On Oct 3, 2007, at 23:48, Clint Webb wrote:
I personally do not see the need to use tags in any other way
except to easily remove (invalidate?) keys that have a particular
tag. Being able to do other things like retrieve all keys that
have a tag, could be useful, but I can see it as complicating things.
It's not that it it's complicated, but it falls under the same
category of listing keys that causes the server to hang while someone
calls it.
First point. When you say 'invalidate_tag' I assume you mean to
invalidate all keys that have that tag.
Correct.
Second point. You ask the question about refcounts on tags, I
think it should, or at the very least provide a command that will
tell the cache to remove all tags that are no longer referenced by
a key. Refcount is probably easier, and just remove the tag when
it gets to zero. Decrementing it when a key is deleted or
expired. Otherwise, memory used by tags will always keep getting
bigger as keys drop out due to LRU. An example, article 50 is
added to the cache, and tags representing that article ID have been
added to a bunch of keys. Over time, article 50 no longer has much
visibility and falls out of the cache due to LRU. If no more keys
are in the cache for the tag, I think the tag should go.
It really depends on how often a tag is *not* used again. If tags
are generally reused, it makes things more expensive and
complicated. It could also lead to a lot of memory churn unless
there's a memory pool for tags as well.
If tags are often not reused, it would be a memory leak.
Third point. I am also assuming that we can assign more than one
tag to a key. If I could only add one tag to a key, then that
would limit its usefulness to me. I see you mention pointers (and
not just pointer) so I am sure that this is correct, just clarifying.
Absolutely. Each item may have zero or more tags.
--
Dustin Sallings
