If you’re preparing for a system design interview, there’s a good chance you’ve already wondered:
Will I need to write code?
It’s a fair question, and one that confuses even experienced candidates. After all, you’re asked to design systems, not implement them, right?
In this guide, we’ll break down:
- Whether system design interviews involve coding (and why it depends)
- What kind of technical depth is expected
- How much pseudo-code or notation should you prepare for
- How to build confidence even if you’re not coding line by line
Let’s set the record straight, so you know what’s expected before you’re caught off guard.
The short answer: Most system design interviews don’t require coding
In standard system design interview questions, especially for mid- to senior-level roles, you’re not expected to write actual code, as in syntax-perfect, compilable code.
Instead, you’re expected to:
- Sketch a high-level architecture
- Discuss component responsibilities
- Evaluate tradeoffs
- Reason through scale, reliability, and consistency
You’ll use diagrams, whiteboards, and spoken reasoning, not IDEs or text editors.
But there are exceptions. And knowing where those live is key.
When system design interviews do involve code (and when they don’t)
Let’s clarify the difference across common interview types:
| Interview type | Involves coding? |
|---|---|
| Traditional system design (mid+ level) | ❌ No |
| L4+ interviews at Big Tech | ❌ Not required |
| Low-level design interviews | ✅ Sometimes |
| Architecture + API design rounds | ✅ Pseudo-code |
| Staff+ roles with infra focus | ✅ Occasionally |
Here’s the nuance:
- For high-level system design interviews, the focus is on architecture, not implementation.
- In low-level design interviews, you may be asked to model classes, interfaces, or pseudo-code specific components, like a rate limiter or cache eviction mechanism.
- In some API design rounds, you might outline endpoints and their interactions in REST or gRPC style. This resembles code but doesn’t require compiling or debugging.
Why some interviewers ask for pseudo-code or class design
System design isn’t just about drawing boxes. It’s also about:
- Thinking in abstractions
- Understanding interface boundaries
- Explaining data flow in tangible terms
So while you may not be asked to implement a database or message queue, you could be asked:
- “How would the class for this queue look?”
- “Can you sketch how the client interacts with this API?”
- “What data structures would you use for this component?”
In these moments, your ability to reason in code matters, even if you’re not writing a full program.
What to expect instead of actual coding
Instead of compiling syntax, you’ll demonstrate your thinking through structured communication. Expect to:
1. Draw diagrams that tell a story
The most fundamental expectation in a system design interview is that you’ll diagram your solution. Whether you’re using a physical whiteboard or a digital tool like Miro or CoderPad, your architecture should be visual, modular, and narrative-driven.
You’ll need to illustrate:
- Core components: databases, queues, APIs, caches, etc.
- How they interact: request-response flow, pub-sub relationships, or async pipelines
- Data direction and frequency: who talks to whom, how often, and why
- Failure points: where things might go wrong and how to handle them
Interviewers are watching for:
- Clarity of flow
- Justification for each component
- Understanding of bottlenecks and latency points
“Think of your diagram like a blueprint: it should explain how the system runs, scales, and fails, all at a glance.”
2. Walk through the interface design or API endpoints
While you won’t write backend logic, you’re often expected to sketch how clients interact with your system. That means outlining APIs, service contracts, or internal interfaces.
For example, if you’re designing a URL shortener, your API sketches might look like:
Or in a low-level component like a cache:
You don’t need to write the implementation—just show that you understand how the system is used, and what the boundaries look like.
Interviewers are watching for:
- Thoughtful input/output design
- Clean abstraction layers
- Realistic use cases and error handling
3. Explain logic using step-by-step reasoning
Sometimes, you’ll need to explain how a component behaves internally, not in code but in logical steps.
For example, if you’re asked to design a rate limiter, you might outline:
- For each user, store a list of timestamps for recent requests
- On each new request, remove timestamps older than the rate window
- If the number of remaining timestamps is below the threshold, allow the request
- Otherwise, reject the request with an error code
Or if you’re designing a cache:
- Check if the key exists
- If yes, return the value
- If no, fetch from source, store in cache, and return
This level of clarity shows algorithmic maturity, even without coding.
Interviewers are watching for:
- Logical consistency
- Awareness of edge cases
- Time and space complexity reasoning
4. Describe data flows and formats
Understanding how data moves through a system is key to high-quality design. You may be asked to describe:
- What a request payload looks like
- How messages are formatted between services
- What schema is used in the database
- How different formats (JSON, Protobuf, Avro) impact performance
Example:
“For real-time chat, I’d use Protobuf to reduce message size and parsing latency between services.”
Or:
“I’d structure the order object with user_id, item_ids, and timestamp fields for downstream analytics.”
This shows you’re not just thinking in boxes, but you also understand the shape and semantics of the data being handled.
Interviewers are watching for:
- Format efficiency
- Schema design
- Impact on speed, storage, and extensibility
5. Talk through scaling, replication, and failure scenarios
Coding isn’t required, but engineering foresight is. You’ll need to discuss how your design holds up under real-world pressures.
Expect to be asked:
- “What happens when traffic spikes 10x?”
- “How do you ensure high availability?”
- “How would you handle regional failover?”
This is your chance to talk about:
- Horizontal vs vertical scaling
- Leader-follower replication
- Circuit breakers and retries
- Sharding strategies
- Eventual consistency patterns
You might not write failover code, but you should talk like someone who has supported a system during failure.
Interviewers are watching for:
- Awareness of operational complexity
- Comfort with distributed systems concepts
- Tradeoff reasoning (latency vs durability, availability vs consistency)
How to prepare without memorizing code
Here’s how to build the kind of technical depth that impresses, even without coding:
Practice breaking down components
Use mock prompts like “design a ride-sharing app” or “design a content delivery network.” Focus on:
- How components talk to each other
- What data is stored and retrieved
- Where bottlenecks might appear
Get fluent in API and object modeling
Even if no code is required, expressing technical concepts clearly matters:
- Sketch how APIs respond to requests
- Model a service’s interface in simple method signatures
Study common building blocks
Know how caches, databases, queues, load balancers, shards, and replicas work at a high level. You don’t need to code them—you need to use them well in design.
What interviewers really care about
It’s not about whether you can code on the spot. It’s about whether you can:
- Think like an engineer
- Decompose large problems
- Make tradeoffs and justify them
- Communicate your design under time pressure
The most common feedback from system design interviewers isn’t:
“They didn’t write code.”
It’s:
“They didn’t explain their choices clearly.”
“They didn’t identify scaling bottlenecks.”
“They didn’t demonstrate depth on core components.”
So your goal isn’t to be a walking compiler—it’s to be a thoughtful architect.
Final answer: Does a system design interview involve coding?
Usually not. But you will need to think like a programmer.
- You won’t write syntax-perfect Java or Python.
- You may sketch pseudo-code, interface methods, or endpoint flows.
- You definitely need to reason clearly and technically.
So don’t worry about brushing up on LeetCode before a system design round. Instead, practice the modern system design interview techniques, architecture, and real-world systems.
The interview isn’t about writing code—it’s about proving you know how systems work, why they’re built that way, and how to scale them when they break.
