Synthetic runs scripted checks from controlled locations and browsers to catch regressions and ensure uptime 24/7. The best real user monitoring tools captures what real users actually experience on real devices, networks, and geographies, ideal for prioritizing work that moves Core Web Vitals (INP/LCP/CLS) and revenue. The best practice is to combine both: synthetics for proactive reliability, RUM for field truth and business impact.
Synthetic Monitoring
Active • ControlledScripted tests (browser/API) from chosen regions and devices.
- Best for pre-prod, uptime/SLA, regression checks
- Works even with low traffic or off-hours
- Stable baselines, proactive alerts on journeys
Real User Monitoring (RUM)
Passive • Field DataMeasures real users on real devices, networks, and geographies.
- Best for prioritizing by impact (conversions, UX)
- Tracks Core Web Vitals at p75: INP, LCP, CLS
- Finds outliers (country, ISP, page, device)
Use Both (Recommended)
Coverage • ConfidencePair proactive reliability with real-world experience.
- Synthetics guardrails critical paths 24/7
- RUM proves real impact and guides prioritization
- Align metrics & alerts; correlate to APM/logs
Updated: October 22, 2025 • INP replaced FID as the responsiveness vital (use RUM to track field performance).
Synthetic Monitoring vs Real User Monitoring (RUM) — Comparison at a glance
Skim this side-by-side to see where each shines. On mobile, scroll the table horizontally.
| Dimension | Synthetic Monitoring | Real User Monitoring (RUM) |
|---|---|---|
| Nature | Active, scripted tests run from chosen regions/browsers/devices. | Passive, field data from real users on real devices & networks. |
| Environment | Great in pre-prod/staging and production canaries. | Best in production (actual traffic & behavior). |
| Traffic dependency | Works with zero traffic. | Needs real traffic (sampling helps at scale). |
| Best for | Uptime/SLA, regression detection, 24/7 journey checks. | Prioritizing by business impact, trends, UX reality. |
| Core Web Vitals | Lab baselines; good for change control & guardrails. | Field CWV at p75: INP, LCP, CLS. |
| Uptime / SLA | Primary use case (HTTP/API + browser flows). | Indirect (errors & availability as experienced by users). |
| Transactions | Deterministic scripted journeys (login, checkout). | Observes real funnels; reveals drop-offs & variance. |
| Alerting | Threshold/availability & step failures (proactive). | Distribution shifts (p75) & outlier segments (geo/ISP/device). |
| Debug depth | Repeatable filmstrips, HAR, controlled repro. | Real-world session context, errors, optional replay. |
| Outlier detection | Limited (unless you simulate many geos/ISPs). | Strong (actual geos, ISPs, devices, pages). |
| Privacy & governance | Lower risk (robots). Data mostly synthetic/logs. | Needs PII masking, consent (CMP), RBAC, EU hosting options. |
| Cost model | By number of checks/locations/frequency. | By sessions/pageviews; sampling controls spend. |
| Limitations | May miss real-world variance & human behavior. | Needs traffic; less deterministic for exact repro. |
| When it shines | Before launch; at night; SLAs; catching regressions early. | After launch; proving impact; SEO/CWV; market/geo insights. |
| Best together | Use both: synthetics for guardrails & early alerts, RUM for field truth, prioritization, and Core Web Vitals outcomes. | |
Tip: align metric names across both (e.g., route names, journey IDs) and correlate to APM/logs for faster root-cause.
Definitions — Synthetic Monitoring & Real User Monitoring (RUM)
What is Synthetic Monitoring?
Scripted checks that simulate user actions from chosen regions, browsers, and devices.
- Runs on schedule (cron) — works with zero traffic, day or night.
- Validates uptime, API responses, and transaction flows (login, checkout).
- Ideal for pre-prod/staging and guarding critical paths in production.
- Produces stable baselines and proactive alerts on step failures.
What is Real User Monitoring (RUM)?
Real user monitoring measures what real users experience on real devices, networks, geographies, and pages.
- Tracks Core Web Vitals at p75: INP, LCP, CLS (field reality).
- Finds outliers by country, ISP, device, or page template.
- Correlates UX with conversions, errors, and backend traces.
- Optional session replay (with PII masking & consent) speeds diagnosis.
When to use each — a practical decision framework
Pick the right tool for the moment. Each scenario card recommends Synthetic, RUM, or Both, with a short “why” and a concrete next step.
Pre-launch & staging checks
SyntheticYou need repeatable, controlled tests to catch regressions before production.
- Script journeys (login, add-to-cart, checkout)
- Run hourly from 2–3 key regions & browsers
- Block release on failures (CI/CD hook)
Low traffic or off-hours coverage
SyntheticRUM can’t alert without users; synthetics give 24/7 availability and baselines.
- Set frequency (5–15 min) for critical flows
- Add API checks for dependencies
- Alert on step failures & latency thresholds
SEO & Core Web Vitals outcomes
RUMOnly field data reflects real user experience at p75 for INP, LCP, CLS.
- Instrument RUM on key templates & routes (SPA)
- Alert on p75 shifts for INP/LCP/CLS
- Segment by geo/ISP/device to find outliers
“It’s slow in country X / ISP Y”
RUMReal traffic exposes variance by geography, ISP, and device mix.
- Drill field metrics by geo & network
- Correlate with errors & third-party tags
- Backtest fixes with A/B or time windows
Checkout reliability (24/7)
BothGuard rails + real impact: synthetics catch outages; RUM shows actual drop-offs.
- Synthetics: scripted checkout from 3 regions
- RUM: monitor conversion + CWV on steps
- Correlate to APM/logs for root-cause
Intermittent JS errors & long tasks
BothRUM finds real sessions & INP outliers; synthetics reproduces with control.
- RUM: upload source maps & track INP attribution
- Replay (masked) sessions to see patterns
- Reproduce via synthetic scripted steps
Measure impact of an optimization
RUMOnly field distributions capture perceived gains across devices & networks.
- Compare p75 before/after (same cohorts)
- Slice by device/geo; watch INP/LCP deltas
- Validate no regression via synthetics
Compliance, privacy & data residency
BothSynthetics minimizes privacy risk; RUM requires strict masking & governance.
- Enable PII masking & CMP hooks (RUM)
- Choose EU hosting / sovereignty options
- Limit capture scope; define retention
How to combine Synthetic Monitoring and RUM — a 6-step playbook
Use this lightweight sequence to pair proactive guardrails (synthetics) with field truth (RUM). It’s vendor-neutral and works for web apps, SPAs, and APIs.
-
Map critical journeys & top traffic pages
List the business-critical flows (login, search, add-to-cart, checkout) and the highest-traffic templates/routes. These will anchor both synthetic checks and RUM dashboards.
-
Instrument RUM for field metrics (INP/LCP/CLS)
Add the browser tag/SDK, enable SPA navigation tracking, and surface p75 distributions for INP, LCP, CLS. Upload source maps to make JS errors readable.
- Segment by geo/ISP/device to catch outliers
- Set baselines per route/template
-
Align naming & sampling across tools
Use the same route names / journey IDs in both RUM and synthetics. Configure sampling and environments (prod/stage) to control cost and noise.
- Tag builds with
release/app.version - Exclude bots / admin traffic from RUM
- Tag builds with
-
Build synthetic guardrails (browser + API)
Script journeys for the flows from step 1 and add API checks for dependencies. Run every 5–15 min from 2–3 regions and at least 2 browsers/devices.
- Fail the build on journey errors (CI/CD)
- Store HAR/filmstrips for repro
-
Correlate and reproduce fast
When RUM flags a regression (e.g., INP spike in country X), jump to session context (and optional replay), then reproduce with a targeted synthetic scenario. Link to APM/logs for root-cause.
- Trace IDs / error IDs clickable from RUM
- Create a synthetic “hotfix” runbook
-
Governance, alerts & continuous review
Enforce PII masking, CMP consent, RBAC, and EU hosting options. Set alerts on synthetic availability/latency and on RUM p75 shifts (INP/LCP/CLS). Review weekly.
- RUM: alert on p75 INP > 200 ms, LCP > 2.5 s, CLS > 0.1
- Synthetics: alert on 2-step failure or > SLAs
Quick configuration checklist
- RUM tag/SDK live on key routes (prod only)
- SPA navigation detection enabled
- Source maps uploaded per release
- Synthetic journeys scripted for login/checkout
- API checks for auth, catalog, payment
- Shared route/journey naming across tools
- RBAC + PII masking + CMP hooks
Alert policy templates
- RUM (field) — INP p75 ↑ 20% (15 min), LCP p75 > 2.5s, CLS p75 > 0.1
- Synthetic (browser) — Step failure (2 of 3), journey > baseline +25%
- Synthetic (API) — 5xx rate > 1%, latency > 95p +30%
- Budget guardrail — RUM sampling cap by env/app
Privacy & data residency (EU)
Default to masking, restrict replay fields, log access, and prefer EU hosting with sovereignty guarantees when required.
Metrics that matter — and where they’re best measured
Use this vendor-neutral matrix to decide whether a metric is best tracked with RUM, Synthetic, or Both. On mobile, scroll horizontally.
| Metric | Best measured in | Why / when | Typical alert / target |
|---|---|---|---|
| INP (Interaction to Next Paint) | RUM | Represents real responsiveness across the whole visit; needs field data and real interactions. | p75 INP ≤ 200 ms (good), > 200–500 (needs), > 500 (poor). |
| LCP (Largest Contentful Paint) | Both | RUM for impact by geo/device; Synthetic for controlled baselines and regression tests. | p75 LCP ≤ 2.5 s (good), > 2.5–4.0 (needs), > 4.0 (poor). |
| CLS (Cumulative Layout Shift) | RUM | Layout shifts are driven by real content/ads/user behavior; field data tells the truth. | p75 CLS ≤ 0.1 (good), > 0.1–0.25 (needs), > 0.25 (poor). |
| TTFB | Both | RUM reveals real networks/CDNs; Synthetic isolates server/regression with fixed nodes. | Watch p75; common guardrail around 0.8–1.8 s depending on stack & region. |
| Uptime / Availability | Synthetic | Deterministic, round-the-clock checks independent of traffic; ideal for SLAs. | Availability ≥ 99.9% monthly; fail on 2 of 3 probe errors. |
| API latency & 5xx rate | Synthetic | Scripted API assertions and multi-region probes to de-risk dependencies. | 95p latency < baseline +30%; 5xx rate < 1%. |
| Transaction (login/checkout) duration | Both | Synthetic = guardrails; RUM = real drop-offs by segment and device mix. | Synth: duration +25% vs baseline; RUM: p75 step times increasing > 20%. |
| JS error rate / stack traces | RUM | Field stacks + source maps locate real crashes; Synthetic can reproduce once identified. | Error rate > baseline +X% or new top error appears. |
| Long tasks / main-thread blocking | RUM | Often device/CPU dependent; field data surfaces segments causing INP regressions. | Time blocked > threshold on target routes; INP p75 ↑ 20%. |
| 3rd-party / tag impact | Both | Synthetic for clean before/after baselines; RUM for real impact on users & conversions. | Alert when tag adds > X ms to LCP/INP or increases JS errors. |
| DNS / Connect / TLS | Synthetic | Best isolated in lab from multiple nodes to detect provider or routing issues early. | p95 connect/TLS spikes vs baseline +30%. |
| Experience availability (as felt) | RUM | Captures real-world outages or blockers users hit despite green synthetics. | Drop in successful sessions or conversion beyond normal seasonality. |
Tip: align route/journey names across tools and correlate to APM/logs. Reminder: INP replaced FID in 2024 — track INP in the field.
Pre-production → Production — the handoff that prevents regressions
Use a two-lane workflow: synthetics to block regressions before release, then RUM to validate real-world impact after go-live. This section shows who does what — and when — so nothing slips through.
Before release
-
Script critical journeys
Login, add-to-cart, checkout, account — with assertions on text, status, and timings.
-
Add API checks
Auth, catalog, payments. Validate 95p latency and error rates against SLAs.
-
Run from 2–3 regions & browsers
Create stable baselines and catch geo/device-specific regressions early.
-
CI/CD block on failures
Fail the pipeline when steps break or exceed thresholds; store HAR/filmstrips.
After release
-
Validate with field metrics
Track INP/LCP/CLS at p75 by route/template and cohort (geo/ISP/device).
-
Correlate to errors & traces
Use JS errors (with source maps) and APM/logs to find and explain outliers.
-
Alert on distribution shifts
Raise alerts when p75 worsens (e.g., INP ↑ +20%) or conversion drops on a step.
-
Feedback to synthetics
Turn new RUM findings into targeted synthetic scenarios to reproduce and prevent.
Tooling landscape — vendor-neutral overview
RUM and Synthetic live across a few clear categories. Skim these neutral cards to understand what type of platform fits your context. Open a card to see common examples and typical strengths.
Unified DEM suites (RUM + Synthetic + APM)
Full-stack
All-in-one observability with RUM, browser/API synthetics, traces & logs, and alerting in one UI.
- Enterprises wanting cross-signal correlation
- Governance, RBAC, compliance & auditability
- One contract, one data platform
Frontend performance specialists (CWV focus)
CWV/INP
Deep diagnostics for INP/LCP/CLS, visual comparisons, and developer-friendly insights.
- Teams optimizing Core Web Vitals as KPIs
- Visual diffs, filmstrips, asset-level timing
- Clear guidance for engineers
Uptime / Status platforms with RUM add-ons
Guardrails
Straightforward synthetic uptime & transactions with optional RUM overlay for websites/APIs.
- Fast setup for incident visibility
- SLAs/SLOs and public status pages
- Basic RUM to complement synthetics
API-first synthetic platforms
APIs/Backends
Programmable probes and assertions for HTTP(s), auth flows, third-party dependencies, and SLAs.
- Microservices & partner integrations
- Contract tests in CI/CD
- Multi-region latency & 5xx budgets
EU-focused platforms (hybrid / self-host options)
EU governance
RUM + Synthetics with options for EU hosting, sovereignty controls, and on-prem/hybrid deployments.
- Regulated sectors & data residency demands
- Granular privacy/PII masking & access logs
- Flexible deployment (cloud, hybrid, on-prem)
Open-source / self-hosted stacks
DIY
Own the pipeline with community agents; shift cost to infrastructure & operations.
- Teams with strong DevOps/SRE capabilities
- Customization & data ownership
- Cost control at very high scale
Note: Examples are illustrative and vendor-neutral. Replace/expand based on your ecosystem and compliance needs.
FAQ — Synthetic Monitoring vs Real User Monitoring (RUM)
What’s the difference between Synthetic Monitoring and RUM?
Synthetic runs scripted tests from controlled locations, browsers, and devices — great for uptime, SLAs, and preventing regressions. RUM measures what real users experience in production — ideal for prioritizing work that improves Core Web Vitals (INP/LCP/CLS) and conversions.
Which is better: RUM or Synthetic?
Neither is “better” in all cases. Use Synthetic when you need repeatable guardrails and 24/7 coverage; use RUM when you need field truth and business impact. Most teams get the best results by combining both.
Do I need both, or can one replace the other?
They are complementary. Synthetic catches issues before users do; RUM confirms how users are affected. Replacing one with the other usually leaves blind spots (either no field reality or no proactive guardrails).
Pre-production vs production — which tool fits where?
- Pre-prod/staging: Synthetic journeys and API checks in CI/CD to block regressions.
- Production: RUM for field metrics at p75, outliers by geo/ISP/device, and real funnels.
- After release: Validate impact with RUM, then reproduce via targeted Synthetic tests.
Does RUM affect SEO and Core Web Vitals?
Yes — RUM provides field measurements for INP, LCP, CLS at the 75th percentile. Improving these for real users strengthens page experience signals and often correlates with better business outcomes.
What changed with INP replacing FID?
INP (Interaction to Next Paint) replaced FID as the responsiveness vital in 2024. INP looks at all interactions across a visit, so you should track it in RUM and add Synthetic guardrails for critical user actions.
How often should I run Synthetic checks?
Common baselines are every 5–15 minutes for critical browser journeys and 1–5 minutes for key API endpoints. Use multiple regions and at least two browsers/devices for coverage.
How do privacy and data residency apply?
RUM: enable PII masking by default, integrate your CMP, apply RBAC, and choose EU hosting when required. Synthetic: lower privacy risk (robots), but still treat credentials and test data securely.
Updated: October 22, 2025