---
name: solution-architecture
description: Designs system structure and makes architectural decisions defensible — boundaries, coupling, trade-offs, and recording why. Use this to design a new system or major component, choose between architectural options, review an existing design, decide where a boundary belongs, or document an architectural decision so it survives the people who made it.
---

# Solution architecture

Architecture is the set of decisions that are expensive to reverse. Everything cheap to change later
is design, and does not need this level of ceremony.

## Start from what constrains, not from what appeals

Architecture is driven by quality attributes, not by features. Establish the ones that actually bind
before drawing anything: expected load and its shape, latency the user notices, availability the
business will fund, data residency and retention obligations, the rate of change the team needs to
sustain, and the size and skills of the team who will operate it.

Vague attributes produce vague architecture. "Scalable" is not a requirement; "20,000 concurrent
sessions with p99 under 300ms" is a constraint that eliminates options.

## Boundaries are the whole game

Draw boundaries where change is independent and where data has a single owner. A boundary in the
right place absorbs change; a boundary in the wrong place turns every feature into a negotiation
across three teams.

The reliable test: if a routine change requires coordinated deployment across two components, the
boundary is wrong regardless of how clean the diagram looks.

Prefer coarse boundaries until you have evidence for finer ones. Distributed systems convert simple
in-process failures into network, partial-failure and consistency problems — a cost worth paying for
independent scaling or independent deployment, and not worth paying for tidiness.

## Trade-offs, stated

Every architecture sacrifices something. The failure is not choosing badly; it is choosing without
naming what was given up, so nobody later understands why the constraint exists.

Present options with their costs: what each makes easy, what each makes hard, what it costs to
operate, and what it would take to reverse. Two credible options and an honest comparison beat one
recommendation.

## Record decisions

Write down the decision, the context at the time, the options considered, and the consequences
accepted. Architecture rots because the reasoning evaporates while the structure remains, and the
next team reads a constraint as an accident.

Keep records short and immutable — supersede rather than edit, so the history of thinking survives.

## Sources

`references/sources.md` in this skill lists the outside authorities that settle the questions
here — what each one is authoritative for, and what you may do with it. Check them before
answering on anything they cover, and cite what you used. Most are free to read and not free
to reproduce; the use note on each is binding.

## Never

- Design against quality attributes nobody has quantified.
- Split a system into services to make the diagram tidy.
- Present a single option as though no trade-off was made.
- Leave an expensive-to-reverse decision undocumented.