The days of monolithic, 100-page architecture documents are over. Agile teams move too fast for static documentation to remain accurate. If documentation isn't treated as code, it becomes legacy debt the moment it's published.
The single most important rule: Documentation should live in the same repository as the code it describes. When developers update an API route, they should update the Mermaid sequence diagram in the same Pull Request. If the diagram is locked away in a corporate Wiki, it will never be updated.
Don't try to cram every detail into a single diagram. Use the C4 model for visualizing software architecture:
Mermaid flowcharts are perfect for creating Context and Container diagrams quickly.
If you can generate architecture diagrams from your source code automatically, do it. But for conceptual diagrams that describe intent rather than current state, Diagram-as-Code tools are essential.
Architecture documentation is most useful when it explains why the system works the way it does. A container diagram can show services, but it should be paired with short notes about ownership, constraints, failure modes, and tradeoffs. Without that context, readers can see boxes and arrows but still miss the design intent.
A common mistake is trying to draw the whole platform in one diagram. Agile teams get better results from smaller diagrams that answer specific questions: how login works, how events move through a queue, how orders are stored, or how a deployment reaches production. Smaller diagrams are easier to review and less likely to go stale.
When diagrams are plain text, they can be reviewed like code. Add a checklist item to pull requests when a change affects architecture, API behavior, data relationships, or user-visible workflow. The documentation update should explain the change at the same level of detail as the code change.
Try this in Mermaid Preview: paste the examples into the live editor, adjust the labels, then export SVG or PNG for your docs.
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