A Primer to LLD Interviews

Most tech interviews these days across all levels have a Low Level Design (LLD) component. It is also called the Machine Coding round. This round entails designing an end-to-end working system, like a parking lot or an elevator control system or an expense sharing app like Splitwise. The objective is very different from the traditional rounds on system design and DSA that we have always prepared for.

Unfortunately, developers are also often under-prepared for this round, and often flunk it. There are primarily three reasons for this:

1. They do not understand the purpose of the interview.
2. They do not understand the fundamental tenets of good software — Good public API design, testability, extensibility, etc.
3. They are unable to code a working system in the allocated time. This is often due trying to implement too many features, or very complicated ones.

What do interviewers actually judge by this interview? While we know they look for “clean, extensible, testable” software, what exactly does it mean? Personally, I have always looked for:

1. How well can you gather the requirements?
2. How thoughtfully do you design the public APIs of your code — Once released to the public, an API is impossible to change (if you care about backward compatibility, that…