A notification stream tells you when a product's sync crosses a health boundary โ it fires on state changes (a run starts failing; a failing product recovers), not on every event, so it stays quiet in steady state. A stream is either a Slack incoming webhook (a short message posted to a channel) or a generic webhook (a JSON POST to your endpoint). It can watch all products in the tool or narrow to one.
Webhook streams are signed: every request carries an HMAC-SHA256 of the raw body in a signature header, keyed with the stream's own signing secret (shown on its edit page). Recompute and compare constant-time; reject mismatches. The secret can be rotated โ the old one stops verifying immediately. A test action sends a sample run.failed payload so you can verify the wiring before trusting it.