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From <>
Subject RE: [VOTE] New Blog Entry
Date Thu, 01 May 2014 23:18:32 GMT
I have made some updates to the blog based on feedback. It's attached here or you can see it
at [1].


- Dave

-----Original Message-----
From: Bill Havanki [] 
Sent: Thursday, May 01, 2014 12:47 PM
To: Accumulo Dev List
Subject: Re: [VOTE] New Blog Entry

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
>>> 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
>>> <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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