Open-Closed Principle in Interviews: Handling a Mid-Design Requirement Change

Twenty minutes into your LLD design, the interviewer smiles: “Also support coupon codes.” Or WhatsApp notifications. Or surge pricing. They are not being cruel — they are testing whether your structure bends or breaks.

That test is the Open-Closed Principle in interview clothing: open for extension, closed for modification. This article shows how to place seams so “one more requirement” stays small.

01What Interviewers Measure with a Twist

• Did you hard-code the volatile decision?
• How many boxes turn red when the twist arrives?
• Can you extend with a new type instead of editing five methods?
• Do you explain the trade-off of the seam you chose?

02Find Volatility Before You Draw

During clarification, ask what is likely to change. Classic volatilities in LLD prompts:

You cannot abstract everything. Abstract the axes the story emphasizes.

03Worked Mini-Example: Pricing Twist

Initial ask: flat fare for rides. Twist: support surge multipliers at peak hours.

// Fragile
FareService.calculate(distance) {
  return distance * RATE;
  // later: sprinkle if (peak) everywhere
}

// OCP-friendlier
FareService depends on PricingStrategy
FlatPricing implements PricingStrategy
SurgePricing implements PricingStrategy

The twist adds SurgePricing (and maybe a factory/selector). FareService stays mostly closed.

04Blast Radius Checklist After a Change

1. List every type you edited.
2. If the list includes stable domain entities for a peripheral rule, your seam was wrong.
3. Prefer adding a type over editing a switch.
4. Re-run the original use case mentally — OCP is not an excuse to break old paths.

Change One Thing on LLDCanvas drops a new requirement mid-design and scores how small your blast radius stays against an OCP-compliant ideal.

05Five Interview Twists and Low-Blast Responses

When you practice, time how long the twist takes. Under five minutes with a clear narration is a strong signal. Fifteen minutes of redrawing usually means the first design hid the volatility inside concrete types.

What to say when you did not leave a seam

Be honest: “I hardcoded email sends inside OrderService, so this change forces an edit there. I would extract a Notifier interface now, migrate the email impl, then add SMS.” Interviewers respect a clean recovery more than a pretend-perfect first draft.

06OCP vs Long-Term Evolution

OCP drills are local: one change, minimal edits. Multi-stage product growth — new markets, debt, scale — needs a longer lens. Continue with evolving software design as requirements grow when you are ready for multi-stage thinking.

07Key Takeaways

• Requirement twists test OCP and calm under pressure.
• Identify volatility early; place interfaces there.
• Measure success by blast radius, not by pattern count.
• Extend with new types when the variation is behavioral.

Practice absorbing one surgical change at a time in Change One Thing.