8:25 a.m.
On Wed, May 18, 2011 at 7:06 PM, Manik Surtani <manik(a)jboss.org> wrote:
Hi guys
Sorry I've been absent from this thread for a while now (it's been growing faster
than I've been able to deal with email backlog!)
Anyway, this is a very interesting discussion. To summarise - as Pete did at some point
- there are 2 goals here:
1. Safe and intuitive use of an appropriate classloader
2. Safe type system for return values.
I think the far more pressing concern is (1) so I'd like to focus on that. If we
think (2) is pressing enough a concern, we should spawn a separate thread and discuss
there.
So, onto the issue of safe classloading.
1) Class loader per session/cache.
I like Jason/Sanne/Trustin's suggestions of a session-like contract, and specifically
I think this is best achieved as a delegate to a cache, again as suggested elsewhere by
Pete, etc. E.g.,
Cache<?, ?> myCache = cacheManager.getCache("myCache",
myClassLoader);
and what is returned is something that delegates to the actual cache, making sure the
TCCL is set and re-set appropriately. The handle to the cache is effectively your
"session" and each webapp, etc in an EE environment will have its own handle. I
propose using the TCCL as an internal implementation detail within this delegate, helps
with making sure it is carefully managed and cleaned up while not re-engineering loads of
internals.
I like the API but I would not recommend using the TCCL for this. I
was able to get a nice perf jump in the HotRod client by skipping 2
Thread.setContextClassLoader() calls on each cache operation (1 to set
the TCCL we wanted and 1 to restore the original TCCL). Setting the
TCCL is a privileged operation, so it has to go through a
SecurityManager and that is very slow.
I think EmbeddedCacheManager.getCache(String name, ClassLoader cl) is
enough ... it is clear enough, and I don't see the need for overloaded getCache(name,
classOfWhichClassLoaderIWishToUse).
I agree, a Cache.usingClassloader(classOfWhichClassLoaderIWishToUse)
overload would have made sense because the method name already
communicates the intention, but a getCache(name, clazz) overload is
too obscure.
2) Class loader per invocation.
I've been less than happy with this, not just because it pollutes the API but that it
adds a layer of confusion. If all use cases discussed can be solved with (1) above, then
I'd prefer to just do that.
The way I see it, most user apps directly using Infinispan would only be exposed to a
single class loader per cache reference (even if multiple references talk to the same
cache).
Frameworks, OTOH, are a bit tougher, Hibernate being a good example on this thread. So
this is a question for Galder - is it feasible to maintain several references to a cache,
one for each app/persistence unit?
3) Can all OSGi requirements be handled by (1)? I would guess so, from what I have read
here, since the class loader is explicitly passed in when getting a handle on a cache.
Yes, the only difference is that OSGi goesn't make any guarantees
about the TCCL, so passing the classloader explicitly will work in all
environments. However,
4) What about storeAsBinary="false"? Threads processing requests from
other nodes are not associated with any CacheManager.getCache(name,
classloader) call, and they also have to unmarshall values with this
setting.
Since Hibernate already mandates storeAsBinary="true" for its 2LC, we
can probably get away with only supporting one classloader per cache
in the storeAsBinary="false" case.
Still, we can't rely on the TCCL of the background threads because
those threads are shared between all the caches in a CacheManager. In
fact we should probably set the TCCL to null for all background
threads created by Infinispan, or we risk keeping the classes of the
first application/bundle that called us alive as long as those threads
are still running.
Dan
Cheers
Manik
On 27 Apr 2011, at 17:39, Jason T. Greene wrote:
> Available here:
> https://github.com/infinispan/infinispan/pull/278
>
> The problem is basically that infinispan currently is using TCCL for all
> class loading and resource loading. This has a lot of problems in
> modular containers (OSGi, AS7, etc), where you dont have framework
> implementation classes on the same classloader as user classes (that is
> how they achieve true isolation)
>
> You can read about this in more detail here:
> http://community.jboss.org/wiki/ModuleCompatibleClassloadingGuide
>
> The patch in the pull request is a first step, and should fix many
> issues, but it does not address all (there is still a lot of TCCL usage
> spread out among cacheloaders and so on), and ultimately it's just a
> work around. It should, however, be compatible in any other non-modular
> environment.
>
> Really the ultimate solution is to setup a proper demarcation between
> what the user is supposed to provide, and what is expected to be bundled
> with infinispan. Whenever there is something the user can provide a
> class, then the API should accept a classloader to load that class from.
> If it's a class that is just internal wiring of infinispan, then
> Infinispan's defining classloader should always be used.
>
> The one case I can think of in infnispan where TCCL really makes sense
> is in the case of lazy deserialization from an EE application. However
> that is only guaranteed to work when you are executing in a context that
> has that style of contract (like in an EE component). There are many
> other cases though where someone would expect it to work from a non-EE
> context (e.g. from a thread pool). What you really want is the caller's
> classloader, but that is not cheap to get at, so it's something that
> should be supplied as part of the API interaction (in the case where
> there is no EE context). Alternatively you can just just require that
> folks push/pop TCCL, but users often get this wrong.
>
> Thanks!
>
> --
> Jason T. Greene
> JBoss, a division of Red Hat
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--
Manik Surtani
manik(a)jboss.org
twitter.com/maniksurtani
Lead, Infinispan
http://www.infinispan.org
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