Checkout Failure Reconstruction: Node.js SaaS Alerting API, Poll Queries, and Webhooks

For Node.js SaaS checkout failures, implement a queryable error alerting API with durable cursors, utilizing polling or webhooks based on latency. A separate heartbeat is essential for checkouts that emit no error events. To detect silent breakage, use error-rate counters for high traffic and last-success timestamps for cron jobs. Effective alerting requires distinguishing failed outcomes from completed requests and ensuring signals can detect work that never started through controlled failure replay tests.