For many large websites, CDNs are the foundation for delivering content quickly to their customers around the world. The ability of CDNs to cache responses close to consumers also allows these sites to operate on a small hardware footprint. However, compared to what they would have to invest if they operated without a CDN and delivered all content through their own systems, this comes at a cost: your CDN may now deliver content that is out of sync with your origin because you changed the content on your own system. This change is not done in an atomic fashion. This is the same “atomic” as in the ACID principle of database implementations.
This is a conscious decision, and it is caused primarily by the CAP theorem. It states that in a distributed data storage system, you can only achieve 2 of these 3 guarantees:
- Partition tolerance
And in the case of a CDN (which is a highly distributed data storage system), its developers usually opt for availability and partition tolerance over consistency. That is, they accept delivering content that is out of date because the originating system has already updated it.
To mitigate this situation the HTTP protocol has features built-in which help to mitigate the problem at least partially. Check out the latest RFC draft on it, it is a really good read. The main feature is called “TTL” (time-to-live) and means that the CDN delivers a version of the content only for a configured time. Afterwards the CDN fetches a new version will from the origin system. The technical term for this is “eventual consistent” because at that point the state of the system with respect to that content is consistent again.
This is the approach all CDNs support, and it works very reliable. But only if you accept that you change content on the origin system and that it will reach your consumers with this delay. The delay is usually set to a period of time that is empirically determined by the website operators, trying to balance the need to deliver fresh content (which requires a very low or no TTL) with the number of requests that the CDN can answer instead of the origin system (in this case, the TTL should be as high as possible). Usually it is in the range of a few minutes.
(Even if you don’t use a CDN for your origin systems, you need these caching instructions, otherwise browsers will make assumptions and cache the requested files on their own. Browsing the web without caching is slow, even on very fast connections. Not to mention what happens when using a mobile device over a slow 3G line … Eventual consistency is an issue you can’t avoid when working on the web.)
Caching is an issue you will always have to deal with when creating web presences. Try to cache as much as possible without neglecting the need to refresh or update content at a random time.
You need to constantly address eventual consistency. Atomic changes (that means changes are immediately available to all consumers) are possible, but they come at a price. You can’t use CDNs for this content; you must deliver it all directly from your origin system. In this case, you need to design your origin system so that it can function without eventual consistency at all (and that’s built in into many systems). Not to mention the additional load it will have to handle.
And for this reason I would always recommend not relying on atomic updates or consistency across your web presence. Always factor in eventual consistency in the delivery of your content. And in most cases even business requirements where “immediate updates” are required can be solved with a TTL of 1 minute. Still not “immediate”, but good enough in 99% of all cases. For the remaining 1% where consistency is mandatory (e.g. real-time stock trading) you need to find a different solution. And I am not sure if the web is always the right technology then.
And as an afterthought regarding TTL: Of course many CDNs offer you the chance to actively invalidate the content, but it often comes with a price. In many cases you can invalidate only single files. Often it is not an immediate action, but takes seconds up to many minutes. And the price is always that you have to have the capacity to handle the load when the CDN needs to refetch a larger chunk of content from your origin system.
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