Synthetic monitoring vs real user monitoring

Synthetic monitoring sends scripted requests to your app on a schedule, so it catches an outage at 3am with zero visitors, while real user monitoring records what actual visitors experience in their browsers, so it catches slow pages and errors no script ever hits.

Logdash is synthetic monitoring. It sends HTTP requests to your URL from its own servers and alerts you when the answer is wrong. It is not real user monitoring and it does not run in your visitors' browsers. Both are worth having, and they fail in opposite directions, so it helps to know which hole each one leaves.

What is synthetic monitoring

A robot visitor on a timer. The simple kind sends one HTTP request and records the status code and response time. The heavy kind drives a headless browser through a script: log in, add to cart, pay. Logdash does the simple kind, one GET per check, every 5 minutes on the free plan, every minute on Builder and every 15 seconds on Pro. It needs no traffic, so it works the same at 3am as at noon.

Synthetic: one scripted request

curl -sS -L -o /dev/null -m 10 \
  -w '%{http_code} in %{time_total}s\n' \
  https://example.com/health

# 200 in 0.084s

What real user monitoring records

A script in your pages measures every real visit, such as how long the largest element took to paint, how fast the page reacted to a click and how much the layout jumped, then sends it home. That is per device, per country and per page, which no single check can give you. The catch is that it needs visitors. No traffic, no data, and an app that is fully down sends nothing at all.

Real user monitoring: what each visit reports

import { onCLS, onINP, onLCP } from 'web-vitals';

function send(metric) {
  const body = JSON.stringify({
    name: metric.name,
    value: metric.value,
    page: location.pathname,
  });
  navigator.sendBeacon('/rum', body);
}

onCLS(send);
onINP(send);
onLCP(send);

Synthetic (Logdash) vs real user monitoring (RUM)

Feature	Logdash	RUM
Where the data comes from	A request from Logdash servers on a schedule	Every real page view, from the visitor's browser
Full outage at 3am, no traffic	Caught within one interval	Silent, no visitors means no data
Slow pages on a phone in another country	Not visible, one request from one place	Visible per device, country and page
Front-end errors and layout shift	Not visible	Measured on every visit
Setup	Paste a URL	A script in every page and an endpoint to receive it

When RUM is the better pick

You care about Core Web Vitals. Google assesses them from real Chrome visits, not from lab or synthetic tests.
The app is up and users still say it is slow. Only field data shows which page, which device and which country.
You already host on Vercel. Speed Insights is free on every plan, with 10,000 events over a rolling 30 days and one overall Real Experience Score.

Synthetic monitoring vs APM

APM instruments your server code and traces each request through handlers, queries and outbound calls. It tells you which query made the endpoint slow. Synthetic monitoring tells you the endpoint was slow, or gone. APM cannot report a dead server, because the agent dies with it. Logdash is not APM: it has logs and metrics from eight SDKs beside each check, and no distributed tracing. If you need a scripted login flow tested, that is a browser check: Checkly runs Playwright scripts and its Hobby plan includes 1,000 browser runs a month.

Which one first

Synthetic. It covers the worst case, fully down, with zero traffic and one URL. Add RUM once you have enough visitors for the numbers to mean something.

Add the URL Create a service in Logdash and paste the address of your health endpoint, or your homepage if the site is static. The first check runs straight away, so a wrong path shows up in seconds rather than during an incident.
Pick the interval Every 5 minutes on the free plan, every minute on Builder, every 15 seconds on Pro. Each check stores the status code and response time, so you get a latency chart from one place to set against your RUM data.
Break it on purpose Connect a Telegram channel, then stop the app or make the endpoint return 503. An alert you never tested is an alert you cannot trust. On the next check the monitor flips to down and a Telegram alert lands on your phone with the monitor name, the status code and the error.

What is synthetic monitoring?

Monitoring that sends scripted requests to your app on a schedule, either single HTTP checks or a headless browser running a script, and alerts you when the result is wrong. It works with zero traffic.

Synthetic monitoring vs real user monitoring: which should I set up first?

Synthetic. It catches a full outage within one check interval even when nobody is on the site, and it takes one URL to set up. RUM needs a script in every page and enough visitors before its numbers mean anything.

Synthetic monitoring vs RUM: can one replace the other?

No. Synthetic sees one request from one place and misses slow pages for real users. RUM sees real visits and goes silent when the site is fully down. Logdash is synthetic only, so pair it with a RUM tool if field data matters to you.

Synthetic monitoring vs APM: what is the difference?