CDN vs Homegrown Caching

Question

My understanding of a CDN (like Akamai or Limelight) is that they are heavy-duty caching services.

I also understand that they are very expensive. So I'm wondering why I can't just create my own cluster of replicated caching servers (using, say, EhCache or Memcached) for all my web app's caching needs (images, URL hits/responses, Javascripts, etc) and basically get the same thing?

In essence, from a developer's perspective, what are the benefits (technical or otherwise) of paying a CDN vs. just using your own caching solution? Or, if I have completely misunderstood what CDNs are, please correct my understanding! Thanks in advance!

Answer 1

The key to this is the N in CDN, Content Delivery Network.

The advantage of a CDN (a good one, at least) is that it's geographically distributed. What the big CDN providers do (the ones you mentioned along with others) is have hundreds or thousands of servers in facilities all over the world. This lets what ever resource the CDN is serving sit as geographically close as possible to the end user no matter where they are in the world.

You can certainly replicate the functionality. At a very basic level most CDN's are simply an object/value store which plenty of software can do - what you can't do, at the scale these guys do it at any rate, is have servers all over the world to serve and replicate these objects.

If all you're interested in is an efficient way to store and serve static files then an object store coupled with something like nginx would do just fine. A CDN is used to make sites load as fast as they possibly can, but they come at a price.

For the record, Amazon's Cloud Front, while not as distributed as Limelight or Akamai is a lot cheaper and a good middle ground compromise on cost vs service.

Answer 2

The advantage of big CDNs is that they own distributed resources all over the world, allowing them to serve the users from a servers near them. Besides caching, this is the major point that makes them fast.

To build your own CDN, you would have to install servers on multiple continents, arrange good Internet connectivity for them, setup caching and make sure you have the servers all synchronized. It's not impossible to do that, but it will be very cost-intensive and might not be profitable.

Answer 3

With all due respect, I disagree with the other two answers given thus far to this question. Just to be clear, I'm not saying they are wrong, I'm just offering a different perspective.

While it is certainly true that the key to CDN performance is the "N," as Bulk eloquently explained, that doesn't mean you can't build your own CDN. The question is whether or not it's worth the time (and by definition, the money).

We live in a world of cheap servers and even cheaper virtual machines. Sure the big CDN networks have thousands/millions of servers all over the world, but that's because they have thousands/millions of sites to serve. Depending on the size of your site/app, all you really need in terms of resources is as much as your site(s) need. If you're small, the minimum might be a VPS on each US coasts, one in Europe, maybe two in Asia, and one in Australia. Sure, the hardware costs are too high for your typical homepage, but they are certainly not extreme, and if you're looking at CDNs in the first place, they are probably within your budget.

To me, commercial CDN services just provide PaaS convenience, but there is nothing preventing you from getting IaaS and building up your own platform.

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One more thing on this topic:

I once read a comment either by David Heinemeier Hansson (the creator of Ruby on Rails) or by someone referring to him that went something along the lines of: The owner(s) of 37 Signals were concerned DHH was using Ruby to build their application. At that point Ruby was still very obscure. Almost all web hosts were offering PHP, Perl, and Microsoft technologies. When asked about the fact that there were only a handful of Ruby hosts in the world, DHH asked, "well how many do you need?"

To me the point is you need to look at what's best for your needs and the needs of your application, and not necessarily what the guys with millions of servers think.