title: Architecture & Design Documentation status: draft version: 0.1 owner: Engineering last_updated: 2026-01-05 ————————

Architecture and design documentation¶

This guide defines how we document architecture and design decisions so systems remain understandable, evolvable, and safe to change over time.

Architecture documentation is not about creating perfect diagrams or heavyweight documents. It is about making structure, intent, constraints, and trade-offs explicit at the right level of detail.

Purpose¶

Architecture and design documentation exists to:

Explain how the system is structured
Make design intent and constraints explicit
Support reasoning about change and impact
Enable informed technical decision-making
Preserve context beyond individual contributors

If a design decision cannot be explained clearly, it is probably not ready.

Scope¶

This guide covers:

System and software architecture documentation
Significant design decisions and trade-offs
Evolution of architecture over time

It does not replace detailed implementation documentation or code-level comments.

Core principles¶

Document decisions, not just diagrams
Match detail to risk and longevity
Prefer simple, stable views over exhaustive detail
Keep documentation close to the code
Update documentation when the architecture changes

Architecture versus design¶

We distinguish between architecture and design to avoid confusion.

Architecture describes the fundamental structure, boundaries, and responsibilities
Design describes how a specific problem is solved within those boundaries

Architecture changes slowly. Design changes more frequently.

When to write architecture documentation¶

Architecture documentation is required when:

Introducing a new system or major component
Changing system boundaries or responsibilities
Adding cross-cutting concerns such as security, data persistence, or messaging
Making decisions that constrain future design choices

For small, local changes, lightweight design notes are sufficient.

Levels of documentation¶

We use multiple levels to keep documentation useful and navigable.

System context¶

The system context shows what the system is and what it interacts with.

External systems
Users and actors
High-level responsibilities
Clear system boundary

mermaid diagram

This view should be stable and easy to understand by non-specialists.

Container and component views¶

These views explain the internal structure at a high level.

Major deployable units
Responsibilities of each component
Key communication paths

Avoid showing every class or function. Focus on responsibility and interaction.

Deployment and runtime views¶

These views show how the system runs in real environments.

Environments and nodes
Network boundaries
Data flows
Scaling and resilience concerns

These views are critical for operability and incident response.

Design documentation¶

Design documentation explains how a specific change or feature works.

A good design document:

States the problem clearly
Describes constraints and assumptions
Explains the chosen approach
Documents alternatives considered
Identifies risks and mitigations

Design documents should be lightweight and time-bound.

Architecture Decision Records¶

Significant architectural decisions must be captured explicitly.

Use a consistent ADR format
Record context, decision, and consequences
Do not rewrite history after acceptance

ADRs complement diagrams by preserving reasoning.

Diagrams as code¶

Diagrams should be treated as maintainable artefacts.

Prefer text-based diagram formats
Keep diagrams close to the documentation they support
Review diagram changes alongside code changes

If a diagram cannot be kept in sync, remove it.

Keeping documentation current¶

Architecture documentation must evolve with the system.

Update docs when responsibilities change
Review architecture docs during significant refactors
Treat outdated architecture documentation as a defect

Trust in documentation is hard to rebuild once lost.

Reviews and governance¶

Architecture documentation should be reviewed with intent.

Does it clearly explain structure and boundaries?
Are trade-offs and constraints explicit?
Is the level of detail appropriate?
Does it align with existing architecture principles?

The goal is clarity, not approval theatre.

Common failure modes¶

Watch for:

Overly detailed diagrams that rot quickly
Missing rationale behind decisions
Architecture frozen on paper but not in reality
Documentation that mirrors code structure too closely
Treating architecture docs as one-off deliverables

If you see these, simplify and refocus.

Ownership and evolution¶

Architecture documentation has clear ownership
Updates are made via pull request
Improvements are encouraged

This guide should evolve as our systems and practices evolve.