Design a Budgeting App

"Design a Mint-like budgeting app: transactions from credit and debit cards, analytics, and a notification when a transaction takes spending over a threshold." That is a verbatim Datadog system design prompt, and the wording asks for three separate surfaces. A connector pushes card transactions in. The app categorizes and stores them, then shows the user where their money went. And it fires an alert the moment a category crosses a budget line. The trap is to hear "notification over a threshold" and design only an alerting system. That leaves most of the prompt on the table, and a design round graded on breadth will mark it down. We build out the full product here, with the alert as one piece of it.

Nail the nouns down first; every later choice leans on them. A transaction is one card charge. It carries a user, the card, an amount, a merchant, a timestamp, plus a status that moves from pending to posted. Each one lands in a category, the bucket of dining or travel or groceries. A budget is a user's limit for one category over a window, "dining under $500 this calendar month." Behind the budget sits a running aggregate. That is the per-user-per-category sum we compare against the limit, and it powers the analytics too. When the aggregate crosses the limit, the system fires an alert. Fix the nouns early: transaction, category, budget, aggregate, alert. The design rotates around one flow. A transaction travels from the card network all the way to that aggregate and the notification it can trip.

The Datadog angle

The budgeting CRUD is standard. The part with Datadog's fingerprints is the alert engine. Most of this design is a normal fintech app: ingest, categorize, store, report. Build all of it, because the reject that comes back on this prompt is thin breadth, and the full app is what gets graded. But one deep dive is pure Datadog. The over-threshold notification is a monitor in disguise: watch a stream of events, hold a windowed aggregate per entity, evaluate it against a threshold, and notify once on the crossing.