"Set" and then "get" don't retrieve the stored value

[Brad Fitzpatrick]

This is another instance of the flush_all-is-only-second-granularity
bug that's existed for awhile.

Because you flush_all and then do a set within the rest of that second,
those sets are also considered non-existent.

We've wanted to fix this by adding a world generation number (and storing
the world version of each item in the item struct), or making the time
higher resolution, but both increase the size of *item for little gain.

Anybody have clever ideas on how to fix this?

(btw, there's already a failing TODO test for this in the test suite, in
the flush-all.t test....)



On Tue, 14 Nov 2006, Kristian Hellquist wrote:

> Here's the dump. The key to look for is
> '65b05a7489ea704c5afb80fee7b029497a80beac'. As you can see I do a
> "set" with response "STORED" followed by a delete which returns "NOT
> FOUND". After that follows various calls that I think you can ignore.
>
> Thanks for your help,
> Kristian
>
>
> 2006/11/14, Brad Fitzpatrick <brad at danga.com>:
> > I'm coming into this late.
> >
> > Why would a "get" right after a "set" not work?  This isn't a known issue,
> > at least not to me.
> >
> > If you can reproduce this easily, send me a tcpdump pcap file of the
> > traffic to your memcached.
> >
> > If you're using the loopback to a 127.0.0.1 memcached on port 11211, run
> > this:
> >
> > # tcpdump -w forbrad.pcap -s 0 -i lo port 11211
> >
> > Then do your (minimal!) failing test, control-C the tcpdump, and email me
> > the forbrad.pcap file.
> >
> >
> > On Tue, 14 Nov 2006, Kristian Hellquist wrote:
> >
> > > The problem is that the the small window unfortunately seems to be
> > > quite large for me. Do you have measure in time of how small the
> > > window is?
> > >
> > > Basically, I have some unit tests on a web application. The web
> > > application relies on memcached heavily. Everything with memcached has
> > > worked like a charm for us so far. We cache sql-finders in memcached
> > > and invalidates the sql-finders when the model has changed.
> > >
> > > In a specific use-case we dont want to update the state of the db
> > > immediately. This data change *a lot*. So when the state of model
> > > change we will cache the sql-finder in memcache directly and later on
> > > we flush the change to the db.  It is in this use-case that I have
> > > experienced the "set" and "get" problem.
> > >
> > > My unit-test successfully ran when I added a sleep call of 0.3 s
> > > between each memcache-call :/ This is a lot. Is there a way of
> > > configurate memcached so I experience greater performance?
> > >
> > > I know that the test-environment doesnt mimick the user behavior of
> > > the webapplication in real use. We use rails and the unit tests
> > > doensn't involve the webserver at all. So in real life more delay will
> > > be inserted between each "set" and "get" call. But, can I *trust*
> > > memcache that data has been stored if I receive a "STORED" response?
> > > Is just a matter of time before the value can be retrieved again? Or
> > > is it bad usage of memcache to cache the sql-finders directly and
> > > later on update the db?
> > >
> > > Thanks for your reply,
> > > Kristian Hellquist
> > >
> > >
> > > 2006/11/14, Jehiah Czebotar <jehiah at gmail.com>:
> > > > > Basically, yes.  There is a small window where this can happen.  There
> > > > > have been numerous discussions about it on the list.  Most people note
> > > > > it with a set and then delete.
> > > > >
> > > >
> > > > tif my memory serves me, the correct order is a delete, set, then a
> > > > get all for the same key
> > > >
> > >
> > >
> >
>