jac_loadtest_cli

jac-loadtest-cli

HAR-based load testing CLI built on Jac. Capture real browser traffic via Chrome DevTools, export it as a .har file, and replay it under load — no scripting required.

The tool registers itself as a jac subcommand, so after installation you run jac loadtest alongside jac start, jac deploy, and the rest of the jac ecosystem.

Compatibility: Works with any HTTP server — jac-scale, Django, FastAPI, Node.js, etc. The only jac-scale-specific feature is auth: if your app uses jac-scale's /user/login JWT flow, credentials are automatically handled. For other auth schemes the raw request from the HAR is replayed as-is.

Testing Modes

Monolith mode (default) — all requests go through a single --url. Use this for production-realistic load testing: it measures what users actually experience end-to-end through the gateway.

Microservice mode — route requests directly to individual service processes by URL path prefix. Use this locally or inside your cluster to isolate per-service latency and identify which service is the bottleneck — without gateway overhead masking the signal.

# Monolith: all traffic through the gateway
jac loadtest recording.har --url http://localhost:8000 --vus 10 --iterations 20

# Microservice: bypass gateway, route by path prefix to individual services
jac loadtest recording.har --mode microservice \
  --url http://localhost:8000 \
  --services-map '{"order_service":"http://localhost:18001","inventory_service":"http://localhost:18002"}' \
  --vus 10 --iterations 20

Note: Microservice mode requires direct network access to service ports. This means it's only usable locally (jac serve) or from inside a Kubernetes cluster — not from outside production. For remote or production load testing, use monolith mode.

Quick Start

# Minimal: 1 VU, 1 HAR replay
jac loadtest recording.har --url http://localhost:8000

# 50 VUs, each replaying the HAR 100 times, with 10s ramp-up
jac loadtest recording.har --url http://localhost:8000 \
  --vus 50 --iterations 100 --ramp-up 10s

# Realistic pacing: replay at recorded think times, halved
jac loadtest recording.har --url http://localhost:8000 \
  --vus 10 --iterations 30 --think-time scaled --think-time-scale 0.5

# Rate-limited stress test: cap global throughput to 50 req/s
jac loadtest recording.har --url http://localhost:8000 \
  --vus 10 --iterations 50 --rps 50

# CI gate: fail if p95 > 500ms or error rate > 1%, stop early on first breach
jac loadtest recording.har --url http://staging:8000 \
  --vus 20 --iterations 100 \
  --fail-on-p95 500 --fail-on-error-rate 1 \
  --threshold-start-delay 10s --abort-on-fail

# Per-request debug output
jac loadtest recording.har --url http://localhost:8000 --vus 2 --iterations 5 --debug

Exit Codes

Code	Meaning
0	Test completed; all thresholds passed (or none configured)
1	At least one threshold failed (--fail-on-error-rate, --fail-on-p95, --fail-on-p99)
2	Config or tool error (missing flag, bad HAR file, auth failure)

Failed thresholds are printed to stderr before exit:

THRESHOLD FAILED: error_rate 3.2% > limit 1.0%
THRESHOLD FAILED: p95 612.4ms > limit 500.0ms

Key Flags

Flag	Default	Description
--vus	1	Concurrent virtual users (asyncio coroutines per worker)
--iterations	1	Stop each VU after N full HAR replays
--ramp-up	0s	Stagger VU startup — prevents thundering herd at test start
--workers	CPU count	Worker processes; each runs its own event loop (capped at --vus and CPU count)
--rps	0 (unlimited)	Global requests-per-second cap across all VUs
--timeout	30s	Per-request timeout; exceeded requests are recorded as TIMEOUT errors
--think-time	none	none / real / scaled — inter-request delay from HAR timings
--think-time-scale	1.0	Multiplier on think times (0.5 = half speed, 2.0 = double)
--fail-on-error-rate	—	Fail (exit 1) if global error rate exceeds N%
--fail-on-p95	—	Fail (exit 1) if global p95 latency exceeds N ms
--fail-on-p99	—	Fail (exit 1) if global p99 latency exceeds N ms
--threshold-start-delay	0s	Ignore threshold checks for this long after the test starts
--abort-on-fail	false	Stop the test immediately when a threshold is first breached
--debug	false	Print one line per request to stderr: VU ID, endpoint, status, latency
--report-format	console	console / json / html
--report-out	—	Write report to file (required for --report-format html)

See docs/COMMANDS.md for the full flag reference including microservice mode, auth, and jac.toml configuration.

Authentication

The tool auto-detects the login request in the HAR by matching --login-path (default /user/login). At test start each VU logs in once and injects the returned JWT into all subsequent requests.

# All VUs log in with the same account used during HAR recording
jac loadtest recording.har --url http://localhost:8000 \
  --username admin@example.com --password secret

Developer Setup

# 1. Navigate to the CLI directory
cd jac_loadtest_cli

# 2. Install the package in editable mode (also installs runtime deps)
jac install -e .

# 3. Verify the command is registered
jac loadtest --help

Running tests

cd jac_loadtest_cli
jac test tests/          # all 148 tests
jac test tests/unit/     # unit tests only
jac test tests/integration/  # integration tests (needs aiohttp servers)

Mock service for local testing

The scripts/mock_service.jac script spins up lightweight HTTP servers to test against without a real backend:

# Start two fake services on ports 8001 and 8002
jac run scripts/mock_service.jac -- order_service:8001 inventory_service:8002

Project Layout

jac_loadtest_cli/          ← Python package (importable as jac_loadtest_cli)
├── plugin.jac             ← registers `jac loadtest` via jaclang entry-points
├── cli.jac                ← argument wiring, run orchestration, exit codes
├── config.jac             ← LoadTestConfig + three-layer config resolution
├── core/
│   ├── har_parser.jac     ← parse HAR 1.2, filter, URL rewrite
│   ├── engine.jac         ← asyncio VU coroutines, RPS cap, threshold watcher
│   ├── metrics.jac        ← RequestResult, MetricsCollector, p50/p95/p99
│   └── process_runner.jac ← multi-process worker orchestration
├── bridge/
│   ├── auth.jac           ← login detection, JWT injection, credential rotation
│   └── topology.jac       ← TopologyRouter, longest-prefix matching
├── output/
│   └── reporter.jac       ← Rich console, JSON, HTML reporters
└── scripts/
    └── mock_service.jac   ← lightweight fake HTTP servers for local testing

HAR Compatibility

Tested with HAR 1.1 and 1.2 (the format exported by Chrome DevTools, Firefox, Postman, and Insomnia). Files from other versions are parsed with a warning — open an issue if something breaks.

Documentation

• Architecture — module map, data flow, design decisions
• Commands — full CLI flag reference
• Roadmap — delivery phases for CLI and web UI
• Testing — test strategy and coverage guide