Approaches to conducting design interviews

Step 1: Formalization of requirements

At this stage you clarify functional requirements (what the system should do) and non-functional requirements (how she should do it). This is a critical step because without it you risk designing the wrong system.

Non-functional requirements (-ilities)

• High Availability — the system must be available 99.9%+ of the time
• Scalability — the system must cope with increasing load
• Performance — latency p99 < 100ms or other SLO
• Durability — data should not be lost
• Consistency — eventual vs strong consistency - an informed choice
• Fault Tolerance — the system experiences component failures

Step 2: System Boundaries (Public API)

Determine how the outside world will interact with your system. This is a contract between the system and its clients.

Step 3: Core Flows and Components

Draw the main data flows in the system. Start with happy path — main use case and then add exceptional flows.

• Read path — how data is read from the system (caches, replicas)
• Write path — how data is recorded (queues, logging)
• Happy path — main successful scenario
• Exceptional flows — error handling, retrays, fallbacks

Step 4: Conceptual Data Diagram

Design a data model without committing to specific technologies just yet. Identify the main entities and the relationships between them.

Step 5: Real Circuit (Technology)

Now we choose specific technologies. It is important to understand the trade-offs of each decision and be able to justify your choice.

Step 6: Scaling

When the basic architecture is ready, discuss how the system will scale with load growth of 10x, 100x, 1000x.

• Vertical scaling — increasing the resources of one machine (easier, but limited)
• Horizontal scaling — adding new instances (more difficult, but unlimited)
• Data partitioning — separating data by key (user_id, region, time)
• Sharding — distribution of data across several databases
• Consistent hashing — minimizing redistribution when adding nodes

Step 7: Advanced Topics

If there is time left, discuss topics for long-term system operation:

• Observability — metrics, logs, traces - how to understand that the system is healthy
• Alerting — what to be alert for and how not to get drowned in noise
• Deployment — blue-green, canary, feature flags
• Disaster Recovery — RTO/RPO, backup strategies, failover
• Security — authentication, authorization, encryption at rest/in transit

Tips for a successful interview

I've put together a short list of tips that I think will improve your efficiency. interview and the likelihood that you will be able to show your best sides:

• Remember the time — The interview time is usually limited to an hour and a lot needs to be discussed during it. That is why you should not get bogged down in any details, but control the timing and move towards solving the problem
• Don't start designing right away — It is important to clearly understand the problem and only then begin to solve it. If you have questions after reading the terms and conditions, it is better to get answers to them first, otherwise you may start a task that is not the one you were asked to do.
• Share your thoughts — This type of interview is needed in order to understand how you solve complex problems and how you think when solving them. If you mostly silently draw a diagram and occasionally tell what you have drawn, then the interviewer will not understand the full depth of your idea
• Show independence in design — this is an open interview where the candidate can show his strengths and show how he designs. But if the interviewer has to guide the candidate through the task, giving structure and helping with the solution, then this will not add points to the candidate
• Respond to leading questions — usually the interviewer with such questions tries to lead the candidate to the right thoughts when he has stalled
• Speak confidently about what you know — You shouldn’t talk about something you’ve only vaguely heard about, as the interviewer may ask for details and then the candidate will have to blush
• Prepare questions for the interviewer — in case there is time left for them - this shows that you are interested in learning about the company and what is happening in it

Long-term training: what to read

For a deep understanding of System Design, we recommend studying professional literature. Books provide a foundation that will not be out of date in a year and help you understand the principles and not just remember ready-made solutions.