A Compact Approach for Each Round

Four steps that work for both

Use the same skeleton for both rounds. (1) Requirements: clarify functional scope and non-functional goals (scale, latency, consistency); write them down. (2) Entities and APIs: for HLD, list core data entities and the main API calls; for LLD, list classes and their key methods. (3) Design: for HLD, draw the request path and data stores, then deep-dive into the hardest part; for LLD, define interfaces and relationships, then implement the core flow. (4) Trade-offs: explain alternatives you rejected, bottlenecks and failure modes. The System Design Interview Prep course covers estimation and building blocks in depth; here we apply them.

A time budget for a 45-minute round

Adjust to the interviewer; these are guidelines, not rules.

HLD (45 min)                         LLD (45 min)
 0-5   requirements + scale           0-5   requirements + use cases
 5-10  entities, APIs, estimates      5-12  classes, relationships
10-25  high-level diagram            12-35  interfaces + core code
25-40  deep dive (1-2 components)    35-42  edge cases, concurrency
40-45  trade-offs, failures          42-45  extensions, trade-offs

Say the plan out loud

Telling the interviewer your plan in one sentence lets them redirect you early, which saves far more time than it costs.

Quick check: What should come first in either round?

• Clarifying requirements and scope
• Writing the database schema
• Picking a message broker
• Implementing the hardest method