In previous article , I show you how to design a URL shortener service. I presented the basics of capacity planning, collecting requirements, DB sharding, etc. Now, I would like to cover another interesting system design interview question - How would you design Twitter?

Requirements and capacity planning

We want a system that allows a user to share short text messages (posts). Those messages will be displayed in 2 places:

1. on user board
2. on home board

User’s X board displays posts that user X published. Every logged-in user Y has also their own private home board. Home board displays posts of users subscribed by user Y.

In previous article , I explained you how to create a service like TinyURL. That kind of service requires storing a lot of data what implies sharding this data across multiple DB instances. I showed a really simple algorithm of how data could be distributed in that case and briefly mention something about replication. Now the question is what would we do in real life?

Real-life distribtued DB

Using existing NoSQL solution would be a good choice as they already solve main problems with distributed DB systems:

A URL shortener is one of the most famous system design interview questions. In my opinion, it is also the most essential one from a scalability perspective and that is why global companies like the famous FAANG ask about it. Let’s go through it like during an interview to show you how you can structure this discussion.

There is no perfect system

Remember, there is no silver bullet to design the perfect system. There are also no good or bad answers to the question. It’s all about trade-offs and all you have to show is that you are aware of those trade-offs.

“You should use NoSQL here”, “SQL doesn’t scale” - those are the top buzz words you can hear at conferences or from candidates while interviewing for a software engineer position. 90% percent of the people cannot explain why is that. They simply repeat those empty buzz words to show that they are up to date with the “modern approach”. Is it really true that SQL does not scale? Let me put some light on the true mindset that stands behind NoSQL and SQL story.