If there is one place where most system design mistakes begin, it’s here:
Designing without fully understanding the problem.
It sounds obvious, but in practice, this is exactly what happens.
We hear a problem statement and immediately start thinking about classes, APIs, or databases.
The result?
We end up solving the wrong problem very efficiently.
Why requirements matter more than design
Imagine building a food delivery system.
If you assume:
- users can pre-schedule orders
- restaurants can reject orders
- delivery must happen within 30 minutes
…but none of these were actual requirements,
then your entire design is based on assumptions, not reality.
And once design is wrong, everything built on top of it will also be wrong.
The three layers of understanding
Before you design anything, you need clarity across three dimensions.
1. Functional Requirements (What the system should do)
These define the core behavior.
Examples:
- Users can browse restaurants
- Users can place orders
- Restaurants can update order status
- Delivery agents can mark delivery complete
This is the visible behavior of the system.
2. Non-Functional Requirements (How well it should work)
These define system quality.
Examples:
- Response time (e.g., < 1 second)
- Availability (system should always be up)
- Scalability (handle thousands of users)
- Consistency (accurate order status)
These are often ignored, but they drive major design decisions.
For example:
- High availability may lead you to relax strict consistency
- Low latency may require caching
3. Constraints & Assumptions
These define the environment in which your system exists.
Examples:
- Users are primarily on mobile devices
- System operates in one city initially
- Payments are handled via third-party providers
Assumptions are especially important in interviews.
If something is unclear, don’t guess silently—state your assumption.
The most common mistake
Jumping to solution mode too early.
You’ll often see this pattern:
“Let’s create a User class… then a Booking class…”
This skips the most important step:
Understanding what actually needs to be built.
A better approach
Before designing, ask questions like:
- Who are the users of this system?
- What actions can they perform?
- What does success look like?
- What scale are we operating at?
- Are there any performance expectations?
You’re not delaying progress—you’re preventing rework.
A subtle mindset shift
Instead of asking:
“How do I design this?”
Start with:
“What exactly am I designing?”
That clarity changes everything.
Closing thought
A well-designed system starts with well-understood requirements.
If you get this step right, design becomes clearer.
If you get this step wrong, no amount of clean code can save you.