4. Design a Notification System

The brief

Design a notification service for transactional and optional messages across email, push and SMS. Follow a request from acceptance to a visible delivery status, respecting user preferences. Explain retries, duplicate provider callbacks and how an urgent message avoids waiting behind a large campaign.

• Accept 10 million notification requests/day, with peaks of 10,000 requests/second.
• Each request can target several channels; providers can throttle, time out or send duplicate callbacks.
• Users can opt out of optional messages, and transactional security alerts have higher priority.

Constraints

Urgent dispatch delay≤ 30 seconds

Declare p95 time from durable acceptance to the first provider attempt under normal provider operation.

Preference enforcement

Define when preferences are checked and how queued optional messages honor a later opt-out.

Delivery semantics

Separate accepted, attempted, provider-acknowledged and delivered states without promising exactly-once delivery.

What to cover

1. 01

   Request and status contract

   Define identity, recipient, channel, template version and observable states.

2. 02

   Delivery pipeline

   Show preference checks, durable queues, provider adapters and priority handling.

3. 03

   Retry and deduplication

   Trace a timed-out send followed by a late callback and a client retry.

4. 04

   Capacity and operations

   Size peak acceptance and worker demand; explain provider quotas, dead letters and sensitive-data retention.

Worked designs

Explore the architecture and decisions, then build on an example with Coach.

Official design

Design a Notification System

Push, email, SMS and in-app for a billion deliveries a day: priorities that never wait, promotions that never arrive late, no duplicates a user would notice. One-hour boards for junior, senior and staff, with the theory behind them.

Explore the design

Review rubric

AI feedback uses these criteria. Scores are practice feedback.