db tsai, i do not think userClassPathFirst is working, unless the classes you load dont reference any classes already loaded by the parent classloader (a mostly hypothetical situation)... i filed a jira for this here: https://issues.apache.org/jira/browse/SPARK-1863
On Tue, May 20, 2014 at 1:04 AM, DB Tsai <dbt...@stanford.edu> wrote: > In 1.0, there is a new option for users to choose which classloader has > higher priority via spark.files.userClassPathFirst, I decided to submit the > PR for 0.9 first. We use this patch in our lab and we can use those jars > added by sc.addJar without reflection. > > https://github.com/apache/spark/pull/834 > > Can anyone comment if it's a good approach? > > Thanks. > > > Sincerely, > > DB Tsai > ------------------------------------------------------- > My Blog: https://www.dbtsai.com > LinkedIn: https://www.linkedin.com/in/dbtsai > > > On Mon, May 19, 2014 at 7:42 PM, DB Tsai <dbt...@stanford.edu> wrote: > > > Good summary! We fixed it in branch 0.9 since our production is still in > > 0.9. I'm porting it to 1.0 now, and hopefully will submit PR for 1.0 > > tonight. > > > > > > Sincerely, > > > > DB Tsai > > ------------------------------------------------------- > > My Blog: https://www.dbtsai.com > > LinkedIn: https://www.linkedin.com/in/dbtsai > > > > > > On Mon, May 19, 2014 at 7:38 PM, Sandy Ryza <sandy.r...@cloudera.com > >wrote: > > > >> It just hit me why this problem is showing up on YARN and not on > >> standalone. > >> > >> The relevant difference between YARN and standalone is that, on YARN, > the > >> app jar is loaded by the system classloader instead of Spark's custom > URL > >> classloader. > >> > >> On YARN, the system classloader knows about [the classes in the spark > >> jars, > >> the classes in the primary app jar]. The custom classloader knows > about > >> [the classes in secondary app jars] and has the system classloader as > its > >> parent. > >> > >> A few relevant facts (mostly redundant with what Sean pointed out): > >> * Every class has a classloader that loaded it. > >> * When an object of class B is instantiated inside of class A, the > >> classloader used for loading B is the classloader that was used for > >> loading > >> A. > >> * When a classloader fails to load a class, it lets its parent > classloader > >> try. If its parent succeeds, its parent becomes the "classloader that > >> loaded it". > >> > >> So suppose class B is in a secondary app jar and class A is in the > primary > >> app jar: > >> 1. The custom classloader will try to load class A. > >> 2. It will fail, because it only knows about the secondary jars. > >> 3. It will delegate to its parent, the system classloader. > >> 4. The system classloader will succeed, because it knows about the > primary > >> app jar. > >> 5. A's classloader will be the system classloader. > >> 6. A tries to instantiate an instance of class B. > >> 7. B will be loaded with A's classloader, which is the system > classloader. > >> 8. Loading B will fail, because A's classloader, which is the system > >> classloader, doesn't know about the secondary app jars. > >> > >> In Spark standalone, A and B are both loaded by the custom classloader, > so > >> this issue doesn't come up. > >> > >> -Sandy > >> > >> On Mon, May 19, 2014 at 7:07 PM, Patrick Wendell <pwend...@gmail.com> > >> wrote: > >> > >> > Having a user add define a custom class inside of an added jar and > >> > instantiate it directly inside of an executor is definitely supported > >> > in Spark and has been for a really long time (several years). This is > >> > something we do all the time in Spark. > >> > > >> > DB - I'd hold off on a re-architecting of this until we identify > >> > exactly what is causing the bug you are running into. > >> > > >> > In a nutshell, when the bytecode "new Foo()" is run on the executor, > >> > it will ask the driver for the class over HTTP using a custom > >> > classloader. Something in that pipeline is breaking here, possibly > >> > related to the YARN deployment stuff. > >> > > >> > > >> > On Mon, May 19, 2014 at 12:29 AM, Sean Owen <so...@cloudera.com> > wrote: > >> > > I don't think a customer classloader is necessary. > >> > > > >> > > Well, it occurs to me that this is no new problem. Hadoop, Tomcat, > etc > >> > > all run custom user code that creates new user objects without > >> > > reflection. I should go see how that's done. Maybe it's totally > valid > >> > > to set the thread's context classloader for just this purpose, and I > >> > > am not thinking clearly. > >> > > > >> > > On Mon, May 19, 2014 at 8:26 AM, Andrew Ash <and...@andrewash.com> > >> > wrote: > >> > >> Sounds like the problem is that classloaders always look in their > >> > parents > >> > >> before themselves, and Spark users want executors to pick up > classes > >> > from > >> > >> their custom code before the ones in Spark plus its dependencies. > >> > >> > >> > >> Would a custom classloader that delegates to the parent after first > >> > >> checking itself fix this up? > >> > >> > >> > >> > >> > >> On Mon, May 19, 2014 at 12:17 AM, DB Tsai <dbt...@stanford.edu> > >> wrote: > >> > >> > >> > >>> Hi Sean, > >> > >>> > >> > >>> It's true that the issue here is classloader, and due to the > >> > classloader > >> > >>> delegation model, users have to use reflection in the executors to > >> > pick up > >> > >>> the classloader in order to use those classes added by sc.addJars > >> APIs. > >> > >>> However, it's very inconvenience for users, and not documented in > >> > spark. > >> > >>> > >> > >>> I'm working on a patch to solve it by calling the protected method > >> > addURL > >> > >>> in URLClassLoader to update the current default classloader, so no > >> > >>> customClassLoader anymore. I wonder if this is an good way to go. > >> > >>> > >> > >>> private def addURL(url: URL, loader: URLClassLoader){ > >> > >>> try { > >> > >>> val method: Method = > >> > >>> classOf[URLClassLoader].getDeclaredMethod("addURL", classOf[URL]) > >> > >>> method.setAccessible(true) > >> > >>> method.invoke(loader, url) > >> > >>> } > >> > >>> catch { > >> > >>> case t: Throwable => { > >> > >>> throw new IOException("Error, could not add URL to system > >> > >>> classloader") > >> > >>> } > >> > >>> } > >> > >>> } > >> > >>> > >> > >>> > >> > >>> > >> > >>> Sincerely, > >> > >>> > >> > >>> DB Tsai > >> > >>> ------------------------------------------------------- > >> > >>> My Blog: https://www.dbtsai.com > >> > >>> LinkedIn: https://www.linkedin.com/in/dbtsai > >> > >>> > >> > >>> > >> > >>> On Sun, May 18, 2014 at 11:57 PM, Sean Owen <so...@cloudera.com> > >> > wrote: > >> > >>> > >> > >>> > I might be stating the obvious for everyone, but the issue here > is > >> > not > >> > >>> > reflection or the source of the JAR, but the ClassLoader. The > >> basic > >> > >>> > rules are this. > >> > >>> > > >> > >>> > "new Foo" will use the ClassLoader that defines Foo. This is > >> usually > >> > >>> > the ClassLoader that loaded whatever it is that first referenced > >> Foo > >> > >>> > and caused it to be loaded -- usually the ClassLoader holding > your > >> > >>> > other app classes. > >> > >>> > > >> > >>> > ClassLoaders can have a parent-child relationship. ClassLoaders > >> > always > >> > >>> > look in their parent before themselves. > >> > >>> > > >> > >>> > (Careful then -- in contexts like Hadoop or Tomcat where your > app > >> is > >> > >>> > loaded in a child ClassLoader, and you reference a class that > >> Hadoop > >> > >>> > or Tomcat also has (like a lib class) you will get the > container's > >> > >>> > version!) > >> > >>> > > >> > >>> > When you load an external JAR it has a separate ClassLoader > which > >> > does > >> > >>> > not necessarily bear any relation to the one containing your app > >> > >>> > classes, so yeah it is not generally going to make "new Foo" > work. > >> > >>> > > >> > >>> > Reflection lets you pick the ClassLoader, yes. > >> > >>> > > >> > >>> > I would not call setContextClassLoader. > >> > >>> > > >> > >>> > On Mon, May 19, 2014 at 12:00 AM, Sandy Ryza < > >> > sandy.r...@cloudera.com> > >> > >>> > wrote: > >> > >>> > > I spoke with DB offline about this a little while ago and he > >> > confirmed > >> > >>> > that > >> > >>> > > he was able to access the jar from the driver. > >> > >>> > > > >> > >>> > > The issue appears to be a general Java issue: you can't > directly > >> > >>> > > instantiate a class from a dynamically loaded jar. > >> > >>> > > > >> > >>> > > I reproduced it locally outside of Spark with: > >> > >>> > > --- > >> > >>> > > URLClassLoader urlClassLoader = new URLClassLoader(new > >> URL[] { > >> > new > >> > >>> > > File("myotherjar.jar").toURI().toURL() }, null); > >> > >>> > > > >> Thread.currentThread().setContextClassLoader(urlClassLoader); > >> > >>> > > MyClassFromMyOtherJar obj = new MyClassFromMyOtherJar(); > >> > >>> > > --- > >> > >>> > > > >> > >>> > > I was able to load the class with reflection. > >> > >>> > > >> > >>> > >> > > >> > > > > >
