What goes in a system design document?

A good design doc states the problem and goals, lists functional and non-functional requirements, estimates scale, describes the high-level architecture and its components, sketches the key APIs, and records the trade-offs and rejected alternatives so reviewers can judge the decision.

How detailed should a design doc be?

Detailed enough that a competent engineer who was not in the room could implement it, and that a reviewer can spot the risky decisions. Skip implementation minutiae that will change; focus on interfaces, data, scale, and the choices that are hard to reverse.

Why document trade-offs and rejected alternatives?

Recording what you considered and rejected, and why, prevents the team from re-litigating the same options later and shows reviewers you weighed the decision. It is often the most valuable section because it captures reasoning that code cannot.

What are non-functional requirements?

Non-functional requirements describe qualities rather than features — latency, availability, throughput, security, cost, and compliance. They drive most architecture decisions, so a design doc that lists only features usually misses the hard parts.

Is my document sent anywhere?

No. The document is assembled in your browser from the fields you complete. Nothing is uploaded, so you can draft designs for confidential systems privately.

System Design Document Builder

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The System Design Document Builder assembles a complete, review-ready design doc from structured inputs. A design doc is the artefact a team reviews before writing code; getting its sections right — especially scale and trade-offs — is what turns a vague feature into an implementable, agreed-upon plan.

How it works

The builder follows the canonical design-doc outline used in engineering reviews. You provide the problem statement and goals, functional requirements (what it must do) and non-functional requirements (latency, availability, security, cost), rough capacity estimates, a high-level design with its components, an API sketch of the key endpoints, and the trade-offs with rejected alternatives. The tool renders these into a numbered document so every reviewer reads the same structure and nothing important — like the scale numbers that drive most architecture decisions — gets skipped.

Tips and example

Lead with requirements, not solutions. Pin down what success means before describing components, or you will design for the wrong constraints.
Put real numbers in capacity. “High traffic” tells a reviewer nothing; “5,000 requests per second, 200 GB/year growth” lets them sanity-check your design.
Make the trade-offs section honest. List the option you rejected and the specific reason — cost, latency, operational burden — so the decision survives scrutiny.

Example: a design for a URL shortener might require sub-50ms redirects at 10k QPS, choose a key-value store over a relational DB for read latency, and explicitly reject a counter-based ID scheme for being a single point of contention.