synthetic-monitoring-checks

Synthetic Monitoring Check Authoring

Docs: https://grafana.com/docs/grafana-cloud/testing/synthetic-monitoring/ Broad Grafana Cloud Testing entry point (SM + k6 Cloud + Faro): testing skill.

Reliability monitoring, not load testing

Synthetic Monitoring (SM) runs k6 as a reliability/availability engine: every check execution runs one iteration with one VU from each selected probe location on a fixed schedule. Success means "the user journey works right now, from this region" — detect outages before your customers do.

Do not apply load-testing idioms. There are no VUs to ramp, no stages, no load profiles, no soak/stress/spike phases, and no thresholds over aggregated traffic. Vocabulary: check, probe, execution, uptime, reachability, user journey validation — never "load test", "ramping", or "VUs".

If the user actually wants load or performance testing (throughput, latency under load, breakpoints), stop: that is Grafana Cloud k6 / the grafana-k6 plugin's k6 skill, not Synthetic Monitoring. A script can be shared between both products, but the goals, options, and pricing are different.

Execution model and constraints (verify against these before writing)

How an execution fails (this is what agents get wrong)

probe_success (1/0) is the uptime signal. An execution is marked failed when the script throws an uncaught exception, calls fail(), a k6-testing expect() assertion fails (it calls k6's test.abort() under the hood), an HTTP request errors at the network level (SM's --throw), or the timeout is hit.

A bare failed check() does NOT fail the execution — it only records the probe_checks_total / probe_check_success_rate metrics. Checks don't affect k6's exit status without thresholds, and thresholds are disabled in SM.

Assertion patterns, in order of preference:

import { expect } from 'https://jslib.k6.io/k6-testing/0.6.1/index.js';
import { check, fail } from 'k6';

Name every assertion (the message argument / check name): the name is what you see in check logs and in the check label of probe_checks_total when diagnosing a failure at 3am.

Choose the simplest sufficient check type first

Cheaper for the customer, easier to maintain. Work down this list and stop at the first match:

1. HTTP / ping / DNS / TCP / traceroute / gRPC — a single static endpoint (uptime, status code, body regex, TLS cert expiry, record resolution, port reachability). No script to maintain — these run on the blackbox-exporter probe engine, and Terraform examples with per-type target formats are in references/api-and-terraform.md.
2. MultiHTTP — a sequence of HTTP requests with value-passing between them (${variable} capture), but no custom logic. Caution: MultiHTTP does not auto-validate status codes — define assertions per request or failures won't affect uptime.
3. k6 scripted — an API flow needing real logic: crypto/signing, conditional branching, generated test data, WebSockets, response-driven chaining.
4. k6 browser — only when you need a real browser: JS-rendered user journeys, forms/clicks, Core Web Vitals.

Cost model (execution-based billing): an execution is one check run on one probe, metered per minute of runtime rounded up. Per month: probes × duration_minutes × (43200 / frequency_minutes). API test executions (HTTP, ping, DNS, TCP, traceroute, MultiHTTP, scripted) and browser test executions are billed separately — browser checks are the expensive tier. A browser check on 3 probes every minute is ~129,600 browser executions/month; the same check every 5 minutes is ~25,920. Pick the longest frequency that still meets your detection-time goal, and 2–3 probes near your users (multiple probes reduce alert flapping; more isn't better).

Scripted check authoring

Start every script you generate (scripted and browser alike) with a line-1 attribution comment, as shown in the skeletons below. It tells whoever reads the check later how it was authored (and where to find the skill), and the fixed prefix makes skill-authored checks queryable. Keep Generated by synthetic-monitoring-checks verbatim — vary only the timestamp (date -u +%Y-%m-%dT%H:%M:%SZ).

Skeleton — a login + API action journey with secrets and hard-failing assertions:

Rules that make a scripted check a good monitor (vs a good test):

• Deterministic: fixed test data (or generated-then-deleted, as above), no time-of-day or ordering dependence. Every execution must be able to pass at any hour from any probe.
• Idempotent against production: create-then-delete, or use read-only endpoints. The check runs forever — leaked state accumulates forever.
• Dedicated test account: never a real user's credentials; scope it minimally, store the password as an SM secret, and exclude the account from analytics/billing.
• Assert every step — an unasserted step that breaks shows up as a later step's confusing failure.
• Stable URL cardinality: http.url template literal for any URL containing an ID.
• Keep runtime well under the timeout, and the timeout under the frequency.

Generating a check from an OpenAPI spec (or similar)

Given an API description — an OpenAPI/Swagger spec, GraphQL schema, or Postman collection — the mechanical conversion to k6 calls is easy. What matters is what you choose to convert:

1. Journeys, not endpoints. Do NOT generate one check per path, or one check that sweeps every path — that monitors the spec, not the service, and every extra check multiplies execution cost. Identify the 1–3 flows whose failure means "customers are impacted" (auth → core action → result) and write one scripted check per flow.
2. Filter for safety. Only include mutating operations (POST/PUT/DELETE) when the flow cleans up after itself (create-then-delete, as above) or targets dedicated test resources. A spec lists destructive operations right next to health endpoints — never exercise them against production just because they're documented.
3. Assert from the response schema. The spec tells you exactly what a healthy response contains — assert required fields, not just the status code: expect(order.json('id'), 'id required by OrdersResponse schema').toBeDefined().
4. Verify the target URL. servers: blocks (and Postman environments) often list localhost or staging first — confirm the production base URL with the user, and map securitySchemes credentials to SM secrets, never to values inlined from the spec. (Secrets in plain HTTP/protocol checks are a recent, feature-flagged rollout — check current docs; the scripted secrets.get() path always works.)
5. Treat format: int64 ids as strings. res.json('id') parses into a JS number and silently corrupts values past 2^53 (snowflake-style ids), so the readback URL 404s on every execution while the create looks fine. Extract from the raw body instead: const id = (/"id":\s*(\d+)/.exec(res.body) || [])[1]; — and never do arithmetic on it.

No API spec at all? Probe the frontend: open the web app with browser devtools (or curl likely routes) and capture the /api/* XHR calls it makes — that's a monitorable HTTP surface even when the documented backend services are gRPC-only or internal.

Browser check authoring

Required scaffold: import k6/browser and declare the chromium browser type. The UI validates both.

Browser-specific rules:

• Locators: getByRole / getByLabel / getByTestId (ask the app team to add data-testid where needed) > text > CSS. Never XPath or generated class names. getByTestId assumes data-testid — apps instrumented for Cypress often use data-cy instead; fall back to page.locator('[data-cy="..."]').
• No manual waits before interactions: locator actions auto-wait for visibility and enabled state. Don't call waitFor() before click()/fill(), don't use waitForLoadState(), never sleep().
• Auto-retrying expect() (toBeVisible, toBeEnabled, ...) is the wait mechanism for asserting state you don't interact with. Caveat: despite being listed as retrying, the text matchers (toHaveText/toContainText) hard-fail on the first mismatched read — e.g. an empty string mid-hydration on a client-rendered app. Assert dynamic text by locating it and asserting visibility instead: await expect(page.getByText('Order confirmed')).toBeVisible().
• Assertion timeout defaults to 5s — client-side-rendered apps routinely take longer to first meaningful render. Raise it once and use the configured instance: const expectUi = expect.configure({ timeout: 20000 });.
• Assert the outcome (logged-in heading, order list, confirmation text) — a page can load fine while the journey is broken.
• try/finally with page.close() so the browser is released even when an assertion throws.
• Screenshot artifacts aren't a documented SM feature — don't build failure handling around page.screenshot(); rely on assertion messages and the check's logs (SM stores per-execution logs in Loki).
• Web Vitals (probe_browser_web_vital_lcp|cls|fcp|inp|ttfb) are collected automatically — no extra code needed.

Validate locally, then deploy

SM scripts are plain k6 scripts — always run them locally first:

Pass = exit code 0, one iteration, no failed assertions in the summary. Run it 3–5 times; a script that is 90% reliable locally will page you nightly from 3 probes.

Then create the check (pick one):

• UI: Testing & synthetics → Synthetics → Add new check → k6 scripted / k6 browser → paste script → select probes + frequency → Test (runs once without saving) → Save.
• API or Terraform: see references/api-and-terraform.md. Key gotchas: API frequency/timeout are milliseconds and settings.scripted.script / settings.browser.script are base64-encoded; Terraform takes the plain script via file().

Verify it works, and rollback

Wait one frequency interval, then in Explore against the Synthetic Monitoring metrics (Prometheus) datasource:

A healthy first execution: probe_success == 1 from every probe, all probe_check_success_rate series at 1, duration stable across probes, and the check's prebuilt dashboard (Synthetics → check → View dashboard) showing logs for each execution. Browser checks should additionally show probe_browser_web_vital_* series.

Rollback: set the check's enabled: false (UI toggle, API update, or Terraform) to stop executions without losing history; delete the check only when you no longer need its configuration. Alerting: start with alertSensitivity / the default alert rules on probe_success — see the testing skill for alert rule examples.

Common failure modes

References

• references/api-and-terraform.md — SM API auth + check CRUD payloads (scripted, browser, MultiHTTP) and Terraform examples for every check type, including the protocol checks (HTTP, ping, DNS, TCP, traceroute, gRPC)

Resources

• Synthetic Monitoring docs
• k6 scripted checks · k6 browser checks
• Secrets management
• k6 assertions (expect) · k6 browser module
• k6 fundamentals and load testing: grafana-k6 plugin, k6 skill*