load-testing-getting-started

Load testing measures how a system behaves under concurrent traffic. The three metrics that matter first are throughput (requests per second the system handles), latency percentiles (p95 and p99 measure tail response time - the experience of your slowest users, not the average), and error rate (the fraction of requests that fail). A passing average can hide a broken p99; always gate on all three. If you are unsure which tool to use for your stack, invoke the load-test-tool-selector agent with your language, infrastructure, and load profile - it returns a reasoned recommendation before you write a single line.

Step 1 - Write your first k6 script

k6 is the recommended starting point for most HTTP services. Install it via your package manager (brew, apt, choco, Docker - see k6 installation), then create script.js (per k6-running):

import http from 'k6/http';
import { check, sleep } from 'k6';

export const options = {
  vus: 10,
  duration: '30s',
  thresholds: {
    http_req_duration: ['p(95)<500'],  // 95th-percentile latency under 500ms
    http_req_failed:   ['rate<0.01'],  // error rate under 1%
  },
};

export default function () {
  const res = http.get('https://your-service.example.com/health');
  check(res, { 'status 200': (r) => r.status === 200 });
  sleep(1);
}

Run it (per k6-running):

k6 run script.js

k6 prints a live progress table and a summary at the end. A threshold failure exits non-zero - the CI gate signal (per k6-thresholds).

Step 2 - Understand the threshold output

The end-of-run summary reports each threshold as ok or failed (per k6-thresholds):

http_req_duration..........: p(95)=432ms   ✓ p(95)<500
http_req_failed............: rate=0.4%     ✓ rate<0.01

If either threshold fails, k6 run exits with a non-zero code. Adjust the numeric budgets to match your service-level objectives, not arbitrary defaults. The k6-load-testing skill covers stages (ramp-up / plateau / ramp-down), abortOnFail, and advanced threshold expressions in full.

Step 3 - Add a CI gate with perf-budget-gate

Once your k6 thresholds are stable locally, add a CI step that fails the build on regressions. The perf-budget-gate skill aggregates verdicts from k6 (and other runners) into a single go/no-go decision with a delta vs the main-branch baseline - use it when you need a unified gate across multiple test suites or want per-PR delta comments.

For a k6-only project, the threshold exit code is sufficient as the initial gate; perf-budget-gate pays off once you add Lighthouse or a second load runner.

Step 4 - When results look bad

If a threshold fails or the system feels slow despite passing thresholds:

• High or unexpected tail latency: use latency-percentile-analyzer to detect bimodal distributions, compute the tail ratio (p99/p50), and check for coordinated omission effects that make naive p99 values optimistic.
• Active production incident or post-deploy regression: use perf-incident-responder to confirm with k6, flame-graph the hot path, and check slow queries in one orchestrated flow.

References

• k6-running - first script, VU model, stages, ad-hoc flags.
• k6-thresholds - threshold syntax, aggregation methods, exit-code CI signal.
• k6-load-testing - full k6 authoring reference (stages, abortOnFail, CI YAML, anti-patterns).
• perf-budget-gate - multi-runner CI gate.
• latency-percentile-analyzer - tail-latency diagnosis when thresholds pass but perf is suspect.
• load-test-tool-selector - tool recommendation for your specific stack.
• perf-incident-responder - on-call orchestrator for active perf incidents.