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. >> > >>> > >> > >>> >> > >> > >
