Well Architected

Well-Architected — 6 pillars to verify before shipping

Every ARCH document for non-nano work must answer the 6 pillar questions below. Skipping a pillar is allowed only if explicitly justified (e.g. "Sustainability: N/A — backend-only, runs in shared infra.").

This is adapted from AWS Well-Architected (lens: small-team SaaS / LLM applications), trimmed to questions that matter at <10 engineer scale.

Pillar 1 — Operational excellence

Questions

1. Observability: What metrics, logs, traces do we emit? How do we tell from a dashboard if this is working in prod?
2. Deployability: How do we ship a change? CI gates? Rollback path?
3. Runbooks: When this breaks at 3am, what does on-call read?

Pass criteria

• ✅ One metric per business outcome (e.g. webhook-deliveries-acked)
• ✅ One log line per request, with request-id correlatable across services
• ✅ Deploy path is documented and tested (rollback dry-run executed)
• ✅ Runbook covers top-3 failure modes from pre-mortem

Common fail

❌ "We'll add monitoring later." Monitoring is part of the feature.

Pillar 2 — Security

Questions

1. Trust boundaries: Where does untrusted data enter? How is it validated/sanitized?
2. Authn / authz: Who can call this? Who can read/write the data?
3. Secrets: Where are API keys, DB passwords, JWT signing keys stored?
4. Data classification: PII? PHI? PCI cardholder data? What's the retention policy?

Pass criteria

• ✅ Every external input has explicit validation at the boundary
• ✅ Authz is enforced at the data layer, not just UI
• ✅ Secrets in env vars or secret manager, never in source
• ✅ Sensitive data classified and retention policy defined

Common fail

❌ "JWT validates the user, that's our authz." JWT is authentication. Authorization is separate (this user can read THIS row).

Pillar 3 — Reliability

Questions

1. Failure modes: What happens when a downstream dependency is slow / down / corrupted?
2. Idempotency: Can a retried request safely re-execute?
3. Backups & recovery: What's the RPO (data-loss tolerance)? RTO (downtime tolerance)? Test plan for both?
4. Capacity: What's the max QPS this can handle? What happens at 1.5x that?

Pass criteria

• ✅ Circuit breakers / timeouts on external calls
• ✅ State-mutating endpoints accept idempotency keys
• ✅ Backups documented + restore tested in the last 90 days
• ✅ Load test exists; results in docs/perf/

Common fail

❌ "Postgres has backups." Backups without a tested restore aren't backups.

Pillar 4 — Performance efficiency

Questions

1. SLOs: What's the p50/p95/p99 latency target? Error rate? Availability?
2. Bottlenecks: Profile the critical path — what's the slowest step?
3. Caching: What's cacheable? Cache invalidation strategy?
4. Scaling: Vertical or horizontal? Auto-scale rules?

Pass criteria

• ✅ SLO numbers in the ARCH doc (not "fast enough")
• ✅ Profile attached for non-trivial requests
• ✅ Cache strategy documented; invalidation explicit
• ✅ Scaling decision justified by data, not "feels right"

Common fail

❌ "Database can handle it." Quantify: queries/sec, row count, index hit rate.

Pillar 5 — Cost optimization

Questions

1. Hot path: What's the most expensive operation per request? Why?
2. Right-sizing: Is the chosen instance type / model / DB tier the smallest one that meets SLO?
3. Cleanup: What happens to old data? Old logs? Old branch environments?

Pass criteria

• ✅ Use skill cost-model to document explicit $ numbers
• ✅ Choose smallest LLM model that meets quality SLO (haiku before sonnet, sonnet before opus)
• ✅ Retention policy for logs, metrics, old data

Common fail

❌ Defaulting to Opus / GPT-4 when Haiku would work. Test on Haiku first.

Pillar 6 — Sustainability (env / energy)

Questions

1. Workload efficiency: Is the code O(n log n) when it could be O(n)?
2. Idle resources: Can dev environments scale to zero overnight?
3. Data minimization: Do we collect / store data we never query?

Pass criteria

• ✅ Hot loop complexity documented
• ✅ Non-prod resources have shutdown schedules
• ✅ Data lifecycle covers ingestion, retention, deletion

Common fail

❌ Logs at debug level in prod, never reviewed. Waste of storage + carbon.

Output format — add to ARCH

## Well-Architected review

### 1. Operational excellence
- Metrics: <list>
- Deploy path: <link to runbook>
- Verdict: PASS | RISKS LISTED

### 2. Security
- Trust boundaries: <list>
- Data classification: <PII / PHI / PCI / none>
- Verdict: PASS | RISKS LISTED

### 3. Reliability
- Failure modes: <link to pre-mortem>
- Idempotency: <yes/no per endpoint>
- Verdict: PASS | RISKS LISTED

### 4. Performance
- SLOs: p99=<ms>, error_rate=<%>, availability=<%>
- Verdict: PASS | RISKS LISTED

### 5. Cost
- Per-request cost: $<amount>
- Verdict: PASS | RISKS LISTED

### 6. Sustainability
- Hot-path complexity: O(<n>)
- Verdict: PASS | N/A | RISKS LISTED

## Open risks (rolled up)

<bullet list of all RISKS LISTED items + mitigation in plan>

When PASS is acceptable with risks listed

Not every architecture is bulletproof. PASS-with-risks is OK if:

• Each risk is explicit (not hand-waved)
• Each risk has either a mitigation in the plan OR explicit acceptance by the user
• The pre-mortem section addresses the top-3 risk-score items

Gate:plan can approve a PASS-with-risks; gate:ship needs the mitigations shipped.