dustin at spy.net
Fri Nov 2 22:44:01 UTC 2007
On Nov 2, 2007, at 16:17 , Brian Beuning wrote:
>> Firstly, I think there are two new commands to implement tags:
>> 1) add_tag (key, tag_name)
>> 2) invalidate_tag (tag_name)
> It took me a little while to figure out that add_tag() does not
> need to talk
> with all nodes of the cache but it seems like invalidate_tag() does
> need to talk to all nodes.
> What is going to happen if one or more nodes are behind a network
> type failure?
> (The Ethernet cable on a server is unplugged but memcached is still
> They could not be notified to increment their tag generation number
> so when
> the partition is fixed and the machines become accessible, those
> nodes have stale data.
It has been pointed out that mass invalidation would not be an
atomic operation. Perhaps it should be stated more clearly on the
>> [It's unclear whether it's worth the effort to ever release a tag
> it's been added.
>> If we assume that tags live forever, we don't have to refcount
>> them and a
> few things get easier. Any opinions?]
> I think the list agreed releasing tags is good.
Right, there were good arguments for this.
> A unified way to handle this is to let a tag age out of the cache
> using the
> LRU and timeout mechanisms already in place. The only twist is
> that accessing a key is also considered accessing all the tags
> associated with that key for LRU purposes. Given this rule, a tag
> should never get flushed until after all the keys using the tag are
> (When a tag has just one key left using the tag and the key has not
> been accessed, there is sort of a race as to which will age out
> first. This is a tricky case to get right.)
We didn't actually consider placing the tags *in* the cache, but it
would be possible. Every object has a reference count, so you could
just keep the reference count high enough to avoid LRU (perhaps the
count would be equal to the things doing the referencing).
> Are the key and tag names one or two namespaces? That is can you
> have a tag
> and key with the same name?
> I suggest making it one namespace for simplicity.
The original thought was that they were completely orthogonal, so
no, there wouldn't be any implicit correlation between an key and a
tag with the same name.
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