In the last blog post I proposed 5 rules for Caching with AEM, how you should design your caching strategy. Today I want to show another aspect of rule 1: Prefer caching at the CDN over caching at the dispatcher.

I already explained that the CDN is always located closer to the consumer, so the latency is lower and the experience will be better. But when we limit the scope to AEM as a Cloud Service, the situation gets a bit complicated, because the dispatcher is not able to cache files for more than 24 hours.

This is caused by a few architectural decisions done for AEM as a Cloud Service:

• The dispatcher is co-hosted with its AEM publish instance to ensure that there is always a 1:1 relation between the AEM publish and its assigned dispatcher.
• All publish instances are re-created every 24 hours.

These 2 decisions lead to the fact, that no dispatcher cache can hold files fore more than 24 hours because the instance is terminated after that time. And there are other situations where the publishs are to be re-created, for example during deployments and up/down-scaling situations, and then the cache does not contain files for 24 hours, but maybe just 3 hours.

This naturally can limit the cache-hit ratio in cases where you have content which is requested frequently but is not changed in days/weeks or even months. In an AEM as a Cloud Service setup these files are then rendered once per day (or more often, see above) per publish/dispatcher, while in other setups (for example AMS on on-prem setups where long-living dispatcher caches are pretty much default) it can delivered from the dispatcher cache without the need to re-render it every day.

The CDN does not have this limitation. It can hold for days and weeks and deliver them, if the TTL settings allow this. But as you can control the CDN only via TTL, you have to make a tradeoff between cache-hit ratio on the CDN and the accuracy of the delivered content regarding a potential change.

That means:

• If you have files which do not change you just set a large TTL to them and then let the CDN handle them. A good example are clientlibs (JS and CSS files), because they have a unique name (an additional selector which is created as a hash over the content of the file.
• If there’s a chance that you make changes to such content (mostly pages), you should set a reasonable TTL (and of course “stale-while-revalidate”) and accept that your publishs need to re-render these pages when the time has passed.

That’s a bit a drawback of the AEM as a Cloud Service setup, but on the hand side your dispatcher caches are regularly cleared.