Reinhard Pötz wrote: <snip/>
But let me broaden the picture: Based on our work from about two weeks ago, I created another aspect which implements the support for conditional GET requests and also takes care that a pipeline isn't executed unless it is really necessary. I was also able to fix all failing test cases. I created an issue that contains a patch: https://issues.apache.org/jira/browse/COCOON3-47 Additionally there is also another feature that I would like to add: The current patch only takes care of 'If-Modified-Since' requests. I also want to support 'If-None-Match' requests that are based on the 'ETag' response header. (see http://en.wikipedia.org/wiki/HTTP_ETag). Using ETag has the advantage that we could support conditional GET requests also in the case where we can't use a timestamp based approach (e.g. when using o.a.c.pipeline.caching.ParameterCacheKey) or to provide conditional GET support in REST controllers. As an ETag value we could use the hash code of a pipeline's cache key.
I don't fully get the context of this conversation, but this last sentence triggered a question to me: how can we validate a cache entry with its _key_? Looking at the code, I see that CacheKey holds both the identifier information (the actual key) and the validity information.
There is a naming issue here which leads to some confusion between key and key-and-validity that we can see it in the code: ExpiresCacheKey doesn't include the validity information in hashcode() and equals() whereas ParameterCacheKey does. What is the right contract?
As a side note, both classes include the class' hashcode in the instance's hash code, which means hash codes will be different a every JVM restart, or across JVM instances in a cluster, and is likely to break persistent and distributed caches.
That being said, I'm wondering if this aggregation of key and validity won't cause other kinds of problems with distributed cache implementations. For example, Java memcached clients serialize the cache key and use this result as the memcache key. If the key includes validity information, the memcache key will change every time the underlying data changes (e.g. a file's timestamp).
At first sight, this can sound good as it means we will have a cache miss when the validity has changed, and will even avoid having to compare the validity of cached content. But this can have a desastrous impact on the cache efficiency in situations where you have some often requested content that changes frequently: the cache will quickly fill up with obsolete versions of this content under different key values, that will lead older content to be evicted, reducing the overall cache efficiency. Whereas a key that's only an indentifier will lead the entry to be _replaced_ and not a new one being added.
So in the end, my feeling is that key and validity information really should be separated.
Now going back to the ETag discussion, using the pipeline's cache key won't work IMHO because of the implementation of some key's hashcode() using only the identifier part of the key and not the validity. Confusion, I told you ;-)
And BTW, what is the "jmxGroupName" property on CacheKey used for? Sylvain -- Sylvain Wallez - http://bluxte.net
