System design interviews: a framework senior engineers use

The framework

1. Clarify. Ask what the system must do, for whom and at what scale. State your assumptions.
2. Define the interface. Sketch the main operations and the data they use.
3. Draw the simple version. One box, one database. Make it work first.
4. Find the bottleneck. Where does it break as load grows? Data size, writes, hot keys, latency?
5. Evolve the design. Add caching, partitioning, queues or replication, each in answer to a named problem.
6. Discuss trade-offs and failures. What does each choice cost, and what happens when a part fails?

What interviewers look for

• Questions before answers.
• Reasoning about trade-offs, not name-dropping technologies.
• Awareness of failure, cost and operations.
• Clear communication and willingness to change course.

Common mistakes

• Starting with a complex diagram.
• Adding components with no stated reason.
• Ignoring data consistency and failure.
• Not checking the design against the requirements at the end.

Practise

Design systems you already use, out loud, with a friend. Write down what you would change after the first user and after the thousandth. See architecture decisions are expensive to undo.

IICTP is a new independent credential and is not ISO/IEC 17024 accredited.