Notification systems

Emails, SMS and push notifications are sent asynchronously from queues, with per-channel providers, retries, user preferences and deduplication so nobody receives the same alert twice.

Where it shows up in real systems

  • Email and SMS provider

    Delivery is slow and can fail, so send from background workers, never while the user waits. Providers throttle senders and track bounces; a burst of password-reset emails can hit the limit.

    In production: Amazon SES, SendGrid, Mailgun, Twilio, Postmark

  • Push notification service

    Apple and Google gateways accept large batches but deliver on their own schedule. Fan-out to millions of devices belongs in a queue with many workers, and invalid device tokens must be cleaned up.

    In production: Firebase Cloud Messaging, Apple Push Notification service, Amazon SNS Mobile Push, OneSignal

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