---
name: system-design-review
description: "Audit an existing or proposed architecture and return a scored verdict across nine axes, from requirements and capacity evidence to observability and rollout, then convert gaps into a prioritized roadmap. Use when reviewing a design doc, auditing a running system, or gating a design."
metadata:
  triggers:
    keywords:
      - design review
      - architecture review
      - design scorecard
      - bottleneck analysis
      - scalability audit
      - evolution roadmap
      - architecture assessment
---

# System Design Review

## **Priority: P1 (HIGH)**

Score claims against evidence appropriate to the declared review scope. A proposed design may support a design-readiness verdict through stated mechanisms and planned validation; it cannot claim operational readiness from plans alone. An implementation review requires implementation evidence; an operations claim requires production runtime/deployment evidence.

## Nine Axes (score each applicable axis 0-10)

| Axis | Scores 10 when | Scores 0 when |
| --- | --- | --- |
| Requirements | Functional, NFR, and out-of-scope written with owners | Only a feature description exists |
| Capacity evidence | Peak QPS, storage, and bandwidth computed and current | Numbers absent or older than the last traffic change |
| Redundancy | Failure domains, recovery mechanisms and ownership fit the stated objective; proposals include validation criteria, operations claims require measured drills | Critical SPOF or no viable recovery for the stated objective |
| Data scaling | Access patterns mapped, ownership single, growth path stated | One shared store, no growth plan, unbounded tables |
| Caching | Hot read paths cached with TTL and invalidation defined | No cache on a proven hot path, or uninvalidatable cache |
| Async offload | Slow/bursty work is isolated when required by stated SLO or failure constraints, with bounded drain/recovery; bounded synchronous work is valid when it meets them | Required isolation is absent or synchronous coupling violates the stated constraints |
| Observability | Signals, alert ownership, runbooks and validation fit the declared scope; operations claims require observed telemetry | No relevant signals, owner or response path for a material risk |
| Rollout | Canary or flag with metric rollback trigger and reversible migrations | Big-bang deploy, irreversible migration |
| Cost proportionality | Spend is sized to the traffic and the risk, and someone can state it | Topology bought for an imagined scale nobody measured |

Report each applicable axis with evidence and a declared profile weighting. Use an applicable-axis denominator
(`10 × applicable-axis count`), not a fixed `/90`, when an axis is justified `N/A`.

## Profile-Aware Scoring

- Declare the system profile and weighting before scoring. An axis may be `N/A` only when the profile and evidence show that it is outside the system's risk envelope; record the rationale, exclude it from the denominator, and do not silently convert it to zero.
- Do not reward adding a cache, queue, replica, or region by vocabulary alone. A component earns credit only when a measured constraint, invariant, owner, cost, and failure/recovery behavior require it; unjustified machinery lowers cost proportionality and operability.
- Review HLD and LLD as one trace: requirements and shaping decisions must resolve into component ownership, contracts, verification, and a stated changed-constraint trigger. A diagram is optional when prose answers the question.
- Separate lifecycle (`proposed|implemented|retired`), source kind (`code|document|runtime|deployment`), and evidence confidence (`unverified|assumed|documented|observed`). Code/document citations are `documented`, not deployment proof; runtime/deployment captures may be `observed`. `assumed` and `unverified` carry no citation; explicit citations require `evidence_kind`.

## Scope-Qualified Semantic Review

Lexical checks are smoke signals, not proof of a sound design. Apply the independent behavioral rubric in
[semantic evaluation](references/semantic-evaluation.md). Record missing calculations, mechanisms,
adverse timelines, invariants, recovery, and scope-specific evidence as findings even when expected vocabulary appears.

## Review Method

1. Establish ground truth first: current traffic, data volume, incident history, and the top pain the owner reports.
2. Score the nine axes against artifacts and metrics; mark any unverifiable claim `UNVERIFIED`.
3. Trace the hottest and the most critical path end to end; the worst hop is the real bottleneck.
4. List findings as `severity - axis - evidence - consequence - smallest fix`.
5. Convert findings into a roadmap: stop-the-bleeding now, structural next, optional later.
6. Check operability: who runs this at 3am, which team owns which piece, and whether that team can actually operate it.

## Common Mistakes to Check

- Architecture drawn before requirements or numbers existed.
- Redundancy claimed but sharing one config plane, credential, or control plane.
- Cache added over a query that was never optimized.
- Sharding adopted before indexing, replicas, and caching were exhausted.
- Queue with no drain-rate budget, no DLQ, and alerting on depth rather than age.
- Alerts on CPU rather than user-visible symptoms or error-budget burn.
- Migration and code shipped as one irreversible step.

## Anti-Patterns

- **No score without evidence**: cite the metric, artifact, or drill; otherwise mark `UNVERIFIED`.
- **No rewrite recommendation by default**: prefer the smallest fix that removes the proven bottleneck.
- **No uniform severity**: rank by user impact and reversibility, not by axis order.
- **No finding without a next action**: every gap gets an owner-ready fix.

## References

- [Scorecard](references/scorecard.md) - scoring rubric, weighting guidance, report template
- [Mistakes Table](references/mistakes-table.md) - failure symptom, root cause, and corrective action