Usage and Billing

Billing is made possible by the logging framework: every model call writes a harpe.metering.usage record, and contextual logging says whose call it was.

To enable billing, simply create a decorator logger to extract usage logging items:

class UsageMeter(inner: Logger, meter: Meter)
  view Logger

  def logEntry(entry: Entry): Unit =
    meter.record(entry)      // update per-session counters / push to a metrics service
    inner.logEntry(entry)    // and still persist

  def close(): Unit = inner.close()
end

Inside the meter, the usage data is reconstructed from the logging item, and entry.context carries the user information the outer scope stamped on it:

if entry.event == Usage.event then
  val usage = Usage.decode(entry)
  val user = User.fromContext(entry.context)
  cents = cents + rate(usage.provider, usage.model).charge(usage)
  // insert into billing database

The same decode works offline over a stored journal, so a live meter and an end-of-month invoice read the identical record.