Cross-platform stateful onboarding sync

WHAT THIS TESTS This evaluates whether you centralize state on the server and design APIs and sync so multiple clients reflect one coherent onboarding journey.

A GOOD ANSWER COVERS Source of truth: the backend stores onboarding state per user, a list of defined steps each with completed status and timestamp; never the client. API: a GET to fetch current state on app load, and an idempotent POST to mark a step complete, idempotent so retries and double-taps do not corrupt progress. Completion should be monotonic: a step, once done, stays done. Sync to other clients: choose by need, real-time push or websocket for instant updates, or refresh-on-focus and short polling if near-real-time is acceptable. Because state is server-side, when mobile marks step 3 done, web fetches or receives the update and re-renders the checklist. Define the step set server-side so both platforms render the same list.

COMMON WRONG ANSWERS Storing progress in shared preferences or local storage per device, so platforms diverge. Non-idempotent completion endpoints that mis-count on retry. No mechanism to update the other client, so web only changes on a manual reload triggered by the user. Letting each client define its own step list, causing mismatched checklists.

LIKELY FOLLOW-UPS Real-time push versus polling, what drives the choice? How do you make step completion idempotent? How do you handle steps that can only be completed on one platform?

ONE CONCRETE EXAMPLE The backend holds steps verify_email, add_profile_photo, invite_teammate for user u_12. On mobile the user adds a photo; the app POSTs complete for add_profile_photo, idempotently. The server updates state and pushes an event over a websocket. The open web session receives it and instantly checks off the item without a manual reload, keeping both surfaces in sync.