Migration

From ad-hoc per-unit pricing code

The typical starting point: a price * quantity in every endpoint, an if (monthlyCount > 10000), an UPDATE balance, and a balance debited twice on a retry. Migrating means moving those pieces, one by one, behind the library’s two doors.

Today	In pricemeter
Unit prices as constants, tiers as ifs	A Rate (data); rate() splits tiers
A price-table query	getRate (or /rates)
A “how many this month” counter	The usedSoFar getRate returns
UPDATE balance … WHERE balance >= ?	Inside commit; throw InsufficientCredit when short
A usage log	The plan’s usage rows, with breakdown and rateId
Double debits on retries	A stable ref → refId; commit skips the repeat
Money in floats	Integer micro-units (usd(0.02) → 20_000)

A suggested order:

1. Write the catalog. Everything you bill is a meter; everything that changes the price is a dimension (country, model). Shared quotas are a pool (feeds).
2. Turn tariffs into data. Move prices scattered through the code into Rate objects and check them with validateRate; rateId ties every row to its tariff.
3. Wire getRate to your existing table. Choose here which period a counter belongs to (calendar month, cycle, lifetime).
4. Wire commit to your existing transaction. If you have ledger and usage tables, add a unique index on (refType, refId); if not, start from the pricemeter/sqlite schema. Verify it with commitContract.
5. Run in shadow. For a while, call metering.plan.observe(...) next to the old code, compare amounts and write nothing. When they agree, switch the writes to pricemeter.
6. Leave the rest in the app. Invoices, tax, coupons and period close do not move into the library; for period-end items there is /adjustments.

Cellonay

Migrating Cellonay’s OTP and WhatsApp billing to pricemeter is planned. The messaging example is the target shape of that migration: a per-account Durable Object ledger, a two-line OTP hold, a quota shared across channels, and a queue for late-published window prices.