Hey Dave, Do you want suggested edits? If so, how would you like them?
-- Sean On Apr 30, 2014 5:51 PM, <dlmar...@comcast.net> wrote: > > I have created a new entry for the blog. The preview feature does not > appear to be working at the moment. I will submit an INFRA issue for this. > I have pasted the text below. For those that have a blog account, you > should be able to see the blog at [1]. This blog entry is set to be > published at 235959 3 May 2014 GMT pending no vetoes. This vote will remain > open for 72 hours, until 2300 3 May 2014 GMT. > > [1] > https://blogs.apache.org/roller-ui/authoring/preview/accumulo/?previewEntry=the_accumulo_classloader > > - Dave > > -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- > > Blog Title: The Accumulo Classloader > > Blog Text: > > First, some history > > > The classloader in version 1.4 used a simple hierarchy of two classloaders > that would load classes from locations specified by two properties. The > locations specified by the "general.classpaths" property would be used to > create a parent classloader and locations specified by the > "general.dynamic.classpaths" property were used to create a child > classloader. The child classloader would monitor the specified locations > for changes and when a change occurred it would replace the child > classloader with a new instance. Classes that referenced the orphaned child > classloader would continue to work and the classloader would be garbage > collected when no longer referenced. > > The only place where the dynamic classloader would come into play is for > user iterators and their dependencies. The general advice for using this > classloader would be to put the jars containing your iterators in the > dynamic location. Everything else that does not change very often or would > require a restart can be put into the non-dynamic location. > > There are a couple of things to note about the classloader in 1.4. First, > if you modified the dynamic locations too often, you would run out of > perm-gen space. This is likely due to unreferenced classes not being > unloaded from the JVM. This is captured in ACCUMULO-599 . Secondly, when > you modified files in dynamic locations within the same cycle, it would on > occasion miss the second change. Out with the old, in with the new > > > The Accumulo classloader was rewritten in version 1.5. It maintains the > same dynamic capability and includes a couple of new features. The > classloader uses Commons VFS so that it can load jars and classes from a > variety of sources, including HDFS. Additionally, we introduced the notion > of classloader contexts into Accumulo. This is not a new concept for anyone > that has used an application server, but the implementation is a little > different for Accumulo. > > The hierarchy set up by the new classloader uses the same property names > as the old classloader. In the most basic configuration the locations > specified by "general.classpaths" are used to create the root of the > application classloader hierarchy. This classloader is a URLClassLoader and > it does not support dynamic reloading. If you only specify this property, > then you are loading all of your jars from the local file system and they > will not be monitored for changes. We will call this top level application > classloader the SYSTEM classloader. Next, a classloader is created that > supports VFS sources and reloading. The parent of this classloader is the > SYSTEM classloader and we will call this the VFS classloader. If the > "general.vfs.classpaths" property is set, the VFS classloader will use this > location. If the property is not set, it will use the value of > "general.dynamic.classpaths" with a default value of $ACCUMULO_HOME/lib/ext > to support backwards compatibility. Running Accumulo From HDFS > > > If you have defined "general.vfs.classpaths" in your Accumulo > configuration, then you can use the bootstrap_hdfs.sh script in the bin > directory to seed HDFS with the Accumulo jars. A couple of jars will remain > on the local file system for starting services. Now when you start up > Accumulo the master, gc, tracer, and all of the tablet servers will get > their jars and classes from HDFS. The bootstrap_hdfs.sh script sets the > replication on the directory, but you may want to set it higher after > bootstrapping. An example configuration setting would be: > <property> > <name>general.vfs.classpaths</name> > <value>hdfs://localhost:8020/accumulo/system-classpath</value> > <description>Configuration for a system level vfs classloader. > Accumulo jars can be configured here and loaded out of HDFS.</description> > </property> > About Contexts > > > You can also define classloader contexts in your accumulo-site.xml file. A > context is defined by a user supplied name and it references locations like > the other classloader properties. When a context is defined in the > configuration, it can then be applied to one or more tables. When a context > is applied to a table, then a classloader is created for that context. If > multiple tables use the same context, then they share the context > classloader. The context classloader is a child to the VFS classloader > created above. > > The goal here is to enable multiple tenants to share the same Accumulo > instance. For example, we may have a context called 'app1' which references > the jars for application A. We may also have another context called app2 > which references the jars for application B. By default the context > classloader delegates to the parent classloader. This behavior may be > overridden as seen in the app2 example below. > <property> > <name>general.vfs.context.classpath.app1</name> > > <value>hdfs://localhost:8020/applicationA/classpath/.*.jar,file:///opt/applicationA/lib/.*.jar</value> > <description>Application A classpath, loads jars from HDFS and local > file system</description> > </property> > > <property> > <name>general.vfs.context.classpath.app2.delegation=post</name> > <value>hdfs://localhost:8020/applicationB/classpath/.*.jar, > http://my-webserver/applicationB/.*.jar</value> > <description>Application B classpath, loads jars from HDFS and HTTP, > does not delegate to parent first</description> > </property> > > > Context classloaders do not have to be defined in the accumulo-site.xml > file. The "general.vfs.context.classpath.{context}" property can be defined > on the table either programatically or manually in the shell. Then set the > "table.classpath.context" property on your table. Known Issues > > > > > > Remember the two issues I mentioned above? Well, they are still a problem. > > * ACCUMULO-1507 is tracking VFS-487 for frequent modifications to > files. > * If you start running out of perm-gen space, take a look at > ACCUMULO-599 and try applying the JVM settings for class unloading. > * Additionally, there is an issue with the bootstrap_hdfs.sh script > detailed in ACCUMULO-2761 . There is a workaround listed in the issue. > > > > I have disabled comments as I see they are being abused in other blogs. > Please email the dev list for comments and questions. >
