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From "Elek, Marton (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDDS-1080) Ozonefs Isolated class loader should support FsStorageStatistics
Date Mon, 11 Feb 2019 11:29:00 GMT
Elek, Marton created HDDS-1080:
----------------------------------

             Summary: Ozonefs Isolated class loader should support FsStorageStatistics
                 Key: HDDS-1080
                 URL: https://issues.apache.org/jira/browse/HDDS-1080
             Project: Hadoop Distributed Data Store
          Issue Type: Improvement
          Components: Ozone Filesystem
            Reporter: Elek, Marton
            Assignee: Elek, Marton
         Attachments: HDDS-1080.001.patch

HDDS-1033 introduced the storage statistics for ozonefs. Unfortunately the isolated classloader
(HDDS-922) doesn't work any more after this change.

To explain the problem, let's use the specific expression _classname[classloader]_ for a class
(named by classname) which is loaded by the classloader.

We have two classloaders the _main_ which is the standard classloader of the application and
the _isolated_ classloader which is created by the OzoneClientAdapterFactory.java.

By default java classloaders delegate the loading to the parent class loader, and they load
all the classes from the parent first (if possible). The isolated class loader does the oposit,
loads all the classes from a specific location of the jar files. With this approach we can
use hadoop3.2+ozone classes together with older hadoop versions.

But back to the problem:

OzoneFilesystem[main] is loaded by the application. In this class an OzoneFSStorageStatistics[main]
is created and with the help of OzoneClientAdapterFactory[main] a new OzoneClientAdapterImpl[isolated!!]
is instantiated which implements the OzoneClientAdapter[main] and will do all the main work[isolated].

OzoneClientAdapterImp[isolated] has a new constructor which requires (String[system], String[system],
OzoneFSStorageStatistics[isolated]). 

And this is the problem, it requires OzoneFSStorageStatistics[isolated] but we have a  OzoneFSStorageStatistics[main].

The fix is very straightforward. In the FilteredClassLoader.java we ha a list for the classes
which should be shared by the two classloaders. For these classes the isolated classloader
will delegate the loading to the parent ([main]) classloader and we will have one (and only
one) OzoneFSStorageStatistics[main] everywhere.



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