System requirements

Functional:

1. give push notification
2. gather contact info
3. send and recieve notifications

Non-Functional:

1. reliability
2. security
3. tracking and monitoring
4. rate limiting

Capacity estimation

• 10 million users × 2 notifications per day = 20 million notifications per day
• Average throughput: ~230 notifications per second
• Peak burst: ~33,000 notifications per second
• Average message size: 1 KB (approx.)

API design

POST https://api.example.com/v/sms/send

Database design

DB: It stores data about user, notification, settings, etc.

SQL database can be used for this purpose

High-level design

Services 1 to N that send notifications via apis provided by notification server

Notification server

Cache

DB

Message Queues for decoupling

Workers which will be pullingfrom the queues constantly

Third party servcies like APM and FCM , sms and email services as well

Request flows

1. a service calls api from notification server to send notifications
2. notification server fetch user meta data from cache and database
3. event sent to the corresponding queue
4. workers pull the events\
5. workers send the events to APM, FCM which will send the notifications to the devices in the end

Detailed component design

Reliability -

keeping a notification log database , retry mechanism implementation also there

Dedup mechanism there for already seen notifiacations if its already seen then the notification is not sent else it is sent

Exactly once delivery is not possible here

Notification template can be kept here in this design

consistent format

many websites and apps give users fine-grained control over notification settings. This information is stored in the notification setting table, with the following fields:

user_id bigInt

channel varchar # push notification, email or SMS

opt_in boolean # opt-in to receive notification

Before any notification is sent to a user, we first check if a user is opted-in to receive this type of notification.

Rate limiter also added in this design

appKey and appSecret are used here for secure push notifications

Monitor queued notifications if queue is large then we have to add more workers

Analytics Service can also be used here to know about the click tracking for the notification

Trade offs/Tech choices

Explain any trade offs you have made and why you made certain tech choices...

Failure scenarios/bottlenecks

Try to discuss as many failure scenarios/bottlenecks as possible.

Future improvements

What are some future improvements you would make? How would you mitigate the failure scenario(s) you described above?