Step 7: Failure Modes and Operations

Most outages are judged by how long they last. A flash sale outage is judged by when it happens. Two minutes of downtime at three in the morning affects almost nobody. The same two minutes starting at the sale's opening second can cover the entire sale, and the stock may already be gone by the time the system recovers.

Two outcomes count as failures here, and they pull in opposite directions. Overselling sells a unit that does not exist. Underselling leaves units unsold at the end of the sale because they were held, lost, or never offered