Imagine teachers exporting classroom posters. Before making a large set, one teacher needs a clear spending limit.
How: work toward a solution
A fictional classroom poster exporter starts with 1,000 credits, spending one per export. Run the shared months of 100, 300 and 600 exports: balances become 900, 600 and zero. Any further export is blocked until a simulated top-up. Credits here do not expire; purchase price, taxes, fees and refunds are omitted.
Buy a balance before consuming units.
Classroom poster exporter · illustrative example
Prepaid credits
Classroom poster exporter · first three months: 100 / 300 / 600 exports
Balance
credits · 1 per export
Exports completed: 0
Insufficient balance blocks the whole request. No expiry here. Credit purchase price omitted.
Fictional amounts. Taxes, fees and refunds omitted.
A purchased allowance starts at 1,000 credits; each export consumes one. After 100, 300 and 600 exports the balances are 900, 600 and zero. A further export is blocked until top-up; purchase price and expiry are unspecified.
A fictional demo within this article. Reset or reload restores its initial state.
What: the concept
Prepaid credits collect payment before usage and reduce a balance as units are consumed. Credit value, expiry and eligible uses depend on the offer.
A poster exporter fits prepayment when a classroom wants a bounded allowance before consuming resources. Postpaid usage billing charges after measurement; subscription access usually follows a time period. Credits define a balance, not necessarily a currency or a renewal interval.
02Applications
A purchased allowance starts at 1,000 credits; each export consumes one. After 100, 300 and 600 exports the balances are 900, 600 and zero. A further export is blocked until top-up; purchase price and expiry are unspecified.
03Implementation & cautions
Keep purchase, reservation, consumption and refunds in a durable ledger. Prevent duplicate deduction and negative balances under concurrency. Define expiration and failed-work refunds explicitly. This local example resets on reload and stores no purchased value.
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