02Key conceptsRequirements
02 · Core method
Requirements
Clarify users, critical flows, constraints, traffic, storage, latency, and availability before drawing boxes.Lesson spine
What you need to understand.
Requirements are the compression step that turns an infinite product into one interview-sized system.
Functional requirements
Name the few user or service actions the design must complete. Prioritize them; do not collect an unbounded wish list.
Non-functional requirements
Attach numbers or ordering to latency, availability, consistency, durability, freshness, privacy, and cost.
Back-of-envelope scale
Estimate peak requests, writes, stored bytes, bandwidth, active connections, and growth. Precision matters less than exposing the dominant resource.
Constraints and non-goals
State what the first version will not solve. A boundary is a design decision, not an apology.
A reliable opening
Confirm users and use cases, pick core features, quantify scale, and restate the contract before drawing.
Before the boxes
Frame the decision.
What must work
Clarify users, critical flows, constraints, traffic, storage, latency, and availability before drawing boxes.
What changes the design
Use peak QPS · concurrency · object size · retention · read/write ratio
What owns the truth
Identify the component that commits authoritative state, then separate synchronous confirmation from derived work.
What stays simple
Do not add global coordination, multi-region writes, or a specialized store until a requirement earns the complexity.
Architecture map
Trace ownership, not just traffic.
Follow the decision from left to right. Every arrow should have a reason.
Name who initiates work
Prioritize must-have flows
Set latency and availability
Estimate peak load and storage
Record privacy and geography
Tie numbers to choices
Walk one representative request across every arrow. Say whether the handoff is synchronous or asynchronous, what identity makes a retry safe, and which step changes authoritative state.
- 01
Actors — Name who initiates work Define the output contract before moving to the next owner.
- 02
Functional scope — Prioritize must-have flows Define the output contract before moving to the next owner.
- 03
Quality bar — Set latency and availability Define the output contract before moving to the next owner.
- 04
Volume — Estimate peak load and storage Define the output contract before moving to the next owner.
- 05
Constraints — Record privacy and geography Define the output contract before moving to the next owner.
- 06
Decision log — Tie numbers to choices Confirm the result and emit the evidence needed to reconcile it.
Decision table
Make the trade-offs explicit.
| Decision | Defensible position | Cost to acknowledge |
|---|---|---|
| Primary mechanism | Estimate only quantities that can change a design decision. | The stronger guarantee usually adds coordination, latency, state, or operational work. |
| Sync vs. async | Keep only correctness-critical confirmation synchronous. Move derived views, notifications, analytics, and cleanup behind a durable boundary. | Async work needs idempotency, lag monitoring, replay, and a product definition for partial completion. |
| Simple vs. scaled | Begin with one logical owner and a clear API. Partition or replicate only the resource proven to be the first bottleneck. | Migration requires stable identities, versioned contracts, backfill, and a rollback path. |
Failure review
Design the recovery path.
Topic-specific risk
Averages hide bursts, fan-out, hot keys, and payload size.
ResponsePersist enough identity and state to distinguish retry, resume, compensation, and operator repair.
Dependency timeout
A timeout is ambiguous: the remote side may have failed, succeeded, or still be running.
ResponseUse deadlines, bounded backoff with jitter, idempotency keys, and a status or reconciliation path.
Overload or skew
Average capacity can look healthy while a tenant, key, partition, region, or expensive request saturates one owner.
ResponseExpose queue depth and hot-key share, apply backpressure, isolate tenants, and degrade optional work before correctness.
Evidence + level bar
Prove the design can be operated.
Health of the promise
Measure user-visible latency or freshness, correctness drift, saturation, retry volume, and time to recover. Alert on the failed promise—not only CPU.
Complete and clear
Finish the happy path, identify the state owner, choose reasonable building blocks, and explain one scale mechanism.
Trade-offs and failure
Separate read and write paths, define consistency, explain partitioning, and make duplicate or partial failure safe.
Evolution and operations
Discuss multi-region boundaries, migration, tenant isolation, capacity, observability, and how the architecture changes over time.
Interview language
Open the deep dive with a claim.
“For Requirements, the decision I want to make explicit is this: Estimate only quantities that can change a design decision. I’ll trace the state-changing path first, show where the result becomes durable, then test the design against the highest-risk failure and our target scale.”
08 · Retrieval check
Can you defend it without the page?
- For Requirements, where is the correctness boundary and which failure would you test first?
- Which component owns committed truth, and what event or response proves the commit?
- Where is the first scaling or coordination bottleneck under the stated envelope?
- What happens after an ambiguous timeout or duplicate operation?
- Which complexity would you remove at one hundredth of the scale?