What Is a Synthetic Check vs Real User Monitoring?

Quick Comparison

What Is Synthetic Monitoring?

Synthetic monitoring runs automated tests that simulate user behavior at regular intervals.

Think of it as a robot that visits your site every minute and reports what it finds.

How Synthetic Works

1. Monitoring service sends requests from probe locations
2. Probes perform checks (load page, call API, run transaction)
3. Results recorded: available/down, response time, errors
4. Alerts fire if checks fail or thresholds exceed

Types of Synthetic Checks

Simple HTTP checks

• Request a URL
• Verify response code
• Measure response time

API monitoring

• Call API endpoints
• Validate response content
• Check authentication flows

Browser tests

• Load page in real browser
• Execute JavaScript
• Measure Core Web Vitals

Transaction monitoring

• Multi-step user journeys
• Login → Search → Checkout
• End-to-end flow validation

Synthetic Monitoring Strengths

24/7 coverage: Runs even when no users are active

Consistent baseline: Same test, same conditions, every time

Early detection: Catches problems before users encounter them

Specific locations: Test from regions you care about

SLA validation: Prove availability with objective data

Synthetic Monitoring Weaknesses

Limited scope: Only tests what you configure

Not real user experience: Controlled conditions differ from reality

Blind spots: Misses issues that only appear under real traffic

Maintenance overhead: Tests need updating as your app changes

What Is Real User Monitoring?

Real User Monitoring (RUM) collects performance data from actual users as they use your application.

Think of it as listening to what your users actually experience.

How RUM Works

1. JavaScript snippet embedded in your pages
2. Snippet collects timing and performance data
3. Data sent to monitoring service
4. Aggregated into dashboards and alerts

What RUM Measures

Page load timing

• Time to first byte (TTFB)
• First contentful paint (FCP)
• Largest contentful paint (LCP)
• Time to interactive (TTI)

User interactions

• Click delays
• Input latency
• Scroll performance

Errors

• JavaScript errors
• Failed network requests
• Broken resources

Context

• Browser and version
• Device type
• Geographic location
• Network conditions

RUM Strengths

Real experience: Data from actual users, not simulations

Full coverage: Every user, every page, every interaction

Diverse conditions: Captures slow networks, old browsers, edge cases

Unexpected issues: Finds problems you didn't know to test for

User segmentation: Break down by browser, region, device

RUM Weaknesses

Requires traffic: No users = no data

Privacy considerations: Collecting user data requires consent

Volume challenges: High traffic means lots of data to process

Delayed detection: Problem must affect users before you see it

Sampling: High-traffic sites often sample, potentially missing issues

When to Use Each

Use Synthetic When:

Uptime monitoring You need to know immediately when your site goes down, even at 3 AM when nobody's visiting.

Pre-launch testing Validating a new deployment before real users hit it.

SLA tracking Proving to customers that you met availability targets.

Third-party monitoring Checking that external dependencies (APIs, CDNs) are working.

Specific user flows Ensuring critical paths (checkout, signup) always work.

Use RUM When:

Understanding real performance How fast is the site for users on mobile in rural areas?

Identifying problem segments Which browser has the highest error rate?

Measuring business impact Correlating performance with conversion rates.

Catching edge cases Finding issues that only appear under specific conditions.

Core Web Vitals Measuring the metrics Google uses for rankings.

Why Most Teams Need Both

Synthetic and RUM aren't competing approaches—they're complementary.

Synthetic catches what RUM misses

• Problems during low-traffic periods
• Issues before users encounter them
• Specific transaction failures

RUM catches what synthetic misses

• Real-world network conditions
• Browser-specific issues
• Unexpected user paths
• Performance at scale

Example: The Complete Picture

Scenario: Your checkout flow breaks

Synthetic monitoring:

• Scheduled test runs every 5 minutes
• Detects failure at 3:15 AM
• Alert fires immediately
• No users affected yet

RUM:

• First user encounters issue at 6:00 AM
• Reports start appearing
• You see it affects Chrome on Windows specifically
• 47 users impacted before fix

With both: You detected at 3:15 AM and fixed before 6:00 AM users arrived.

With only RUM: You found out at 6:00 AM when users started failing.

With only synthetic: You might have fixed it but not known it was Chrome-specific.

Implementation Strategy

Start With Synthetic

1. Uptime checks on critical endpoints
2. API monitoring for key transactions
3. Multi-location checks to verify global availability

This gives you baseline coverage with minimal setup.

Add RUM for Depth

1. Page load performance across your site
2. Error tracking for JavaScript issues
3. User flow analysis for conversion paths

This shows what users actually experience.

Correlate the Data

Look at both together:

• Synthetic says site is up and fast
• RUM shows specific user segments struggling

That gap is where the insights live.

Cost Considerations

Synthetic Costs

• Per-check pricing (more checks = more cost)
• Location multipliers
• Check frequency

Typical: $20-200/month for basic coverage

RUM Costs

• Per-pageview or session pricing
• High traffic = high cost
• Sampling can reduce costs

Typical: Free tier to thousands/month depending on traffic

Budget-Friendly Approach

If you can only afford one:

• Low traffic site: Start with synthetic (RUM has no data anyway)
• High traffic site: RUM may be more valuable (but synthetic is cheaper)

Ideally, use both at appropriate levels.

Summary

Synthetic monitoring simulates users with automated tests. It's proactive, consistent, and works without traffic.

Real User Monitoring observes actual users. It shows real experience, catches edge cases, and requires actual traffic.

Most mature teams use both:

• Synthetic for proactive detection and baselines
• RUM for understanding real user experience

Together, they provide complete visibility into your service's health.