Synthetic Monitoring vs Real User Monitoring: When to Use Each

Synthetic monitoring and real user monitoring (RUM) are fundamentally different approaches to understanding your service's health. One simulates users; the other measures them. Both have important blind spots that the other fills. The most reliable engineering teams use both, but understanding when each is most valuable helps you deploy them effectively.

What Is Synthetic Monitoring?

Synthetic monitoring (also called active monitoring) involves scripted checks that simulate user behavior. A monitoring agent — not a real person — makes requests to your endpoints on a schedule and reports the results.

Types of synthetic monitoring:

• Simple HTTP checks (is the endpoint responding with 200?)
• Browser-based checks (does the page render correctly?)
• Multi-step transaction checks (can a user complete checkout?)
• API checks (does the API return correct data?)

Key characteristics:

• Runs on a schedule (every 1 minute, every 5 minutes)
• Consistent, reproducible checks
• Works before you have real users
• Detects problems proactively
• Zero privacy concerns (no real user data)

AzMonitor is primarily a synthetic monitoring tool, running scripted checks from 20+ global locations on schedules you configure.

What Is Real User Monitoring (RUM)?

Real user monitoring captures performance data from actual users as they interact with your site. A lightweight JavaScript snippet (or mobile SDK) collects metrics as pages load and reports them to your monitoring platform.

What RUM captures:

• Page load times for every user, every page
• Core Web Vitals (LCP, INP, CLS) from real browsers
• Performance broken down by device type, browser, geography
• JavaScript errors that affect real users
• Time to interactive from real network conditions

Key characteristics:

• Reflects actual user experience (not simulated)
• Requires real traffic (useless for new sites)
• Can't predict future issues (only measures what happened)
• Contains user-derived data (privacy implications)
• Provides aggregate performance trends

Core Differences

| Dimension | Synthetic | Real User Monitoring | |-----------|-----------|---------------------| | Data source | Scripted agents | Real user browsers | | Proactive/reactive | Proactive | Reactive | | New site coverage | Works immediately | Needs traffic | | Geographic coverage | Configurable | Wherever users are | | Performance granularity | P50, P95 estimates | Actual distribution | | Privacy concerns | None | Data collection required | | Availability data | Yes | Limited | | User segment data | No | Yes (device, browser, location) | | Cost of failure detection | Immediate | After users experience it |

When Synthetic Monitoring Is the Right Choice

Use synthetic monitoring for:

Availability detection: RUM can't tell you your site is down — if no users can connect, no RUM data comes in. Synthetic monitoring fires alerts for availability failures regardless of traffic.

Pre-launch validation: Before launch, you have no real users. Synthetic monitoring validates your infrastructure is functioning before the first real user arrives.

API and backend monitoring: APIs often don't have user-facing browser interfaces. Synthetic monitoring works for any HTTP endpoint. RUM requires a browser.

Off-peak monitoring: User traffic drops overnight and on weekends. RUM data thins out, and low-traffic periods become invisible. Synthetic monitoring runs on a consistent schedule regardless of traffic volume.

SLA measurement: SLAs require consistent, objective measurement. Synthetic monitoring provides consistent check-by-check data suitable for SLA calculation.

Scheduled and authenticated flows: Monitoring flows that require authentication (login, checkout with real payment data) is easier with synthetic monitoring than with RUM.

When Real User Monitoring Is the Right Choice

Use RUM for:

Understanding actual user experience: Synthetic monitoring runs from well-connected servers; your users have 3G connections, old devices, and geographic diversity. RUM captures what they actually experience.

Core Web Vitals measurement: Google's ranking factors (LCP, INP, CLS) are measured from real users in the wild. Chrome User Experience Report (CrUX) data comes from real users, not synthetic tests. If you care about search rankings, RUM data matters.

Identifying user-specific issues: A bug that affects only mobile users on iOS Safari, or users in India on slow connections, won't appear in synthetic tests that run from desktop Chrome on fast connections. RUM catches these.

Performance trend analysis: Real user performance trends reveal things synthetic tests miss — are users getting faster or slower experiences as your user base grows? Are certain pages slower for logged-in users?

Business correlation: RUM lets you correlate performance metrics with business outcomes — does slower LCP correlate with lower checkout conversion? This analysis requires real user data.

The Blind Spots of Each Approach

Synthetic monitoring blind spots:

• Doesn't capture real browser variations (old devices, slow connections)
• Misses JavaScript errors that only affect certain user agents
• Can't detect problems only affecting specific user segments
• Check-based data misses issues between checks

RUM blind spots:

• Zero visibility during low-traffic periods
• Can't detect outages (no users = no data)
• Less useful for APIs and backend services
• Privacy constraints limit data collection (GDPR, CCPA)
• High-cardinality data can be expensive to store and analyze

Using Both Together: The Complete Picture

The most effective monitoring combines synthetic and RUM:

Synthetic monitoring (AzMonitor):
- Availability: Is the site up? (every 30 seconds)
- Functionality: Does checkout complete? (every 5 minutes)
- API health: Do APIs respond correctly? (every 1 minute)
- SSL and DNS: Are certificates valid? (continuous)

Real User Monitoring:
- Core Web Vitals: What do actual users experience?
- Performance trends: Is experience improving or degrading?
- Segment analysis: Which users have the worst experience?
- Business correlation: How does performance affect conversion?

Together:
- Proactive alerts when something breaks (synthetic)
- Understanding of user impact and experience (RUM)
- Complete coverage of availability AND performance

When your synthetic monitoring fires an alert, RUM data helps you understand user impact. Was this outage during a high-traffic period? Were users in specific regions more affected? Did conversion drop during the incident?

Getting Started

Most teams should start with synthetic monitoring — it's simpler to set up, provides immediate value, and doesn't require existing user traffic. Add RUM once your site has meaningful traffic and you want to understand real user experience.

Set up synthetic uptime monitoring with AzMonitor as your foundation. For RUM implementation, see our complete RUM guide.