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From Bill Havanki <>
Subject Re: [VOTE] New Blog Entry
Date Thu, 01 May 2014 16:47:02 GMT
Overall the article is great! I have suggested edits, so I'd like to know
where I can stick them (don't be rude now ;) ). We've used Review Board for
doc feedback in the past ... that's an OK way. Dave, I can just email them
to you to avoid spamming. Let me know.

On Wed, Apr 30, 2014 at 10:43 PM, Josh Elser <> wrote:

> Ah ok. I was just looking through the link you provided and didn't notice
> an author at all.
> Just found it now in tiny letters at the bottom :)
> On 4/30/14, 10:18 PM, dlmarion wrote:
>> I believe that the author is shown. Well, at least the person who posts
>> it is shown. In this case it is one in the same.
>> Sent via the Samsung GALAXY S®4, an AT&T 4G LTE smartphone
>> -------- Original message --------
>> From: Josh Elser <>
>> Date:04/30/2014  9:51 PM  (GMT-05:00)
>> To:
>> Subject: Re: [VOTE] New Blog Entry
>> It would be nice to include yourself as the author of the post. That
>> would be nice to help users identify who created the content.
>> On 4/30/14, 6:51 PM, 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]
>>> 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 F
>> r
>  o
>> m HDFS
>>> If you have defined "general.vfs.classpaths" in your Accumulo
>>> configuration, then you can use the 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 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
>>> 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.

// Bill Havanki
// Solutions Architect, Cloudera Govt Solutions
// 443.686.9283

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