[CC-ing the list, by permission from the author]
> Now the problem is that I do want to have a long term cache, but,
> I wouldn't like it to be on a file system (for various reasons, one
> being also efficiency).
> So my idea was to download the source code, and see if I can
> somehow make polipo use a BerkeleyDb database instead of the file
> system.
I'd think about it twice. Given a good underlying filesystem
(Reiserfs, XFS or a recent release of Ext3, if you happen to be
running Linux), it should beat any generic DB implementation hands on.
> Thus my questions are:
> 0) Is it feasable to implement such a feature?
It's quite doable.
> 1) Would you ever implement a "pluggable" cache support? So that
> users can switch the backend? (Guess not :) )
No, I won't. I couldn't possibly make sure I don't break such code in
the future. Experience shows that people don't help much with testing,
so I'm not willing to include code that I don't regularly test myself.
> 2) Could you provide me with some documentation that would help me
> understand how such a feature would be implemented?
Sure.
An open on-disk file is represented with a DiskCacheEntry structure.
You need the following functions to be implemented:
- makeDiskEntry, which opens an on-disk entry, checks whether it
validates against an in-memory object, and if so, fills up the
headers;
- destroyDiskEntry, which closes such an entry; the second
parameter specifies whhether to delete it;
- objectFillFromDisk, which fills up part of an in-memory object
from disk;
- writeoutMetadata and writeoutToDisk, which write out in-memory
data to disk.
There's also a second, more light-weidght data structure called
DiskObject, which is only used by the expiry process.
Hope this helps,
Juliusz
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