Design Yelp

Question 1

Who is asking what is nearby?

Accepted Answer

A phone that knows its own position. The location comes from the client and the answer has to arrive before the map finishes moving.

Question 2

What does the app connect to?

Accepted Answer

A gateway is the entry point for a read-only, extremely cacheable workload — and where you would put the rate limits that stop someone scraping your entire place database one radius at a time.

Question 3

Two hundred million places. How do you find the ones within 5km without measuring the distance to all of them?

Accepted Answer

By never looking at most of them. Space is divided into cells — geohash, S2 or a quadtree — and each place is filed under its cell, so a search reads the handful of cells covering the radius. A `WHERE distance < 5` over every row is the answer that does not survive contact with 200M places, and this index is the answer that does.

Question 4

Where do the places themselves live?

Accepted Answer

Names, opening hours and addresses are relational data queried by id once the geo index has told you which ids matter. The index answers "which", the store answers "what".

Question 5

Who expands the radius when a cell comes back empty?

Accepted Answer

Cell lookups return candidates, not answers — they still need filtering by true distance, widening in sparse areas, and truncating in dense ones. That logic is a service, and it is where the quality of the results is decided.

Question 6

Your app queries the places store directly. What stops someone downloading all 200 million rows?

Accepted Answer

The place database is the asset. A client with a direct connection can page through the whole thing, and there is nowhere to put a rate limit.

Question 7

How many nearby searches a second?

Accepted Answer

500,000,000 / 86,400 is about 5,800 a second, against a few thousand place updates a day. A read-only workload by any measure — which is what justifies an index that is rebuilt rather than updated in place.

Question 8

Manhattan and the Sahara are the same size on a grid. How do you handle both?

Accepted Answer

Fixed cells make one query return fifty thousand results and another return none. Either the cells subdivide with density — a quadtree — or the service expands ring by ring until it has enough. Naming which, and what it costs, is the answer.

Blueprint — system design