[1,060 words, 6 minute read time]
If you’re building a monitoring stack for a SaaS product, ecommerce store, or growing website, you’ll quickly run into two terms that sound similar but solve different problems:
- Uptime monitoring (synthetic monitoring): automated checks that simulate a visitor or request
- RUM (Real User Monitoring): measurements collected from actual users as they browse
Here’s the simplest way to remember it:
Synthetic tells you fast; RUM tells you truth at scale.
Most growing teams end up using both, because they answer different questions—and cover each other’s blind spots.
For the broader uptime monitoring roadmap, see the complete guide.
Definitions: what each one measures
Uptime monitoring (synthetic monitoring)
What it is: Your monitoring tool sends automated requests to your site on a schedule—every 1, 5, or 10 minutes—from one or more locations.
What it measures:
- Is the site reachable? (HTTP/HTTPS checks)
- Is a key page returning expected content? (keyword checks)
- Is a service reachable? (ping/port)
- Is a flow working? (multi-step synthetic checks: login, checkout)
What it’s best for:
- Fast detection and alerting
- Clear “up/down” signals
- Verification from multiple locations
- Testing critical flows proactively
You can go deeper on multi-step checks and API patterns in advanced monitoring.
RUM (Real User Monitoring)
What it is: A small script (often in the browser) collects performance and error data from real visitors.
What it measures:
- Actual load and interaction performance (by device, browser, network)
- Real error rates (JS errors, resource failures)
- Geographic and ISP-specific issues
- Impact by page/template and user segment
- Often, Core Web Vitals-style metrics
What it’s best for:
- Understanding real user experience
- Finding issues that only affect certain browsers/devices
- Seeing how widespread a problem is
- Prioritizing fixes by actual impact
What uptime monitoring catches that RUM misses
Uptime monitoring is proactive. It can catch issues before you have enough real traffic data to notice them.
Synthetic catches:
- Total outages (site doesn’t respond)
- Early warnings during low traffic periods (nights/weekends)
- Broken flows if you set multi-step checks (login/checkout)
- Regional outages if you monitor from multiple locations
- Dependency failures that show up as HTTP errors/timeouts
Example: outage in one region
If your CDN or routing has a regional failure, uptime monitoring from that region can alert you quickly—before a large number of users complain.
This is one reason multi-location checks matter in synthetic monitoring.
What RUM catches that uptime monitoring misses
RUM is observational. It’s the “truth serum” of user experience.
RUM catches:
- Browser-specific breakages (Safari/Firefox quirks, mobile issues)
- Device/network-related slowness (slow 4G, older phones)
- Front-end errors that don’t show up as server downtime
- “Up but unusable” experiences (UI loads, but clicks don’t work)
- User-impact distribution (who is actually affected, and how many)
Example: checkout broken for Safari users
Your synthetic checks might hit checkout successfully on a default browser environment, while real Safari users fail due to a JavaScript compatibility bug. RUM will show Safari error spikes and conversion drops even though your site is “up.”
The blind spots (what each can’t do well)
Synthetic blind spots
- Can miss issues tied to specific browsers/devices unless your synthetic tool supports that diversity
- Can miss real-world network variability and user behavior patterns
- Can produce false positives if blocked by WAF/bot protection
- Tells you something broke; doesn’t always tell you who’s impacted
RUM blind spots
- Doesn’t help much during low traffic (no users, no data)
- Can be slower to detect issues unless you have alerting set up
- May miss backend failures that prevent the RUM script from loading at all
- Often requires more setup and analysis
The hybrid approach (recommended for growing sites)
For most SaaS and ecommerce teams, the best stack is:
1) Synthetic (uptime) for detection + alerting
- HTTP/HTTPS monitor for critical URLs
- Keyword checks for “is it working?”
- Multi-step checks for login/checkout
- Multi-location confirmation for regional issues
2) RUM for impact + prioritization
- Track real performance and errors
- Segment by browser/device/geo
- Tie issues to revenue outcomes (conversion rate, drop-offs)
Why hybrid works:
Synthetic answers “something is wrong” fast.
RUM answers “who is affected and how bad is it” accurately.
If your monitoring strategy is still developing, your “home base” is the complete guide.
Budget and complexity guidance (how to choose what to start with)
If you’re early-stage or resource-constrained
Start with synthetic uptime monitoring first. It’s:
- simpler to set up
- easier to interpret (“down” is down)
- immediately actionable
Add RUM when:
- you have enough traffic to generate meaningful data
- you’re optimizing conversion/retention
- you’re seeing “slow but up” complaints
If you’re a growing SaaS/ecommerce business
Start with a minimal hybrid:
- synthetic monitoring on critical journeys
- RUM for key performance/error metrics
If you’re an agency
Synthetic monitoring is usually the backbone (clear SLAs, client reporting). RUM can be added selectively for higher-tier clients or performance-heavy sites.
Common misconceptions (that lead to bad monitoring setups)
“RUM replaces uptime monitoring.”
No. If your site is down and the RUM script can’t load, RUM may go quiet right when you need visibility. Synthetic still catches outages reliably.
“Uptime monitoring tells me everything about user experience.”
No. A 200 OK response doesn’t mean a real user can complete checkout, interact with the UI, or avoid browser-specific breakage.
“One synthetic check equals full coverage.”
Not even close. You need to monitor the journeys that matter (login/checkout) and validate content (keyword checks) as you grow—see advanced monitoring.
“Response time monitoring is the same as RUM.”
Not exactly. Response time monitoring is often synthetic (server response timing). RUM measures the real user experience end-to-end. For the difference, see response time monitoring.
Practical starting metrics (what to track first)
If you want a clean starting point without overbuilding:
Start with 1 synthetic metric
Choose one:
- Availability of your most important page (HTTP + keyword check)
- Success of a critical flow (login or checkout multi-step)
And 1 real user metric (RUM)
Choose one:
- Error rate (JS errors, failed requests)
- Real-user load performance (a Core Web Vitals-style metric, if available)
- Conversion drop-off for checkout/login funnel (if instrumented)
Choose one “synthetic” + one “real user” metric to start (CTA)
If you’re building a monitoring stack and don’t want to overthink it:
- Pick one synthetic check that represents success (login works, checkout loads, pricing page serves correct content).
- Pick one RUM metric that represents real user pain (error rate or real-user performance).
CTA: Choose one synthetic + one real user metric to start—then expand based on what breaks most often and what costs you most when it does.