The Zend Framework provides an interesting set of PHP5 libraries for caching. There’s a nice architecture to it, providing a number of different backends and frontends for caching. However, I recently found it very frustrating to try and figure out a decent caching strategy for the new version of a site I was working on.

And the documentation did not help at all. So, allow me to elaborate for the benefit of the huddled masses.

The introduction in the caching section of the manual gives a decent enough overview of the basics, if I want to cache a page with a nice simple ID, such as “Page1″ with a set lifetime I can do so in a few lines of code.

However, another page goes on to mention how you can also “tag” records with multiple tags. Another page talks about how to clear the cache by a single tag, or combination of tags.

But nothing explains the relationships between tags and ids, and how the clear works with tags or ids.

Now, I’m working on a system which has two views of a music catalogue for a radio station. There is the requests system and there is the discography system. They both present differing views of the same data. The request system filters the list of artists, albums and tracks on the station to those that can be requested and displays a “request optimised” set of screens with some of the data. The discography section shows everything about an artist, all their albums, all tracks, reviews and so forth, without the cruft needed for the request sub-system.

We cache these screens for obvious reasons. What we need is the ability to clear the cache of an artist, album or track possibly within the requests or the discography, or both. So, I figured on a system of unique keys like artist_123 and album_123 etc then to use the tags to “lump” things together. So album_123 would have the artist_123 tag in both discography and request view plus the discography tag in the discography and the requests tag in the requests view. Something like:

$cache->load("album_123", array("album_123", "artist_123", "requests", "en_GB", "album");

I could then simply invalidate the cache of all albums, request pages, artist_123 pages or British English generated pages or any combination of those items

This does not work.

The first important thing is that the ID is the unique key. Not the combination of the ID and the tag(s). So if you save page1 to the cache with the tags tag1 and tag2, then try and load page1 from the cache with tags tag3 and tag4, you’ll get the result of saving with tag1 and tag2!

Insane, but true. Try it. The tag has no effect whatsoever it seems on the load code. If it can find an item by ID, then it loads it, irrespective of tags. I’m not sure if this is behaviour by design, or a bug on my system using the file backend, but it is consistent. I just think it’s mad.

To get what I desire, I’m going to have to cache with:

$cache->load("requests_artist_123_album_123_en_GB", array("requests", "artist_123", "album_123", "en_GB", "albums");

Then I can still do a clear on the appropriate tags, if for example I want to remove all albums from the requests cache:

$cache->clean(Zend_Cache::CLEANING_MODE_MATCHING_TAG, (array("requests","albums"));

It’s a bit of a pain in the rear, but, once you’ve figured out the $tags argument on the load method is pointless, it’s fine.

Of course, figuring all this out was further compicated by the fact that the examples in the manual often use load() with $key and $tags and save() with no arguments. I assumed therefore that the point of the $tags argument on load() was to set the tags that would be used auto-magically on save(). Only, if you don’t pass $tags to save() it saves with no tags. Which is also silly, since it does respect the $key used in the load() method.

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