant A and merchant B, so support context does not merge naturally across sites. CHIPS provides per-top-level-site state, not shared cross-site login.
Step 3: Design Explicit Authorization for Cross-Site Identity
If the product truly needs one account visible on multiple sites, use top-level navigation or the Storage Access API as an exception path. The Storage Access API lets third-party content request state that is normally unpartitioned and inaccessible; explain why access is needed, request it at an appropriate time, and provide a useful denial path. Bind authorization to the requesting top-level site, make it short-lived, and make it revocable.
Step 4: Build a Compatibility and Privacy Matrix
Test browsers that support CHIPS, browsers without partitioned-cookie support, denied storage access, site-data clearing, multiple top-level sites open at once, and iframe replacement or navigation followed by reload. Assert cookie isolation, session recovery, repeated prompts, and whether the stateless mode still completes the core action.
6. High-Quality Sample Answer
I would first clarify whether the support widget really needs to identify the same user across merchants. My default answer is no: each merchant’s session should be independent because cross-site linking creates privacy and compliance risk.
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On the default path, the widget server sets a Partitioned; Secure cookie. The browser forms an independent storage partition from the top-level site and widget origin, so a session on merchant A is absent on merchant B. Static scripts and images can use ordinary caching, but identity is not placed in state that can be read across sites.
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For a genuine unified-account case, I provide a Sign in button. It navigates to the account’s top-level page, completes authentication, and returns a short-lived one-time code to the original merchant. If supported, the widget may request unpartitioned access through the Storage Access API after user interaction. If access is denied or unsupported, it falls back to an anonymous per-site session and top-level login. I would test two top-level sites, single-site data clearing, and denied authorization to prove there is no account mix-up or repeated prompt loop, while tracking partition hit rate and login failure rate.
7. Common Failure Modes
- Treating CHIPS as a shared cross-site cookie instead of partitioned state.
- Moving all state to unpartitioned access, expanding tracking and failure surface.
- Discussing only Chrome and skipping Firefox, Safari, or capability-detection failures.
- Requesting access silently at iframe load without user action, explanation, or denial fallback.
- Putting a long-lived token directly in a return URL, exposing it to logs, history, or referrers.
8. Follow-Up Questions and Responses
Follow-up 1: Why not put the user ID in the top-level site’s URL?
URLs can enter logs, history, analytics systems, and referrers. Use a short-lived one-time code and exchange it server-side for a constrained session.
Follow-up 2: When can a CHIPS cookie make a user appear signed out?
The first visit to a new top-level site has a new partitioned cookie. That is intentional isolation. Provide a clear top-level sign-in flow instead of trying to read another site’s cookie.
Follow-up 3: How do you preserve core functionality when Storage Access API is denied?
Treat cross-site identity as an enhancement. Anonymous support, help content, and a local merchant session still work; guide the user to top-level login only when historical account data is needed.