System design interview question

Design a news feed

Twitter's home timeline, Facebook's News Feed, Instagram's feed: “design a news feed” is one of the most-asked system design interview questions, because a feature that looks small hides the hardest part of the job. The read path has to feel instant, and a single write can touch millions of rows. Here is how to walk into it — and then how to practise it until the answer is yours.

Practise News Feed

Multiple choicePick the design from four
Fill the gapsFill the gaps in one drawn for you
Build from scratchBuild it on a blank canvas

Practise it on a canvas

Free — no account needed.

The question, as an interviewer would set it

A social app lets people follow each other and post short updates, some with photos. Opening the app shows a home feed: the newest posts from everyone you follow, and it must appear in well under a second, because people open it dozens of times a day. Reads outnumber posts by about a hundred to one. When someone posts, it is fine if followers see it a few seconds later, but the post itself must never be lost, and the person posting must not wait while the app works out who should see it. Photos are large and are viewed far more often than they are uploaded, by people all over the world.

What it is really testing

Every phrase that matters in the brief is a requirement in disguise. Spot them before you draw a single box.

“it must appear in well under a second”
A feed that is already assembled when it is asked for, held in memory.
“Reads outnumber posts by about a hundred to one”
Do the expensive work once per post, not once per read.
“the post itself must never be lost”
A durable store the post is written to first.
“the person posting must not wait while the app works out who should see it”
Hand the fan-out to a line of work done in the background.
“viewed far more often than they are uploaded, by people all over the world”
Serve photos from locations near each viewer.

Back of the envelope

Daily active users: 300 millionAn assumption to say out loud. If the interviewer has a number in mind, they will give it to you.
Feed reads: ~10 opens a day each → ~3 billion a day, ~35,000 a secondThat is the average. Peaks run several times higher.
New posts: ~1 per 100 reads → ~30 million a day, ~350 a secondThe hundred-to-one read ratio is what makes this a read-heavy design.
Feed updates: ~350 posts a second × ~200 followers → ~70,000 a secondAnd one post from an account with 50 million followers is 50 million updates on its own.

Questions to ask first

Is the feed strictly newest-first, or ranked? Ranking adds a scoring step; newest-first keeps the question about how data flows.
How fresh must it be? “A few seconds late is fine” leads to a very different design from “instantly”.
What is in a post — text only, or photos and video too? Media is a separate storage and delivery problem from the post itself.
How many accounts does a typical user follow, and how many followers can the biggest account have?
How many people use it daily, and how often does each of them open the feed?

The trade-offs you will be pushed on

1.Do you assemble each feed when a post is written, or when the feed is read? What does each choice cost, and who pays for it?
2.What happens when an account with tens of millions of followers posts?

Now answer it

The same design always gets the same score, with the reason behind every requirement — then defend it against the trade-offs above.