Prepaid budget

A prepaid budget is money paid in before delivery. A campaign can never spend beyond it, whatever budget figure is configured.

Why prepay rather than invoice?

Because it makes overspend structurally impossible rather than merely monitored. A campaign with a million-unit budget and no funded balance delivers nothing at all, which is the correct behaviour.

How does this interact with pacing?

Available budget is capped by the funded balance before any pacing decision is made. Nodes then lease slices of that capped amount, so distributed serving cannot collectively exceed what was paid.

What happens to unspent balance?

It remains yours. Unspent prepay is a liability on our books, not revenue, and it is refundable rather than forfeited.

What happens as the balance runs down?

Delivery degrades rather than stopping abruptly, because the available amount is recalculated against the funded balance before each auction rather than at the end of a period. A campaign approaching zero simply wins less often, then stops entering. Nothing is served and then found to be unpayable, which is the failure mode an invoiced model has and a prepaid one cannot.

Why do distributed systems need this to be explicit?

Because several serving nodes can each believe there is budget left at the same moment. Capping the available amount at the funded balance and then handing out short leases against that cap means the sum of what every node can spend is bounded by what was actually paid. Without the lease step, concurrent nodes reconcile after the fact, and after the fact is exactly when the overspend has already happened.

Does prepay disadvantage a small advertiser?

Less than it looks. A funded balance is a floor on commitment rather than a minimum spend: the campaign delivers against whatever was paid and stops, so a small budget buys a small, complete test rather than a partial one. It also removes the credit check and the invoicing relationship, which are usually what keeps a small advertiser out of a new channel.