Synthetic monitoring vs real user monitoring

Synthetic monitoring is active: a machine you control runs a scripted check against your site or API on a schedule, from locations you pick, whether or not a single real user is online. Real user monitoring (RUM) is passive: a script in the page reports what actual visitors experienced, but only while actual visitors are there. Put bluntly, synthetic tells you the site is down. RUM tells you the site is slow for real people.

They answer different questions, and most mature teams end up running both. The order is not a coin flip. You buy synthetic first, because RUM cannot page you at 3am, and the rest of this post explains exactly why.

Synthetic monitoring vs real user monitoring: the comparison

The blind spot that decides the buying order

Here is the argument nobody selling RUM puts on their homepage. RUM depends on a beacon fired from a page that loaded in a real browser. An outage is the event that stops pages from loading. The failure mode and the reporting mechanism share a single point of failure.

Two ways that bites you:

• No traffic, no signal. Your B2B SaaS goes hard down at 3:04am on a Sunday. Nobody is on it. RUM records nothing, because nothing happened to record. You find out at 8:40am from a customer.
• The outage silences the reporter. If your origin returns 502 or DNS stops resolving, the page never renders, the RUM script never executes, and no beacon is sent. What you see in the RUM dashboard is not a spike of errors. It is a hole in the chart. A hole is indistinguishable from a quiet night, and you cannot alert confidently on an absence.

Some RUM products bolt on traffic-anomaly alerting to paper over this, and it half works at high volume. It is still inference from missing data, and it degrades exactly when your traffic is naturally low, which is exactly when synthetic checks are doing their most valuable work. Synthetic monitoring has the opposite property: the check runs whether the internet cares about you tonight or not, and a failed check is a positive signal you can route to a phone.

What is synthetic monitoring?

Synthetic monitoring is active testing: probes you control execute scripted requests against your service on a fixed schedule from chosen locations, then assert on the result. Because it manufactures its own traffic, it works with zero visitors, on staging, and at 3am. It is how you learn a service is unavailable or broken before a customer does.

In practice, a serious synthetic setup covers more than "is it 200":

• Uptime checks. HTTP and HTTPS requests validating status code and response time from multiple regions, so one flaky network route does not get logged as your outage.
• Keyword and JSON assertions. The most underrated check there is. A page can return 200 and render a white screen. An API can return 200 with {"data": null} after a bad deploy drops a field. HTTP status alone says both are healthy. Asserting that the page still contains "Add to cart", or that the JSON body contains a price key with a number in it, is the only thing that catches a soft failure.
• Multi-step transaction checks. A browser script that logs in, adds an item to the cart, and reaches the payment step. Your homepage being up is not the same as your revenue path being up, and the two fail independently.
• API contract checks. Custom headers, auth tokens, POST bodies, expected schema, latency thresholds. See our API monitoring tool for how those checks are configured.
• SSL certificate expiry. An unrenewed certificate is a self-inflicted total outage with a known date on it, and it still takes down sites every week.
• Ping and port checks. ICMP for reachability, TCP for the services with no web page at all: SMTP, database ports, message brokers.
• Cron heartbeats. Inverted synthetic checks. Your job pings a URL when it finishes, and the absence of that ping within the expected window raises the alert. Silent nightly-backup failure is a classic disaster that no browser-based tool can ever see.

That "200 but wrong" problem is not unique to web requests, incidentally. The same active versus passive split has taken over data engineering: an ETL job can exit 0 and still write garbage into the warehouse, which is why teams now run freshness, volume and schema checks on the data itself rather than trusting the pipeline's own success flag. Same principle, different layer: a green status code is not evidence of a correct result.

What is real user monitoring?

Real user monitoring is passive measurement: a small JavaScript snippet in your pages records what each visitor's browser actually experienced, then sends that telemetry back for aggregation. No synthetic traffic, no scripts, no assumptions about devices. It reports reality, with the hard constraint that it only sees the reality your visitors create.

What RUM gives you that synthetic never will:

• Real Core Web Vitals. LCP, INP and CLS as Google's field data measures them. A synthetic run from a data center with a gigabit link and a fresh CPU is a lab number. It is useful for regression detection and useless as a description of your users.
• Device and network truth. The mid-range Android on a congested LTE connection in Ohio, running your bundle on a CPU four times slower than your MacBook. That user's 6-second LCP does not exist in any synthetic report you will ever run.
• Geographic reality at your actual distribution. Synthetic gives you three or five vantage points you chose. RUM gives you every city your customers are actually in, weighted by how many of them there are.
• JavaScript errors in the wild. The exception that only fires in Safari 16 with a specific extension installed, on a session that never reported a support ticket.
• Conversion-path slowness. Which step of your funnel is slow for the segment that abandons. That is a revenue question, and synthetic cannot answer it.

None of that is decoration. If frontend performance is tied to your revenue or your rankings, you will want RUM eventually. It is just not the thing that wakes you up.

Active vs passive monitoring: when you need which

Match the tool to the failure you are actually afraid of.

• Pre-launch, or low-traffic B2B SaaS. Synthetic only. RUM with 40 visitors a day is a chart with too few points to mean anything, and half of them are your own team. You cannot compute a p75 LCP from noise. Meanwhile, a website monitoring tool checking every 30 seconds gives you 2,880 data points a day from day zero.
• Ecommerce at scale. Both, in that order. Synthetic to catch the outage and the broken checkout step at 2am. RUM to find which product pages are slow enough to cost you conversions on mobile. Both budgets are justified because both failures cost real money.
• Internal tools and machine-to-machine APIs. Synthetic, and there is no debate: there is no browser to instrument. A payments webhook or a partner API has no page, no DOM and no RUM script. It has an endpoint, a contract and a latency budget, and only an active check can test them.
• Marketing sites. Synthetic for availability and SSL, plus page speed testing if Core Web Vitals rankings matter to you. Traffic is often too spiky and too shallow for RUM to earn its price.
• High-traffic consumer apps. Both, comfortably. You have the volume to make RUM statistically honest and the blast radius to make synthetic non-negotiable.

The cost shapes are completely different

Synthetic pricing is bounded and predictable: you pay per monitor and per check frequency. Twenty monitors at 30-second intervals costs the same on the day you go viral as it did the day before. Our uptime monitoring pricing works exactly that way, and it is the cheapest insurance in your stack.

RUM pricing almost always scales with pageviews or sessions ingested. That means the bill grows with your success, and it grows fastest during the traffic spikes where you most want full visibility. Plenty of teams respond by sampling their RUM data down, which quietly reintroduces the blind spots they bought the tool to remove. Neither model is wrong. You just need to know which one you are signing, because one of them has a ceiling and the other does not.

Do I need both synthetic monitoring and RUM?

Eventually, if you have real traffic and revenue tied to frontend speed. Not on day one. Start with synthetic, because it is the only thing that will tell you the site is down, and downtime is the failure with the highest cost per minute. Add RUM once you have enough visitors for the field data to be statistically meaningful and a performance problem worth the ingest bill.

What is the difference between synthetic monitoring and real user monitoring?

Synthetic monitoring actively runs scripted checks on a schedule from infrastructure you control, so it works with zero visitors and detects outages immediately. Real user monitoring passively collects performance data from actual visitors' browsers, so it shows real device, network and Core Web Vitals experience, but only when real users are on the site. Synthetic finds downtime. RUM finds slowness.

Where AlertPing sits, honestly

AlertPing is a synthetic monitoring product. We run HTTP checks with keyword and JSON assertions, SSL expiry monitoring, ping and port checks and cron heartbeats, confirm every failure from three regions before firing, and send downtime alerts by SMS, Slack, email and webhook, with a status page for your customers.

We do not do real user monitoring. There is no JavaScript snippet, no Core Web Vitals field data, no session replay, and we are not going to pretend otherwise to win a comparison. If what you need is p75 INP on mid-range Android, buy a RUM tool. If what you need is to know, within 30 seconds and at any hour, that your checkout is returning 500 to everyone, that is the job we built for. Read how uptime monitoring works under the hood, or follow the setup walkthrough on how to monitor website uptime to get the first checks live.

The teams that get this right are rarely the ones with the most tools. They are the ones who bought the outage alarm before the performance microscope.