Cache Management: Configurable Caching

Performing some computations in Zope can take a long time or use a lot of resources. One way to deal with expensive tasks is to cache them. The first time the computation is requested, the results are stored in a table or cache. Subsequent requests get the results from the cache. This can result in a dramatic speed increase.

There are so many possible strategies for caching that no one could possibly come up with them all. Caches can be stored in memory, on disk, on other computers, or by other means. Caches can be limited in size or unconstrained. They can be made to work with only specific types of objects. They can be tuned in different ways.

So instead of trying to provide for every possible caching strategy, Zope defines an API called cache management that lets developers write their own caching strategies, or cache managers, and lets site administrators easily connect cacheable objects to those cache managers.

You can use caching to speed up access to often-requested pages, reduce disk access and network traffic, and deal with heavy loads. All these benefits come with risks of excessive caching, however, so it's important to fine-tune the cache settings. More on this later.

How to set up caching

The first thing you need to do is create a cache manager instance. In the Zope management interface, go to a folder containing objects that would benefit from caching. From the add list, select a cache manager type such as RAM Cache Manager. Use an ID that describes the purpose of the cache manager.

Next, visit one of the objects that you want to cache. A new tab labeled Cache should be visible. Select it. From the drop-down box, select the cache manager you just created and press the Change button.

The object is now ready to be cached. Visit the View tab. If the object is a script that takes a long time to render, the first view will still take just as long as before. But if you're using a RAM cache manager or similar, the second view should be much faster. Press the reload button in your browser to try it out.

You can associate many objects to a cache manager at once using the Associate tab of all cache managers. Visit the cache manager object you created and select the Associate tab. Press the Locate button. Zope will locate all cacheable objects in the folder. Select the checkboxes next to the objects you want to cache and press the Save changes button.

Inherent risks

Cache managers generally don't know the nature of what is being cached, so here are some issues that can surface:

• Data that is intended for authorized viewers only can be inadvertently cached in public caches.
• Data is cached for too long a time.
• If more than one cache is involved, data is purged from one cache but not the other.
• A method that makes up part of the page sets the caching headers for the entire response, fooling downstream caches into thinking the whole response should be cached.
• Result data can depend on any number of objects. Early on it was decided that the standard cache managers will not try to deduce dependencies, but instead rely on the user for configuration of simple dependencies.

Because of these risks, you should be careful when setting up caching. You'll need to fine-tune the cache settings. Sometimes you'll find that you can't cache one of your major pages, but that you can cache pieces of it.

Also remember that caching can actually slow down Zope if it is applied unscrupulously. You should perform speed tests to verify that caching really does speed up your site.