External Resource Monitoring
Your website loads scripts, fonts, and stylesheets from third-party CDNs and external services. When any of them go down, your site breaks — but your origin server still says "200 OK". Upsonar's third-party dependency monitoring checks what your users actually see, not just what your server reports. Also known as external resource monitoring, CDN monitoring, or third-party script monitoring.
CDNs · Scripts · Fonts · Stylesheets · Per-region checks
What is external resource monitoring?
External resource monitoring (also called third-party dependency monitoring or CDN monitoring) is the practice of tracking the availability and performance of every asset a web page loads from outside its origin server — JavaScript libraries from CDNs, web fonts, payment widgets, analytics scripts, chat tools, ad networks, embedded videos, maps, and third-party APIs. A dependency monitor parses the HTML returned by the origin, extracts every external URL the page references, and checks each one independently from multiple regions on every monitoring cycle. When any dependency fails, becomes slow, or returns an error status, an alert is sent with the specific resource, the failure mode, and which regions observed the problem. This is categorically different from traditional uptime monitoring, which only checks whether the origin server itself responds and cannot see the dependency graph at all.
Why External Resource Monitoring Matters
CDN Outages Break Your Site
Cloudflare alone handles over 20% of all web traffic. When it or Google Fonts or jsDelivr has issues, your page breaks even though your server is fine. Users see a broken layout, missing functionality, or blank page.
Invisible to Traditional Monitors
Ping-based monitors check your server, not your page. They report "up" while your users see a broken site. Only dependency monitoring catches these failures.
Automatic Discovery
upsonar parses your HTML and finds every external resource automatically. No manual configuration — just add your URL and we discover all dependencies.
Real Outages Where a Dependency Broke the Web
These are not hypothetical scenarios. Each of them took major websites offline because of a single failing external dependency, and in each case origin-only uptime monitors reported nothing wrong.
June 8, 2021 — Fastly CDN global outage. A single customer pushing an invalid configuration triggered a latent bug in Fastly's Varnish-based edge, taking down Reddit, Amazon, The New York Times, The Guardian, CNN, Twitch, Shopify, Stack Overflow, and the UK government portal Gov.uk for approximately one hour. Fastly's post-mortem confirms that the impact was not a server failure at any of those sites — it was a failure at a shared CDN layer they all depended on.
February 28, 2017 — AWS S3 us-east-1 outage. A typo in a debugging command removed more servers from a subsystem than intended, cascading into a 4-hour failure of S3 in the largest AWS region. The outage broke Slack, Trello, Quora, Medium, Expedia, Imgur, and dozens of other services that stored static assets, images, or JavaScript bundles in S3 — including sites whose own infrastructure was completely unaffected.
July 2, 2019 — Cloudflare global outage. A bad regex in a WAF rule deployed to all Cloudflare edge servers simultaneously pinned CPUs and returned 502 errors for every site behind Cloudflare for 27 minutes. Per Cloudflare's own post-mortem, every Cloudflare-fronted site appeared down to users regardless of whether the origin itself was healthy — another failure mode that only a dependency monitor, not a server monitor, can catch.
The pattern is the same in every case. Your origin server is fine. Your uptime monitor says "200 OK". Your users see a broken page, missing content, failed JavaScript, or outright errors. External resource monitoring is the only category of tool that catches these failures at all, because it is the only category that actually checks the dependencies your page loads.
What We Monitor
Scan Your Website's Dependencies Now
Enter any URL to see all external resources it loads — CDNs, scripts, fonts, and more. Free, no signup required.
Frequently Asked Questions
Monitor What Your Users Actually See
Free plan includes external resource monitoring for 3 websites. Know when CDNs and third-party services affect your site.
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Last updated: April 2026