This does not apply to the filesystem and memory caches.
Most in-memory caches only allow the user to store simple key/value pairs. For small applications that cache only a few values, this is acceptable. But when the system grows, it can become very expensive to clear the whole cache when an element has changed.
BabelCache uses the idea outlined in the Memcache Wiki:
Assign every namespace an internal version number and only increment this number when the user flushes the cache. This does not remove the data, makes it unavailable.
Let’s see how this works:
Every namespace consists of one or more parts, separated by dots: my.super.namespace. When BabelCache generates the final key to use for storing the data, it splits the namespace up into its parts. Then, beginning by the first, it looks for the current versions of the namespaces.
So in the first step, the version for my is searched. Versions are stored like normal values in the cache, but are prefixed with version/. We look for version/my, to be exact. If the version is found, it is attached to the namespace, else a fresh one is generated (and stored). Let’s suppose we found a version for my and that it is 1234. The namespace now looks like my@1234.
The next step is to look for the version of the following namespaces. That’s where the magic happens: We don’t look for version/my.super, but for firstname.lastname@example.org. This shows that the version of any sub-namespace is directly related to its parents namespaces.
This loop goes on until we have constructed the complete cache key: email@example.com@5.namespace@842 (example). In the final step, the element key is attached to the string, resulting in firstname.lastname@example.org@5.namespace@842$key.
Now let’s see how a flush() works.
Flushing means nothing more than incremeting the version number of a specific namespace. If we take the key from the example above and try to flush my.super (making all data in my.super and my.super.namespace unavailable), the flush method will get the current version (5) and just increment it.
After this, if we try to access my.super.namespace$key again, we can’t find it. When we perform the same algorithm as outlined above, we won’t find the version for email@example.com@6.namespace. Since we can’t find it, a fresh version is generated and used from then on: firstname.lastname@example.org@6.namespace@1 (for example).
The major drawback of this technique is, that flushes are always recursive. The re-assignment of a new version in the middle of the cache key makes all data in deeper namespaces unavailable.
This can be a problem if you’re using really, really deep namespaces with very expensive to create objects. In that case you might want to flatten your structure a bit. In most cases, a flat structure combined with more than one flush can help improving the performance.
Also, the maximum length of cache keys (combination of namespace and key) is reduced, as there needs to be enough space to include the version numbers.
As we can see, flush()ing is a really simple operation. No data is touched, only one version has to be incremented. The drawback of course being that the flush is always recursive and you may lose too much data.
Another thing should be mentioned: Since no data is really removed, your cache will fill up until it reaches its maximum size. Old elements will then be garbage collected. This is normal behaviour for all in-memory caches and will not result in a slowdown.