test-load

---
name: test-load
description: >
  Design and run a k6 or Artillery load profile: throughput, latency
  percentiles, error rate, breaking point. Use when "load test this", "will it
  handle launch traffic", or "find the breaking point". Code-level resilience
  → audit-resilience.
license: MIT
---

# test-load — Measured concurrency, not a guess

**Degree of freedom: MIXED** — journey design `[HIGH freedom]`; env choice
and profile order `[LOW freedom]`. **Never production without explicit
sign-off and rate caps.**

Turn "I think it'll hold" into numbers: at N concurrent users, p95 is X, error
rate is Y, it falls over at Z.

**`audit-resilience` tells you retries exist; this proves whether they save you
when 500 people arrive at once.**

## This skill vs neighbors

| Skill | Owns |
|---|---|
| **test-load** (this) | Concurrent traffic profiles + measured SLO |
| `audit-resilience` | Timeouts/retries/idempotency *in code* |
| `audit-infra-cost` | Consumes these capacity numbers to right-size |
| `audit-performance` | Single-user Web Vitals / bundle |
| `backend-db-performance` | Query/index fixes after this names the bottleneck |
| `plan-llm-cost-guardrails` | Model-token spend, not HTTP concurrency |

Load against production only with explicit sign-off and rate caps — an uncapped run is a self-inflicted outage. Prefer prod-like staging.

## How to reason

1. **Observe** — profile, VUs, p95/p99, error types, host/DB metrics
2. **Interpret** — SLO miss vs capacity vs cascade (pool / limiter / memory)
3. **Classify** — pass / degrade / breaking-point / invalid (unsigned prod)
4. **Severity** — named failing resource + whether failure is graceful

## Worked example

> **Observe:** load profile 200 VU, p95 2.4s (SLO 500ms), 8% 529s; DB
> `remaining connections = 0`.
> **Interpret:** pool exhaustion, not the Node process.
> **Classify:** SLO fail; breaking resource = DB pool.
> **Handoff:** `backend-db-performance` (pool + queries); numbers → `audit-infra-cost`.

---

## Phase 0 — Detect stack and set targets  [HIGH freedom]

- Tool: **k6** preferred (scriptable, CI-friendly); Artillery as fallback
- Target env + auth method
- **Goal first** (without it the test is noise):
  - Expected peak concurrency (ads / launch)
  - Acceptable p95/p99 and max error rate (SLO)
  - Capacity (hold at peak) vs breaking point (where it fails)

---

## Phase 1 — Model realistic load  [HIGH freedom]

- **Journeys, not one URL** — signup → browse → action, with think-time
- **Mix** — weight by real traffic (mostly reads)
- **Data variety** — pool of users/inputs, not one cached row
- **Auth** — real login or pre-provisioned tokens (unauthenticated misses RLS cost)

---

## Phase 2 — Graduated profiles  [LOW freedom — smoke → load → stress → spike]

1. **Smoke** — few VUs; script + baseline healthy
2. **Load** — ramp to expected peak, soak several minutes (SLO pass/fail)
3. **Stress** — past peak until degrade; name the failing resource
4. **Spike** — instant jump (viral ad); does it recover?
5. Optional **soak** — 30+ min for leaks / pool exhaustion

---

## Phase 3 — Measure the right things  [HIGH freedom]

- Latency **percentiles** (p50/p95/p99) — averages hide pain
- Error rate **and types** (timeout vs 5xx vs refused)
- Throughput (req/s) per level
- Bottleneck: DB pool, function concurrency, rate limiter, memory, cold start
- Correlate Supabase / Sentry / host metrics during the run

Keep the reusable script in the repo as a reviewable change (commit only when asked). Hand capacity numbers to `audit-infra-cost`.

---

## Definition of Done

- [ ] Tool + env chosen; prod excluded or signed + capped
- [ ] SLO stated before running
- [ ] Journeys weighted, think-time, varied data, real auth
- [ ] Smoke → load → stress → spike run
- [ ] Percentiles, errors, throughput captured
- [ ] Breaking point + failing resource named
- [ ] SLO verdict + fix handoff
- [ ] Script in the repo, ready to commit

## Self-critique before reporting  [LOW freedom — do not skip]

1. **SLO stated first** — a run without a target is noise
2. **Not unsigned prod** — if only prod exists, stop
3. **Percentiles, not averages**
4. **Breaking resource named** — not "it got slow"
5. **Journeys, not one URL**

## Output format

1. **Design** — SLO, journeys, env, profiles
2. **Results** — profile | VUs | throughput | p50/p95/p99 | errors | vs SLO
3. **Breaking point** — where, why, graceful vs cascade
4. **Handoff** — `backend-db-performance`, `backend-patterns`, `audit-resilience`, `audit-infra-cost`

## Related

- `audit-resilience` — code-level NFRs
- `audit-infra-cost` — right-size from these numbers
- `backend-db-performance` / `backend-patterns` — fix the named bottleneck
- `audit-performance` — single-user frontend