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---
name: api-chaos-runner
description: "Builds a workflow that runs the project's existing API tests under injected network chaos - latency, timeouts, dropped connections, bandwidth caps, packet loss - using Toxiproxy as the proxy layer (with notes on alternatives Pumba / Gremlin / LitmusChaos). Defines a chaos matrix per test scenario, runs each, and reports which assertions break under which conditions. Use when the API surface needs to verify resilience patterns (retry, circuit-breaker, timeout, fallback) actually work."
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# api-chaos-runner
## Overview
Most API tests run against perfect networks: <1ms latency, no
packet loss, infinite bandwidth, deterministic ordering. Real
production isn't like that. **Network chaos testing** drives the
existing tests under controlled network impairment - the team
discovers which retry / circuit-breaker / timeout patterns actually
hold up before real customers find out.
The canonical open-source primitive is **Toxiproxy** - Shopify's
"TCP proxy to simulate network and system conditions for chaos and
resiliency testing" ([toxiproxy-readme][toxiproxy]). The pattern:
sit Toxiproxy between client and upstream; manipulate **toxics**
(latency, timeout, bandwidth, etc.) during test execution.
[toxiproxy]: https://github.com/Shopify/toxiproxy
This skill is **build-an-X** - the workflow chains the team's
existing API tests (Postman / Karate / RestAssured / Tavern /
Schemathesis) through a Toxiproxy-managed connection and orchestrates
a per-scenario chaos matrix.
## When to use
- The API has documented resilience requirements (retry on 5xx,
circuit-break on 3 consecutive failures, timeout at 5s, etc.) and
the team needs to **verify** them, not just document them.
- A new external dependency just got added; the team wants to
pressure-test fallback behavior.
- An incident postmortem identified a "we should have caught this in
testing" item; chaos coverage is the prevention mechanism for that
class.
- The team has integration tests already and wants to multiply their
signal value via fault injection.
If the team is just starting API testing and has no resilience
patterns to verify, this skill is overkill - start with happy-path
coverage via `postman-collections`
or the language-native equivalents first.
## Step 1 - Pick the chaos primitive
| Tool | Layer | Best for |
|---------------------|-----------------------------|-------------------------------------------------------|
| **Toxiproxy** | TCP proxy | Per-connection latency / bandwidth / timeout / drop. Most precise. |
| **Pumba** | Docker container | Container-level chaos (kill, pause, network). |
| **Gremlin** (commercial) | Multi-platform | Production-grade chaos with audit / approval flow. |
| **LitmusChaos** | Kubernetes operator | Cloud-native; experiments declared as CRDs. |
| **`tc qdisc`** (Linux native) | Network interface | Lowest level; most setup; CI-friendly only with `--cap-add NET_ADMIN`. |
Default recommendation: **Toxiproxy** for per-API chaos in CI. The
others fit when the team is already in those ecosystems
(Docker-Compose-heavy projects, Kubernetes-first projects).
## Step 2 - Define the chaos matrix
For each existing API test scenario, define a matrix of conditions
to run it under:
| Scenario | Toxic | Expected behavior |
|-----------------------|------------------|--------------------------------------------|
| Order create (POST /orders) | None (control) | 201 in <500ms |
| Order create | latency=1000ms | 201 in <2s (within timeout budget) |
| Order create | latency=10000ms | 504 with retry-after, OR client gives up |
| Order create | bandwidth=10kbps | 201 (eventual) OR 408 timeout |
| Order create | reset_peer | 502 with retry attempted |
| Order create | timeout | 504; circuit-breaker opens after 3rd |
Per [toxiproxy-readme][toxiproxy], the canonical toxic types include
**latency**, **down** (forced failure), **bandwidth**, **slow_close**,
**timeout**, **slicer**, **limit_data**, **reset_peer**.
The matrix is the load-bearing artifact: what the team **expects**
under each condition is what differentiates resilience verification
from "did the test pass?" The Expected column drives the assertions.
## Step 3 - Wire Toxiproxy into the test environment
### Setup (Docker example)
```yaml
# docker-compose.test.yml
services:
toxiproxy:
image: ghcr.io/shopify/toxiproxy:latest
ports:
- 8474:8474 # control API
- 5432:5432 # proxied DB
- 8080:8080 # proxied API
app:
build: .
environment:
DATABASE_URL: 'postgres://user:pass@toxiproxy:5432/db'
EXTERNAL_API_URL: 'http://toxiproxy:8080'
```
Per [toxiproxy-readme][toxiproxy], the application points at
Toxiproxy's listen ports rather than the upstream. Toxiproxy
forwards to the real upstream when no toxic is active.
### Define proxies via the control API
```bash
# Register the upstream
curl -d '{"name":"orders-api","listen":"0.0.0.0:8080","upstream":"orders-api-real:8080"}' \
http://toxiproxy:8474/proxies
```
### Add a toxic during a test
Per [toxiproxy-readme][toxiproxy]:
```bash
# 1000ms latency on every request through this proxy
toxiproxy-cli toxic add -t latency -a latency=1000 orders-api
# Bandwidth cap at 10 KB/s
toxiproxy-cli toxic add -t bandwidth -a rate=10 orders-api
# Forced timeout
toxiproxy-cli toxic add -t timeout -a timeout=5000 orders-api
# Remove all toxics
toxiproxy-cli toxic remove orders-api -n <toxic-name>
```
For a stateless add-test-remove cycle, the language-native client
libraries (`toxiproxy-python`, `toxiproxy-node`, `toxiproxy-ruby`,
`toxiproxy-go`) wrap the HTTP API.
## Step 4 - Run the matrix
A minimal runner shell script:
```bash
#!/usr/bin/env bash
# scripts/chaos-matrix.sh
set -e
PROXY=orders-api
TEST_CMD="npx newman run collections/orders.postman_collection.json -e environments/chaos.json -r cli,junit --reporter-junit-export results-$1.xml"
run_with_toxic() {
local label="$1"; local type="$2"; local args="$3"
echo "=== $label ==="
toxiproxy-cli toxic remove "$PROXY" -n latency 2>/dev/null || true
toxiproxy-cli toxic remove "$PROXY" -n bandwidth 2>/dev/null || true
toxiproxy-cli toxic remove "$PROXY" -n timeout 2>/dev/null || true
if [ -n "$type" ]; then
toxiproxy-cli toxic add -t "$type" $args "$PROXY"
fi
$TEST_CMD "$label" || true # don't bail; we want the matrix
}
run_with_toxic 'control' '' ''
run_with_toxic 'latency-1s' latency '-a latency=1000'
run_with_toxic 'bandwidth' bandwidth '-a rate=10'
run_with_toxic 'timeout' timeout '-a timeout=5000'
```
The matrix produces one JUnit XML per scenario. Aggregate them in
the report stage.
## Step 5 - Report what broke under what
A successful chaos run produces a **resilience matrix** report:
```markdown
## API Chaos Matrix - verdict: REVIEW
| Scenario | Control | Latency 1s | Bandwidth 10k | Timeout 5s | Reset peer |
|------------------|:-------:|:----------:|:-------------:|:----------:|:----------:|
| POST /orders | ✅ | ✅ | ✅ | ✅ | ❌ |
| GET /orders/:id | ✅ | ✅ | ❌ | ✅ | ✅ |
| DELETE /orders/:id | ✅ | ✅ | ✅ | ✅ | ✅ |
### Failures
| Test | Toxic | Expected | Actual |
|------|-------------|---------------------------------------------------|--------|
| POST /orders | reset_peer | 502 + retry attempted; second attempt succeeds | 502; no retry observed in client logs |
| GET /orders/:id | bandwidth=10k | 200 in <30s | 408 timeout at 10s |
```
A green matrix isn't the goal - finding where resilience is **missing**
is the goal. A failure under a chaos scenario is a feature request,
not a bug in the test.
## Choosing what to inject
Match toxics to documented resilience requirements:
| Resilience pattern documented | Toxic to inject |
|------------------------------------------|----------------------------------------|
| Retry on 5xx | `down` (forced 5xx) or `reset_peer` |
| Timeout after Nms | `latency=N+500` (force the timeout) |
| Circuit-breaker after 3 failures | `down` for ≥3 requests |
| Fallback to cache when upstream unreachable | `down` indefinitely |
| Bulkhead under load | `bandwidth=very-low` |
| Slow-loris client | `slow_close` on response |
Run only the toxics that map to a documented expectation; running
every toxic against every endpoint is noise.
## Anti-patterns
| Anti-pattern | Why it fails | Fix |
|--------------------------------------------------------------|------------------------------------------------------------------|-----|
| Chaos in production | Real users observe; oncall pages. | CI / staging only. Production chaos requires the team's full chaos engineering practice (Gremlin / Litmus + approval flow). |
| Per-PR chaos matrix | Adds 10+ minutes; team disables. | Nightly chaos runs; PR runs only the control row. |
| Asserting "chaos must not break anything" | Every system has a breaking point; the test trivially fails. | Assert specific resilience behavior under specific conditions; document the breaking point as accepted. |
| Using `down` for everything | `down` forces 5xx; doesn't model real-world latency / bandwidth. | Mix `latency`, `bandwidth`, `timeout`, `reset_peer` for realistic mixes. |
| Skipping the control row | Without control, the matrix can't distinguish chaos failures from test bugs. | Always run a no-toxic scenario as the baseline. |
## Limitations
- **Toxiproxy is TCP-level.** UDP flaws (DNS resolver weirdness,
QUIC) need different tooling.
- **Doesn't model partial failures within a single connection.**
Toxiproxy treats each connection uniformly; for "retry on the 3rd
request after 2 successes" patterns, layer a counting middleware.
- **Per-test-suite isolation.** When tests run in parallel against
the same Toxiproxy instance, they fight over the same toxic
state; serialize chaos scenarios or use one Toxiproxy instance per
worker.
## References
- [toxiproxy][toxiproxy] - main repo: install, control API, toxic
types, language-native clients.
- Pumba - https://github.com/alexei-led/pumba
- LitmusChaos - https://litmuschaos.io/
- Principles of Chaos Engineering - https://principlesofchaos.org/
- `postman-collections`,
`tavern-testing`,
`karate-testing`,
`restassured-testing` -
example-based test suites that this skill drives through chaos.