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by acdha
1592 days ago
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To be clear: CDNs are still highly useful. This is just about using them to host third-party libraries rather than serving them yourself. It's worth noting that even a decade ago, using a CDN for third-party libraries often failed to produce the envisioned benefits. I tested this a far number of times and the cost of doing multiple additional DNS lookups, negotiating HTTPS, etc. was often higher than expected and the proliferation of versions meant that while it's true that the average browser hitting your site had a copy of jQuery in the cache it wasn't likely to be the same version from the same CDN. Since HTTP/2 went mainstream half a decade ago, that's become an even harder competition to win since the browser can efficiently request a large number of files simultaneously without any of that setup overhead. There's also a security concern: unless you use Sub-Resource Integrity (SRI) you're giving the CDN owner full control of your site and the users' data. That's not a catastrophic risk but it's probably more than a potential minor performance benefit is worth. The last reason not to use them is increasingly the most important: if you include something large like many sites do, you probably have a ton of code which is going to have to be downloaded but isn't actually used by your site. A custom bundle (with tree-shaking, etc.) is going to save a substantial amount of transfer and it's usually the case that transferring less will overwhelm all other concerns. |
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This always seemed like a lot of hopeful mythology. As you said, there were so many versions, and then if you start combining the modules, and the various public CDNs, it becomes a combinatorics problem that you can only when if the browser cache is enormous. But even a pretty large browser cache becomes pretty small when auto-play video is a thing and 2mb seems small for a landing page.
CDNs are certainly useful, if you can serve your content from closer to the user, that reduces latency which also increases potential bandwidth early in a connection. It's often helpful to serve content through a CDN, even if it's not cached, as reducing the latency makes a big difference (and the edge <-> origin connections can often be longer lived so that latency has less of an effect on the bandwidth available)