Design a Notification System

Design a Notification System

Step 1 of 11

users5 M

One event, three possible channels, and a user who does not want all three.

The Idea

Turn one event into the right messages on the right channels — push, email, SMS — honouring each user's preferences. Assume 5 million users and bursts of 200 000 notifications a minute after something big happens.

Real-World Example

A school announcing a snow day: one decision, then the app, the text and the phone tree. A parent who gets all three at once is irritated, not better informed, so somebody decides which channels fire.

The Tradeoff

A queue makes third-party outages survivable and makes duplicates possible, so every notification carries an idempotency key. Batching and quiet hours cut the volume people actually feel, and they also delay the one message somebody was waiting up for.

Your turn

Put the steps in the right order.

1. 1Hand the payload to the channel provider and record the result↑↓
2. 2Accept the event and write it to the queue↑↓
3. 3Look up the user's channel preferences and quiet hours↑↓
4. 4Skip the job if this idempotency key was already sent↑↓

Check order

Mini quiz

1 / 3

An idempotency key on each notification prevents:

• AA provider outage
• BThe same notification being sent twice after a retry
• CUsers opting out
• DQuiet hours being ignored