tchatzi at arx
Mar 27, 2009, 11:31 AM
Views: 4757
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Re: using memcached as a StateDB.. getting there
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O/H Gregory S. Youngblood έγραψε:
> I, for one, am interested, though I haven't been able to do much or get
> involved.
>
> "Namespace" - I suggest making this an option that is user tunable, but pick
> a good default value that hopefully won't need to be tweaked all the time.
>
It would vaguely resemble ``StateDir''. Each separate StateDir signifies
a unique application.
Thus far, using StateDir in shmfs/tmpfs I'd use
/dev/shm/apache/website1 and /dev/shm/apache/website2 for two separate
applications. Something similar would apply to using a single memcached
for multiple applications, no ?
Of course, the default StateDir ``.state'' would mix things up a bit.
> "Lock/Unlock" - Each individual operation of memcached is atomic.. if two
> processes attempt to write to one location at the same time, they will be
> serialized and one will not corrupt the other. The problem is you don't
> necessarily know which one will actually win. This means you can't guarantee
> state if you don't do locking to ensure the right one wins.
Well, the original point of Lock and UnLock was to avoid corrupting the
on-disk files, but your point is valid..
> Consider an
> incrementing counter. Assuming you do a get to read the value, increment it
> by one, and then do a set to save it; each individual operation is safe
> (get, set), but not the combination (get+set). If two hit at the same time,
> the number would increment by 1 and not by 2 (one for each). I'd worry that
> making lock and unlock a no-op would create new problems, especially as site
> volume increases and chances of simultaneous updates increase.
>
Each user gets his own key (session-id in Apache::ASP) to write, apart
from the generic ``application''.
The case you're describing isn't handled in Apache::ASP already and it's
probably because the developer (end user in our case) should concern
himself with that.
But we can work on that one too if you feel so inclined.
> "No list of keys" - Usually I know what keys I'm stuffing into memcacheb and
> I don't need to walk through the keys, or if I do I have a place outside of
> memcache that has the keys to lookup. This one could get tricky. How would
> you keep the list of keys in a second entry intact, especially if two
> processes wanted to add a key at the same time?
>
According to perltie, tying hashes pretty much expects ``FIRSTKEY'' and
``NEXTKEY'' to work. But this is an inadequacy of Cache::Memcached::Tie
which we may or may not address.
I was (indirectly) asking Josh if these are actually needed for
Apache::ASP Sessions to work, seeing that they're defined in
Apache::ASP::Session but never actually called - not from within
Apache::ASP at least. But someone is bound to have used / wants to use
keys(%$Session) already..
> Other thoughts/suggestions:
>
> Having not looked at your code or design this may not be applicable, but
> consider making this generic, something that memcache or another cache
> engine could be plugged into. If you're interested, and my time permits, I'd
> be interested in working on part of this with you.
>
The thing is, Apache::ASP::State is too tightly bound to on-disk dbm
files and would require a major rewrite to facilitate other storage
engines. I suspect that's what held Josh from implementing
Apache::Session storage. Come to think of it, there already is an
Apache::Session::Memcached thing out there, so perhaps we should focus
on making Apache::ASP::State work with an Apache::Session and friends
back-end instead of hacking around.
I'd feel more comfortable if we had help from the original author of
Apache::ASP for this (or at least his blessing ;)
Let's think this through the weekend and decide on Monday.
Best Regards,
Thanos Chatziathanassiou
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