Stop Saying This in System Design Interviews
System design interviews aren’t about showing off buzzwords.
They’re about showing clear thinking, trade-offs, and real-world judgment.
Yet many candidates fail not because they don’t know enough —
but because they repeat popular myths that signal shallow design thinking.
Here are 10 of the most common system design mistakes — and what interviewers actually want instead.
1. “Let’s Use NoSQL Because It Scales More”
This is the most common myth.
Truth:
NoSQL is not “more scalable” by default. It trades:
- Consistency
- Transactions
- Joins
for horizontal scale.
If your system needs:
- Strong consistency
- Financial accuracy
- Complex relationships
Then SQL may be the better choice.
Interviewers want:
Your reasoning, not your buzzwords.
2. “We’ll Just Add Caching to Fix Performance”
Caching is not a silver bullet.
Blindly adding Redis without:
- Cache invalidation strategy
- TTL policy
- Consistency plan
creates data bugs.
Interviewers want:
You to ask what is slow before adding layers.
3. “Microservices Are Always Better”
Microservices introduce:
- Network latency
- Distributed failures
- Debugging complexity
Starting with microservices for a small system is overengineering.
Interviewers want:
Monolith first → extract later.
4. “We’ll Shard the Database for Scale”
Sharding increases:
- Operational complexity
- Data movement
- Cross-shard queries
It should be the last scaling step.
Interviewers want:
Vertical scale → read replicas → caching → sharding.
5. “Let’s Use Kafka for Everything”
Kafka is not a queue, not a database, and not a cache.
It’s great for:
- Event streaming
- Analytics pipelines
- Asynchronous workflows
But terrible for:
- Request/response
- User-facing latency
- Simple tasks
Interviewers want:
Right tool for the right job.
Fun fact : For async processing, SQS might be a better choice in a lot of cases while keeping the complexity low and providing error handling / alerting automatically via DLQs (Dead letter queues)
6. “Eventually Consistent Is Fine”
This sounds smart but is dangerous.
Ask:
- What happens when users see stale data?
- What business rules break?
- What guarantees are required?
Interviewers want:
You to think about correctness, not just scale.
7. “We’ll Add Load Balancers and Auto-Scaling”
This is infrastructure — not architecture.
Without:
- Stateless services
- Idempotent APIs
- Health checks
auto-scaling will fail.
Interviewers want:
System behavior, not cloud features.
8. “We’ll Just Store Everything in One Service”
This creates:
- Tight coupling
- Deployment bottlenecks
- Scaling pain
But splitting too early is also wrong.
Interviewers want:
Logical boundaries with a growth plan.
9. “Security Can Be Added Later”
Security is not a feature — it’s a requirement.
Skipping:
- Authentication
- Authorization
- Rate limiting
- Encryption
is a red flag.
Interviewers want:
Security thinking from the start.
10. “We’ll Handle Failures If They Happen”
In distributed systems, failures always happen.
Ignoring:
- Timeouts
- Retries
- Circuit breakers
- Graceful degradation
means your system will collapse under load.
Interviewers want:
You to design for failure, not perfection.
Final Advice
System design interviews test judgment, not vocabulary.
If your answer starts with:
“We’ll use X because it’s modern”
You’re already in trouble.
Start with:
- Requirements
- Constraints
- Trade-offs
That’s how real systems are designed.
If you want to practice real system design problems (not buzzword answers), InterviewTruth is built for you.