The right website-monitoring script depends on the failure you need to detect. Use a lightweight HTTP probe for reachability, status codes, latency, and response content. Use a real browser (such as Playwright) when you must prove that JavaScript, login, checkout, or another multi-step journey works. In both cases, define an explicit pass condition, record enough evidence to diagnose failures, and send the result to an alerting system.
A single successful request only describes what one probe runner could reach from its network location. For meaningful coverage, document the URL, cadence, timeout, success assertions, and probe locations.
1. Decide what “healthy” means
Write the failure definition before writing code. Common checks include:
- Availability: DNS, TCP, TLS, and HTTP connection complete.
- Correct response: the status is in the range or exact value you expect (for example, 200).
- Content assertion: the body contains a stable marker such as a release name or signed-out heading.
- Performance: response or journey latency stays below your documented threshold.
- User workflow: a browser can load, authenticate, submit, and observe the expected result.
Match the probe to the risk. HTTP/TCP checks are cheap and fast but cannot prove that client-side rendering or a checkout flow works. Browser checks provide deeper coverage and therefore need more CPU, secrets handling, and careful test-data cleanup.
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2. A small HTTP monitor in Python
This script checks status, elapsed time, and a required text marker. It emits a non-zero exit code for an operator, cron, CI job, or container health check.
#!/usr/bin/env python3
import sys
import time
import requests
URL = "https://example.com/health"
EXPECTED_TEXT = "OK"
TIMEOUT_SECONDS = 15
MAX_SECONDS = 3.0
started = time.perf_counter()
try:
response = requests.get(
URL,
timeout=TIMEOUT_SECONDS,
headers={"User-Agent": "site-monitor/1.0"},
)
elapsed = time.perf_counter() - started
text_ok = EXPECTED_TEXT in response.text
status_ok = response.status_code == 200
passed = status_ok and text_ok and elapsed <= MAX_SECONDS
print({
"url": URL,
"status_code": response.status_code,
"elapsed_seconds": round(elapsed, 3),
"status_ok": status_ok,
"content_ok": text_ok,
"passed": passed,
})
if not passed:
print("failure_detail: status, content, or latency assertion failed", file=sys.stderr)
sys.exit(1)
except requests.RequestException as exc:
elapsed = time.perf_counter() - started
print({"url": URL, "elapsed_seconds": round(elapsed, 3), "passed": False}, file=sys.stderr)
print(f"request_error: {exc}", file=sys.stderr)
sys.exit(2)Install the only dependency with python -m pip install requests. Keep the marker stable; a marketing headline or rotating timestamp creates false alarms. Do not print cookies, authorization headers, or response bodies that may contain personal data.
Quick cURL probe
For a basic status and timing check, use:
curl --fail --silent --show-error --max-time 15
--output /dev/null
--write-out 'status=%{http_code} total_seconds=%{time_total}n'
https://example.com/health--fail makes HTTP errors fail the command, while --write-out preserves useful timing for logs. Add a separate content check when status alone is insufficient.
3. Preserve evidence and alert on action
Every run should record at least timestamp, target identity, probe location, pass/fail state, status code, duration, and a bounded error detail. Send failures to the system your team already watches—an incident platform, email, chat, or ticket queue. Alert after a sensible number of consecutive failures when transient network errors are common, but do not hide a sustained outage behind aggressive retries.
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4. Prometheus Blackbox Exporter for external probes
Prometheus’ multi-target exporter guide demonstrates passing a target and configured module to Blackbox Exporter:
curl 'localhost:9115/probe?target=prometheus.io&module=http_2xx'The exporter exposes metrics for DNS lookup, HTTP phases, status code, TLS details, and certificate expiry. Prometheus periodically scrapes that endpoint; the exact interval is a workload decision, not a universal five-second rule.
Keep the website as the target label
A common production mistake is allowing the exporter address to become the Prometheus instance label. Preserve the URL being tested with relabeling (or the equivalent target-preservation configuration) so dashboards and alerts identify the failed website, not merely the exporter host. A minimal scrape job is:
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- job_name: blackbox-http
metrics_path: /probe
params:
module: [http_2xx]
static_configs:
- targets:
- https://example.com/health
- https://status.example.net/
relabel_configs:
- source_labels: [__address__]
target_label: __param_target
- source_labels: [__param_target]
target_label: instance
- target_label: __address__
replacement: blackbox-exporter:9115Configure alerts from the resulting metrics, for example when the probe success metric is false or latency exceeds your service objective. Blackbox modules determine what “success” means; review the module’s status, TLS, and content settings rather than assuming every HTTP 200 is healthy.
5. Browser journey monitoring with Playwright
Use a browser when the important assertion is visible page content or a sequence of actions. This example checks a stable heading and writes a machine-readable result:
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import { chromium } from "@playwright/test";
const browser = await chromium.launch({ headless: true });
const page = await browser.newPage();
const started = Date.now();
try {
await page.goto("https://example.com", { waitUntil: "domcontentloaded", timeout: 30000 });
await page.getByRole("heading", { name: "Example Domain" }).waitFor({ timeout: 10000 });
console.log(JSON.stringify({
passed: true,
elapsed_ms: Date.now() - started,
url: page.url()
}));
} catch (error) {
console.error(JSON.stringify({
passed: false,
elapsed_ms: Date.now() - started,
error: String(error)
}));
process.exitCode = 1;
} finally {
await browser.close();
}Install Playwright with npm install @playwright/test and its browser binaries according to the project’s installation instructions. Prefer accessible roles, labels, and other stable selectors over CSS classes generated by a build. Capture a screenshot or trace only when needed for diagnosis, and redact secrets before storing artifacts.
Authentication and side effects
- Store credentials in the runner’s secret manager, never in source control or console output.
- Use a least-privilege synthetic account and rotate it.
- Prepare deterministic test data and clean it up, or use a non-mutating path.
- Wait for a specific UI state, not an arbitrary long sleep; retain a short timeout margin for slower regions.
- Decide whether a third-party dependency is part of your success criterion. Otherwise, mock or isolate it to avoid blaming your site for an external outage.
A practical scheduled example
The Playwright Synthetic Monitoring project illustrates running tests periodically in Docker, exposing Prometheus metrics and a health endpoint. Its README describes a default five-minute interval that can be configured and notes that the project does not deliver alerts itself. Treat those as properties of that example, not Playwright defaults; connect its metrics or health endpoint to your own alerting.
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| Approach | Best for | Trade-offs |
|---|---|---|
| Local script or cron | One endpoint, a small team, or a private network | You operate scheduling, history, redundancy, and alert delivery. |
| Prometheus plus Blackbox Exporter | Teams already storing metrics and writing PromQL alerts | Excellent protocol metrics; browser workflows require a separate runner. |
| Managed synthetic service | Hosted probes, retained results, and multiple locations | Review service limits, privacy, configuration, and regional behavior. |
| Self-hosted Playwright worker | Login, JavaScript, and multi-step journeys | Higher runtime and maintenance cost; secrets and test data need operational controls. |
Relevant documented options include Prometheus, Google Cloud synthetic monitoring, Grafana Cloud Synthetic Monitoring, and the New Relic Synthetics REST API. Their current prices and plan limits are not established here, so verify those details before choosing.
7. Location, cadence, and interpretation
A check reports what one runner could reach from one network location. Run from more than one region when geography matters, and label each result with its location. Google Cloud documents public checks from multiple global locations and selectable checker regions, but says request data is not guaranteed to remain in a specific geographic location. For Cloud Run-function synthetic monitors, invocation can occur from any region supported by uptime-check servers; selecting a function region does not configure the entire monitoring path.
Google Cloud’s documentation describes synthetic monitors as periodically issuing simulated requests, recording whether they succeeded, and recording additional request data such as latency. Set cadence according to user impact, rate limits, and cost. A five-second scrape may be suitable for a tutorial or a narrowly scoped target, but it can create unnecessary load in production.
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8. Troubleshooting common failures
DNS or connection timeout
Check resolver behavior, firewall rules, proxy settings, and IPv4 versus IPv6 from the runner. Compare from a second location before declaring a global outage.
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HTTP 200 but the monitor fails
Your content assertion may be absent, personalized, compressed differently, or rendered only by JavaScript. Inspect a bounded response sample, verify redirects, and move to a browser assertion if the user-visible content is client-rendered.
Browser test times out
Capture the failing URL and console/network error, then replace brittle selectors and arbitrary sleeps with explicit state waits. Check that the runner has the required browser dependencies and enough CPU.
Intermittent alerts
Separate connect, TLS, server, and content timings. Use a small consecutive-failure window, but preserve every failed sample so an operator can distinguish a regional network problem from an application regression.
Prometheus labels show the exporter
Inspect relabeling. The target URL must be copied into __param_target and then into instance before replacing __address__ with the exporter endpoint.
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Request a WebP image with cURL:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpPython:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)Node.js:
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FAQ
Should I monitor the homepage or a health endpoint?
Prefer a purpose-built health endpoint for dependency and status checks, then add a separate browser journey for the user experience your team must protect.
Can a screenshot prove uptime?
No. A screenshot demonstrates that a rendering request produced an image; pair it with HTTP or browser assertions and explicit alerting for availability monitoring.
What’s actually slowing this PC down?
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How many probe locations do I need?
Use one location for a private, single-region service only when that matches your users. Add locations where regional reachability or latency is part of the requirement.
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