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From Clinton Begin <clinton.be...@gmail.com>
Subject Re: Possible race condition using org.apache.ibatis.cache.Cache#hasKey ?
Date Thu, 31 Dec 2009 18:53:44 GMT
LOL... it was pseudocode in the middle of a sentence dude.  :-)



On Thu, Dec 31, 2009 at 11:29 AM, Simone Tripodi
<simone.tripodi@gmail.com>wrote:

> Hi Clinton,
> sure, I'll add the Jira ticket for this, but since here in Italy,
> because of the timezone, started celebrating the new year, I'll do it
> tomorrow :P
> BTW, I discourage the use of the test
>
> value == Cache.NULL_OBJECT
>
> but rather
>
> Cache.NULL_OBJECT.equals(vaue)
>
> I had to patch iBatis 2 (and reported to the dev ML, see
> http://markmail.org/message/lm752wxm2jwcpg5l), with this last check to
> use a distribuited cache. My scenario was: the same iBatis application
> replicated on more than one application server (on different servers)
> more than one memcached node server as cache storage. Moreover,
> Cache.NULL_OBJECT has to be Serializable.
>
> Happy new year and tanks for your replies!!! :)
> Best regards,
> Simone Tripodi
>
>
> On Thu, Dec 31, 2009 at 5:11 PM, Clinton Begin <clinton.begin@gmail.com>
> wrote:
> > PS:  I just had a look, and I doubt there's any chance of a race
> condition
> > here... there's only two usages of hasKey().  One is thread local, and
> the
> > other uses a read/write lock scope.
> >
> > That said, I think your point about performance is perfectly warranted,
> and
> > quite simple to solve.  For example:
> >
> >  if (cache.hasKey(key)) {
> >               return (List) cache.getObject(key);
> > }...
> >
> > Becomes:
> >
> > Object value = cache.getObject(key)
> > if (value != null) {
> >     return  Cache.isNull(value) ? null : value;
> > }...
> >
> > Where Cache.isNull(value) simply checks if value == Cache.NULL_OBJECT.
> Of
> > course you could just do that outside too, but I think having a method
> like
> > this makes it nicer to read.
> >
> > Do you want to add a Jira ticket for this?
> >
> > Clinton
> >
> > On Thu, Dec 31, 2009 at 9:04 AM, Clinton Begin <clinton.begin@gmail.com>
> > wrote:
> >>
> >> Good points.
> >>
> >> I think the only reason hasKey exists is to support cached null values.
> >> But that said, I believe in iBATIS 2 I used a NULL_OBJECT value to
> represent
> >> the difference between "yes I'm cached, and I'm null" vs. "I'm not
> cached".
> >>
> >> So I think there definitely is something to look at here.
> >>
> >> Clinton
> >>
> >> On Thu, Dec 31, 2009 at 5:38 AM, Simone Tripodi <
> simone.tripodi@gmail.com>
> >> wrote:
> >>>
> >>> Hi all guys,
> >>> since I've been integrating 3rd part caching solutions[1] in iBatis3,
> >>> I started thinking about the use of method in the Cache interface:
> >>>
> >>> org.apache.ibatis.cache.Cache#hasKey()
> >>>
> >>> Honestly, I'm a little scared about the use for a key check, as it may
> >>> expire between checking for the key, and whatever you want to do with
> >>> the stored object, and produce a race condition :(
> >>> I'd propose to remove this method, and let to the layer built on top
> >>> of cache interface checking if the retrieved object is not null.
> >>>
> >>> Indeed, some distribuited and scalable caching servers like
> >>> memcached[2] don't provide methods to key checking, because of the
> >>> reason above. Moreover, in this scenario, the only way I have to check
> >>> if a key is present in the cache, is getting the object, and checking
> >>> it is null. But let's suppose I've a cached Object of 10M size,
> >>> checking first and then getting if present, causes 20M net traffic :(
> >>>
> >>> Please don't get me wrong, I don't want to criticize the excellent
> >>> work you've been doing - I don't use different persistence layer than
> >>> iBatis! - but since iBatis is largely used in production environments,
> >>> I would encourage the community to be sensible to this kind of
> >>> potential issues.
> >>>
> >>> What do you think about it? Have a nice end of the year party and see
> >>> you next year! :D
> >>> Best regards,
> >>> Simone Tripodi
> >>>
> >>> [1] http://ibaguice.googlecode.com/svn/site/1.0-SNAPSHOT/caching.html
> >>> [2] http://memcached.org/
> >>>
> >>> --
> >>> http://www.google.com/profiles/simone.tripodi
> >>>
> >>> ---------------------------------------------------------------------
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> >>>
> >>
> >
> >
>
>
>
> --
> http://www.google.com/profiles/simone.tripodi
>
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