FYI, I signed a ZF CLA today and sent it in. Looks like they took care of it, so I'm all set on that front now.
Fabien, in my opinion this cache id prefix is only necessary for the memcached and apc backends.
The file backend already has a prefix of sorts, a "file name" prefix. Doesn't really matter though, cause it serves the same purpose. Even so, might not be a bad idea to change that to a cache id prefix for consistency. Whether its a file name prefix or a cache id prefix, it doesn't really matter for the File backend, because the cacheid is used to create the final file name anyway. Less work if we don't fool with it, but more consistent with the other backends if we do change it ...
The only other two backends are sqlite and Zend Platform. Both of those support tags though, so I don't feel the prefix is necessary. The whole point is to create namespaces with the cache, that's one of the things tags do for you.
I'm willing to finish this task when I have some answers. I'll provide up to date patches for memcached and apc backends, and I'll do unit tests and any docs too.
Thanks
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Assign to Fabien Marty.