The Ultimate Guide to Engineering Design Docs

The most expensive mistake in software engineering is writing weeks of code for the wrong solution. The strict discipline of writing a Design Document (also known as an RFC - “Request for Comments”) is the antidote to this common failure mode.

“Code is easy. It’s the thinking that’s hard.”

A Design Doc is not documentation; it is a tool for thinking.

Why Write Them?

Async Consensus: Avoiding 2-hour meetings where nothing is decided.
Historical Context: 6 months from now, you will ask, “Why on earth did we choose NoSQL?” The doc answers that.
Force Multiplier: It allows senior engineers to scale their impact by reviewing architecture without reading every line of code.

The Anatomy of a Perfect Design Doc

A great design doc follows a predictable structure.

1. Context & Scope

Objective: A 2-sentence summary of what we are building.
Background: Why are we doing this? Link to product specs or tickets.
Non-Goals: Explicitly state what you are not doing. This prevents scope creep.
Success Criteria: How will we measure if this design worked? (e.g., “P99 latency stays under 200ms at 10k RPS”).

2. The Proposed Design

This is the meat of the document.

High-Level Architecture: A diagram (Mermaid.js or system diagram) showing how components interact.
API Design: Define the endpoints (GET /users/:id), payloads, and error codes.
Data Model: Schema definitions, table relationships, and storage choices.

3. Alternatives Considered (The Most Important Section)

This is where you prove you did your homework. Don’t just list your choice; list the other valid ways to do it and why you rejected them.

Approach	Pros	Cons	Verdict
Option A: Use Redis	fast, simple	data loss risk	Rejected
Option B: Postgres	reliable, relational	slower writes	Selected

4. Operational Excellence

Often ignored, but critical for keeping the system running.

Rollback Plan: If we deploy this and the database CPU spikes to 100%, how do we undo it immediately?
Scalability: How does this design handle a 10x traffic spike? Where is the first bottleneck?
Cost Estimate: Will this increase our cloud bill significantly? (e.g., “Adding 5TB of SSD storage = +$300/mo”).

5. Cross-Cutting Concerns

Do not skip these.

Security & Privacy: AuthZ/AuthN, PII handling. Vital in 2026: Include Data Residency (GDPR/CCPA) and AI compliance.
Observability: What metrics will we track? How do we know it’s broken?
Migration: How do we move from the old system to the new one with zero downtime?

A Template for You

Feel free to copy this markdown for your next RFC.

# [RFC] Title of Feature

## Summary
Brief explanation...

## Motivation
Why are we doing this?

## Detailed Design
### API
### Database

## Alternatives Considered
1. ...
2. ...

## Security & Privacy
...

The Review Process

Sending the doc is just the beginning.

Rule of 24h: Reviewers should aim to provide feedback within 24 hours.
Comment on Logic, Not Grammar: Focus on race conditions, scalability bottlenecks, and data integrity.
Resolve Conflicts: If a comment thread gets too long, hop on a call to resolve it, then document the decision back in the doc.

Writing is the highest leverage skill for a software engineer. Master the design doc, and you master the ability to influence technical direction.