A design starts with users, operations and quality constraints. Clarify scale and correctness before drawing infrastructure.
Before you start
You should understand API requests, storage and basic capacity estimates. Begin with a concrete user action and its correctness requirement. Draw data flow and failure boundaries before selecting infrastructure; a technology name by itself does not explain why a design meets the requirement.
The practical goal is to reason through this situation: A read-heavy catalog and a financial ledger have different consistency and recovery needs. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Define core user actions
Identify the operations the system must actually support.
Step 2: Clarify correctness constraints
Ask what may be stale, lost, duplicated or unavailable.
Step 3: Estimate workload
Use explicit scale and latency assumptions before selecting components.
Worked scenario
A read-heavy catalog and a financial ledger have different consistency and recovery needs.
A catalog can serve briefly stale descriptions, while a ledger must preserve financial entries under failure. Both may have high read traffic, but that similarity does not justify identical storage and recovery rules. Write the important guarantees beside the request flow before discussing caches or replication.
Common mistake
Listing popular technologies does not explain whether requirements are met.
Verify the behavior
Trace one user action and confirm every component supports a stated requirement.
Interview exercise
Open a design interview.
Answer and reasoning
Ask about core operations, expected load, latency, availability and which data must never be lost or stale.
Continue learning
Compare the scenario with the System Design interview questions and test your understanding with the System Design MCQs. For terminology and implementation details, consult the reference material.