Scaled a Distributed System
Designed a Backend System
I thought this YouTube feature is useless but later found out to be very smart and solved an important issue.

Let's consider 2 famous YouTubers pewdiepie and MrBeast who have 110M and 63.2M subscribers respectively.

Now, pewdiepie uploads a video, and 110M accounts are sent notifications that "Hey! pewdiepie has uploaded a video. You are subscribed to the channel."

Now 110M notifications are in the queue to be sent which is really huge!

And now consider coincidentally MrBeast uploads his video at the same time pewdiepie did. Now 173.2M notifications are lined up to be sent!!

This is an insane number of notifications to be sent.

Sending 173.2M notifications will break the system and cause scalability issues for YouTube.

YouTube smartly tackles this problem by identifying that the notification should be sent to subscribers who want to know immediately that their favorite YouTuber has uploaded.

Considering an educational youtube channel, you won't immediately want to watch it. So, sending a notification about it won't have any impact on you. It's useless.

By introducing the bell icon YouTube identified the power users and avid subscribers.

Let's consider 20% of the subscriber base of pewdiepie and MrBeast press the bell icon. Now, the problem statement of YouTube dealing with the notification has decreased significantly.

This helps them scale their notifications effectively and doesn't irritate their user.