While there are many compelling reasons to use a Content Delivery Networks (CDN), you should consider the following situations before placing essential files on other domains…
1. Additional complexity
A link to a CDN file won’t work if you’re developing offline. That’s rarely an issue, but some developers have temperamental internet connections or need to work on the move.
2. Files may not be optimized
Consider a modular library such as Modernizr or YUI. Free CDN-hosted files are available but they generally provide all the common functionality within a large package that may exceed several hundred Kb.
That said, you could upload optimized files to a private CDN but you would need to consider the additional effort and cost that would incur.
3. There are no pre-caching guarantees
While there’s a higher probability a popular CDN file has been pre-cached, it’s not a given. In particular, mobile devices tend to have small and fairly inefficient caches. The advantages may be negligible, especially if you can host a smaller file on your local server.
4. Blocked access
We live in a world with geographic, legal, political, and commercial boundaries. It’s not uncommon for organizations or whole countries to block the domains or IP address of popular free CDN services.
Similarly, companies such as Google and Microsoft must adhere with US export laws. There are known restrictions on use of data in Cuba, Iran, North Korea, Sudan and Syria. The list of blocked countries can change on the whim of any government.
5. Two points of failure
It’s catastrophic when your site goes down and, despite a good reliability record, CDNs aren’t infallible. There’s little you can do but wait for the service to resume.
It would be possible to use a CDN and fall-back to local files if the service isn’t available. It’s a solution which provides a good level of redundancy — at the expense of further development and complexity.
Similarly, your CDN options will be more limited if you require HTTPS.
7. Loss of control
Are you happy handing control of website files to Google, Microsoft, Amazon or any other large web company? Are you concerned they’re collating information about your website and systems? Do those companies have too much control over the net? Perhaps it’s paranoia, but you have a reason to be paranoid when everyone’s out to exploit you!
If you’re feeling dizzy from all these pros and cons, come back soon for the final installment: Should You Use a Content Delivery Network?
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Craig is a freelance UK web consultant who built his first page for IE2.0 in 1995. Since that time he's been advocating standards, accessibility, and best-practice HTML5 techniques. He's created enterprise specifications, websites and online applications for companies and organisations including the UK Parliament, the European Parliament, the Department of Energy & Climate Change, Microsoft, and more. He's written more than 1,000 articles for SitePoint and you can find him @craigbuckler.