HTTP Cache and the need for cachability

One of the often cited advantages of REST implemented in HTTP is the easy access to caching which can improve performance and reduce the load on the servers. Now with breaking app engine quota regularly around Mandel Map the obvious solution is to turn on caching. Which I just have by adding the line

self.response.headers['Cache-Control'] = 'max-age=2592000'

Which basically means "don't come and ask me again for a week". Now part of the problem is that hitting reload in a browser forces it to go back to a server anyway but there is a second and more important problem as you mess around with the Map. With the map, double click and zoom in... then hold shift and zoom out again.



Notice how it still re-draws when it zooms back out again? The reason for this is that the zoom in calculation just works around a given point and sets a new bottom left of the overall grid relative to that point. This means that every zoom in and out is pretty much unique (you've got a 1 in 2916 chance of getting back to the cached zoom out version after you have zoomed in).

So while the next time you see the map it will appear much quicker this doesn't actually help you in terms of it working quicker as it zooms in and out or in terms of reducing the server load for people who are mucking about with the Map on a regular basis. The challenge therefore is designing the application for cachability rather than just turning on HTTP Caching and expecting everything to magically work better.

The same principle applies when turning on server side caching (like memcache in Google App Engine). If every users gets a unique set of results then the caching will just burn memory rather than giving you better performance, indeed the performance will get slower as you will have a massively populated cache but have practically no successful hits from requests.

With this application it means that rather than simply do a basic calculation that forms the basis for the zoom it needs to do a calculation that forms a repeatable basis for the zoom. Effectively those 54x54 blocks need to be the same 54x54 blocks at a given zoom level for every request. This will make the "click" a bit less accurate (its not spot on now anyway) but will lead to an application which is much more effectively cachable than the current solution.

So HTTP Cache on its own doesn't make your application perform any better for end users or reduce the load on your servers. You have to design your application so the elements being returned are cachable in a way that will deliver performance improvements. For some applications its trivial, for others (like the Mandelbrot Map) its a little bit harder.

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