A design round is a conversation that candidates treat as a lecture. The lecture feels safe because nobody can interrupt a person who never breathes. It is also how you spend twenty minutes on the load balancer and never hear which part they cared about. Three to seven minutes is enough to show a shape. It is not enough to finish the system, and that is intentional.
What belongs in the first pass
Who uses it, and the one request that must be fast. The API for that request, in words, including what the client sends and what it gets back. The data you store for that request, and what you refuse to store yet. One bottleneck, named out loud, with the thing you would add next if they want to spend the rest of the hour there. Caching, queues, and regions come after they point.
A clock check: at two minutes you should have said the user and the request. At four minutes you should have said the data. If you are still estimating QPS at minute five, park the number and draw the box. Estimates matter, and they can be one sentence with the assumption labeled.
How to practice
Set the type to System design so the range reads 180–420 seconds. Answer a prompt from the hub, or paste the prompt you were given last time. Stop at seven minutes even if the diagram in your head is unfinished. The unfinished part is what they will ask about. If you cannot stop, you do not have a first pass. You have a script.
Pace can be a little slower here, around 110–150 words per minute. Pauses under two seconds are useful before a tradeoff. Pauses past four seconds, with no bridge, sound like the design left you. Say “The part I would cut first is” and name it.
Luna can sit on a real design round and hand you a line when you freeze. This page is where you learn the length of your own first pass before you need that.
FAQ
Is seven minutes too short for system design?
It is too short for the whole round. It is the right length for the opening pass. The rest of the time belongs to their questions.
Should I start with capacity numbers?
One labeled estimate is enough inside the first pass. A five-minute calculation before the API is a way to avoid the design.
What if they want me to go deeper immediately?
Follow them. The range is a default for practice, not a rule you recite over their question.