Architecture Decision

Architecture Decision Skill

Core Philosophy: "Everything is a Trade-off"

There are no "best solutions", only solutions with the right set of trade-offs for the specific context.

Decision Making Process (RFD / RFC)

1. Context: What is the problem? Why now?
2. Constraints: Budget, Time, Skillset, Legacy.
3. Options: Propose at least 3 viable options.
4. Trade-off Analysis: Compare constraints vs options.
5. Recommendation: Choose one and justify.

Trade-off Analysis Matrix (Template)

Architecture Note

When a decision is made, document it.

# Architecture Note: Use PostgreSQL for Main Database

## Status
Accepted

## Context
We need a reliable, relational database for user data...

## Decision
We will use PostgreSQL 16.

## Consequences
- Positive: ACID compliance, rich ecosystem.
- Negative: Scaling writes requires sharding later.

Critical Factors to Evaluate

• Reliability (Availability, Fault tolerance)
• Scalability (Throughput, Latency)
• Maintainability (Simple vs Easy, Ecosystem)
• Security (AuthN/Z, Encryption)
• Cost (Infrastructure, License, Cognitive load)

Rules

• MUST present at least 3 viable options, including "status quo / do nothing" when relevant
• MUST state constraints (budget, time, team skillset, legacy) before enumerating options — options without constraints are arbitrary
• NEVER recommend an option without explicitly naming its failure mode and what would trigger revisiting the decision
• CRITICAL: the output is a document (ADR / RFC / RFD), not code changes. Code work belongs in /plan or /refactor-plan.
• MANDATORY: Consequences section names both Positive and Negative effects. A decision with only upside is not honestly analysed.

Gotchas

• "Status quo" is a genuine option and often the right one for constrained teams. Omitting it biases the analysis toward change for change's sake.
• ADRs are append-only. A reversed decision gets a new ADR with Status: Supersedes ADR-N, not an in-place edit of the original — the history of thinking matters more than the current state.
• Qualitative scales (High / Medium / Low) without anchors are hand-waving. When scoring, pin each level to a measurable threshold (e.g., "High scalability = 10k req/s at p95 < 100ms on 2 nodes").
• "We might use X" and "We will try X" are not decisions — they are deferrals. A valid ADR states the chosen option unconditionally; if you cannot, the decision is not ready.
• Political constraints (team expertise, vendor relationships, executive preferences) are first-class constraints. Hiding them behind technical language produces ADRs that get silently ignored — name them explicitly in the Constraints section.

When NOT to Load

• To find what architectural problems exist — use /architecture-audit (discovery) before this skill (decision)
• To plan implementation of an already-decided option — use /plan or /refactor-plan
• To design a single module's interface — use /design-an-interface
• When fewer than 2 options exist — there is no decision to make; document the constraint instead
• For runtime configuration choices (flag values, timeouts) — those are operational, not architectural