Synthetic Monitoring vs Real User Monitoring (RUM)

Two Approaches to Performance Monitoring

Website performance monitoring is divided into two fundamental approaches: Synthetic Monitoring and Real User Monitoring (RUM). They solve different problems and provide different data. Understanding the differences will help you choose the right strategy for your project.

Synthetic Monitoring

Synthetic monitoring uses automated scripts that simulate user actions. Tests run on a schedule from fixed locations (data centers) under controlled conditions.

How It Works

1. You configure a check: URL, frequency, geography, test type
2. The system regularly sends requests to your website
3. Response time, availability, and response correctness are measured
4. Notifications are sent when problems are detected

Advantages

• Proactive — detects problems before users notice them
• Controlled conditions — results are stable and reproducible
• Baseline metrics — ideal for establishing baselines and tracking degradation
• Availability monitoring — 24/7 uptime checks even during off-hours
• Pre-deployment testing — can test staging before going live

Limitations

• Does not reflect actual user experience
• Tests only predefined scenarios
• Does not account for device and network diversity
• Limited number of monitoring locations

Synthetic Monitoring Tools

The Heartbeat Monitor on Enterno.io is an example of synthetic monitoring. It regularly checks your website's availability and notifies you of problems. The Ping test and speed test are also synthetic checks.

Real User Monitoring (RUM)

RUM collects performance data from real users in real time. JavaScript code on your pages measures metrics directly in the visitor's browser.

How It Works

1. A JavaScript SNI is added to the website
2. The script collects Performance API документацию data from the user's browser
3. Metrics are sent to an analytics server
4. Data is aggregated and visualized

Advantages

• Real data — reflects actual user experience
• Full coverage — all devices, browsers, networks, geographies
• Scale — millions of data points instead of individual checks
• Business context — can correlate performance with conversion
• Anomaly detection — identifies issues for specific segments

Limitations

• Requires traffic — no users means no data
• Reactive approach — problems are discovered after affecting users
• Noisy data — high variance due to different conditions
• Performance impact — the monitoring script itself consumes resources
• Privacy — must comply with GDPR and other regulations

Comparison Overview

• Data: Synthetic — controlled, stable; RUM — real, variable
• Device coverage: Synthetic — limited; RUM — complete
• Availability: Synthetic — 24/7 without traffic; RUM — only with users
• Problem detection: Synthetic — proactive; RUM — reactive
• Reproducibility: Synthetic — high; RUM — low
• Business correlation: Synthetic — no; RUM — yes
• Cost: Synthetic — fixed; RUM — depends on traffic

When to Use Each Approach

Choose Synthetic Monitoring When

• You need uptime and availability monitoring
• You want to establish performance baselines
• You're testing staging/pre-production
• You have low traffic
• You need SLA monitoring

Choose RUM When

• Real user perspective is important
• You need to optimize Core Web Vitals
• You want to correlate performance with business metrics
• You have a diverse audience (different devices, regions)

Use Both

The best strategy is a combination of both approaches. Synthetic monitoring for uptime and baseline performance, RUM for understanding actual user experience.

Practical Approach

Start with synthetic monitoring — it's easier to set up and provides immediate results. Configure uptime monitoring for your key pages and APIs. When traffic grows, add RUM for the full picture.

Summary

Synthetic monitoring and RUM are not competitors but complementary tools. Synthetic monitoring provides proactive control and stable baseline metrics, while RUM gives a real picture of user experience. For comprehensive monitoring, use both approaches.