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I was on a new Web 2.0 application last nite. The Ajax was flowing nicely through this app, and it was sure pretty.

But then I started to notice some weird behaviour. If I added something it showed up fine on one page, but didn't show up on another. As I navigated around this world I kept seeing inconsistencies from area to area.

I see this from time to time, and normally it smells like aggressive page caching.

I have nothing against caching at the page level. It makes a LOT of sense for many things, as the closer you get to the user, the less work you are repeating.

However, you always pay a price in this balanced world of performance and scalability. In this case, there is a lot more to keep in sync, and a lot of people ignore that side of the equation.

This is why I really like to have a caching layer for my applications which are further towards the DB than the web page itself. This cache does the hard work of keeping all of the info that I need in sync, but when it does change, the dynamic web pages automatically get that update.

This means that you get a nice balance of all worlds:

• Data is cached closer to the user, yet not too far from the DB
• Access times to this data cache are almost in-memory, and very fast
• You have consistent data showing up on all of your pages

As always, this will depend on what you are doing, and it is a tricky balancing act.... but let's try to not just turn on page caching and walk around, expecting everything to Just Work (tm).