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 <[email protected]> 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 <[email protected]> >> Date:04/30/2014 9:51 PM (GMT-05:00) >> To: [email protected] >> 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, [email protected] 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 F >>> >> r > > o >> m 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. >>> >>> -- // Bill Havanki // Solutions Architect, Cloudera Govt Solutions // 443.686.9283
