Should You Memorize System Design Answers?

Short answer: no. Memorizing a finished design lets you repeat a problem you have already seen, and nothing more. Interviewers change one requirement partway through, and then you have nothing to say next. What you need instead is a repeatable method, plus enough knowledge of the parts to rebuild any design under time pressure.

This is not an argument against studying worked designs. Reading how other people solved Uber or Instagram is one of the fastest ways to learn. The problem starts when reading is the only thing you do, because reading a design and producing one are different skills.

Quick Overview

Why a memorized answer stops working

The interviewer changes the numbers. You give the design you studied, and they say the write rate is now a hundred times higher. A memorized answer does not cover that case. With a method you return to the requirements and redesign the part that no longer fits.

They ask why. Every design you memorize contains decisions somebody else made. When the interviewer asks why a queue sits there, or why that store and not another, the reasoning was never yours to give.

The question is not in any course. Companies invent problems from their own product. If your preparation was a set of finished answers, an unfamiliar question leaves you with nothing to start from.

This is also the difference between a candidate who is offered a senior role and one who is not. More on the other common causes in why people fail system design interviews.

The ten minute test

There is one reliable way to find out which of the two you have. Do this with any design you believe you know.

1. Cover the solution completely.
2. Spend ten minutes writing your own requirements, your rough capacity numbers, and a first architecture.
3. Uncover the solution and compare.

Now read the comparison correctly. The point is not whether your design matches theirs. It almost never will, and it does not need to.

The question is whether each difference was a decision you can defend. If you can say why you chose a different store, and what you traded away, you learned the method. If your page is blank until you look, you learned a diagram.

Run this test before you finish any course, including ours. A course that leaves you unable to reach its own designs on your own has not finished teaching you.

What is worth memorizing

Some memorizing helps, and pretending otherwise is not useful. Three short lists are worth knowing by heart.

Rough numbers. Common latency figures, and the arithmetic for turning users per day into writes per second. These come up in the first five minutes of most answers, and deriving them live wastes time you need later.

The building blocks. Load balancers, caches, queues, blob stores, rate limiters, and the handful of others. Know what each one solves and what it costs, because these are the pieces every design is assembled from.

Your own order of work. Requirements, then scale, then a first design, then the deep dive, then trade-offs. Knowing your order by heart is what stops a hard question from producing a blank first minute.

What does not belong on the list is a finished architecture for a named product. That is the output of the method, and it is the one part you should be able to produce rather than recall. Practicing trade-off reasoning is a better use of the same hour.

How to Prepare

• Practice producing, not reading. For every design you read, do one cold from a blank page. Two designs you can rebuild are worth more than twenty you have read.
• Learn the parts before the products. If load balancers, caches, and queues are still fuzzy, start with Grokking System Design Fundamentals rather than with case studies.
• Use a repeatable method. Grokking the System Design Interview teaches the interview approach and the trade-offs first, then works through fifteen complete designs. Each design is practice for the method, not an answer to store.
• Rehearse the requirement change. Ask a friend to alter one number partway through your answer. Getting used to redesigning under pressure is the single highest value thing you can practice.
• Pick a course by whether it makes you produce. Before you buy anything, apply the ten minute test to its free material. The comparison of the main system design courses covers what each one gives you and what it costs.
• Work from a wide question bank. The system design interview library gives you enough unfamiliar problems to keep testing yourself on questions you have not already read.