See who can design a system that scales
System design rounds are time-consuming, and they usually need a senior engineer in the room. Codility’s system design tasks show you before the interview: candidates build infrastructure on a board, send a rush of traffic through it and get a score automatically, with no code to write. The demo below is an interactive reproduction of those tasks: watch it play for a minute, then try a mission yourself.
What the demo shows
A reproduction of one mission for a fictional food delivery app, start to finish: a design that fails, the troubleshooting and a design that passes.
- 01
A brief from the business
Each mission sets the traffic to handle, the success rate to hit and a budget for the infrastructure.
- 02
Built on a board, with no code
Candidates add servers, load balancers, caches and databases from a catalog, then wire them up so requests flow from the internet to the database.
- 03
Test the design under traffic
The Test system button sends the mission’s traffic through the design, like a load test. Candidates can test as often as they like.
- 04
A failed test shows symptoms
It names each overloaded machine, with its load and its capacity. Finding the fix is up to the candidate.
- 05
Cost is part of the answer
Every machine has a price. A passing design under the cost target earns a bonus, so a lean design scores higher than an oversized one.
- 06
The same scoring for every candidate
The score counts the missions passed, plus the cost bonus. Every candidate is scored by the same rules, so you can compare results across a whole hiring round.
Two system design tasks in the Codility library
Both use the same board, and each asks a different question.
InfraLab: will the infrastructure hold up under traffic?
Built for DevOps, cloud and site reliability engineers (SRE).
Candidates decide what goes where, what happens when a machine fails and what it costs.
TicketDrop: does the system get every order right at peak traffic?
Built for backend engineers who will own a whole system.
Candidates run a concert ticket shop, and test results show symptoms such as seats sold twice or a slow checkout. Working out why is the candidate’s job.
Before each submit, the candidate writes notes for the reviewer, so your team sees their thought process as well as the result.
The AI Assistant can explain concepts such as queueing or idempotency. It cannot see the board, so the design stays the candidate’s own.
Prefer to talk a design through live? Codility Interview has a whiteboard for system design interviews.
Built on almost two decades of technical assessment
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Questions about system design assessments
A system design assessment checks whether a candidate can design infrastructure that meets a goal, such as a traffic target, within limits such as a budget. In Codility’s system design tasks, candidates build the design on a board, and a test run shows whether it holds up.