Why a ticket-availability monitor is a state machine, not a scraper

A ticket calendar looks like an easy automation target: request a page, search for a date, and send an email when it appears. That implementation works until the first queue, partial response, stale cache or provider outage. Then it can quietly turn "I do not know" into "sold out" — or generate a false alert. I learned this while building MachuPing , an independent monitor for official Machu Picchu ticket availability. I am the maker. It does not sell, hold, reserve or buy admission; the official booking platform remains the source of truth. The useful abstraction is a small state machine: UNKNOWN -> CONFIRMED_UNAVAILABLE -> RETURNED_AVAILABLE ^ | | | v v +------------- PROVIDER_ERROR ------ ALERTED The exact labels will vary, but three rules matter. 1. Unknown is not unavailable Queues, timeouts, malformed payloads and incomplete calendars are observations about the monitor, not evidence about inventory. Persist them separately. A provider error should never close a date or trigger a reassuring "still sold out" message. 2. Match the user's real constraint "Machu Picchu is available" is too broad to be useful. Inventory is split by route, date, entry time and capacity. A valid transition requires a match for the selected combination, including the requested party size. This also prevents a common analytics mistake: counting every polling response or every seat-like value as a unique ticket. A state change is a state change, not proof of inventory volume. 3. Alert on a confirmed transition, not a snapshot The valuable event is not simply available . It is a move from a previously confirmed unavailable state to confirmed available. Persist an idempotency key for that combination so retries do not create duplicate email. Before sending, revalidate the observation when the provider permits it. The alert should still state the limitation plainly: availability can disappear before the traveller reaches official checkout. A practical event record At a module boundary, I pr