Notifications

A notification system is the service that sends emails, SMS messages and push alerts on behalf of the rest of a product: “your order shipped”, “someone replied to you”. Other services drop a request onto a queue, and workers pick it up, apply user preferences and rate limits, and call outside providers like Apple, Google or an email service. Those providers fail and time out regularly, so workers retry with backoff, and messages that keep failing are parked in a dead-letter queue for inspection. Because retries can resend something that actually went through, each notification carries a unique ID that lets the system skip duplicates, so a user doesn’t get the same text five times.