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https://issues.apache.org/jira/browse/HDFS-1742?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13005092#comment-13005092
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Allen Wittenauer commented on HDFS-1742:
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The namenode, secondary nn, etc should never be running user code directly. It
won't scale and it will introduce an incredible amount of instability.
It would be much better if this was designed in such a way that it was a
completely separate process (or gang of processes). This process could be fed
by receiving the edits stream similar to how Checkpoint and Backup nodes work
today.
> Provide hooks / callbacks to execute some code based on events happening in
> HDFS (file / directory creation, opening, closing, etc)
> -----------------------------------------------------------------------------------------------------------------------------------
>
> Key: HDFS-1742
> URL: https://issues.apache.org/jira/browse/HDFS-1742
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: name-node
> Reporter: Mikhail Yakshin
> Labels: features, polling
>
> We're working on a system that runs various Hadoop job continuously, based on
> the data that appears in HDFS: for example, we have a job that works on day's
> worth of data and creates output in {{/output/YYYY/MM/DD}}. For input, it
> should wait for directory with externally uploaded data as
> {{/input/YYYY/MM/DD}} to appear, and also wait for previous day's data to
> appear, i.e. {{/output/YYYY/MM/DD-1}}.
> Obviously, one of the possible solutions is polling once in a while for
> files/directories we're waiting for, but generally it's a bad solution. The
> better one is something like file alteration monitor or [inode activity
> notifiers|http://en.wikipedia.org/wiki/Inotify], such as ones implemented in
> Linux filesystems.
> Basic idea is that one can specify (inject) some code that will be executed
> on every major event happening in HDFS, such as:
> * File created / open
> * File closed
> * File deleted
> * Directory created
> * Directory deleted
> I see simplistic implementation as following: NN defines some interfaces that
> implement callback/hook mechanism - i.e. something like:
> {code}
> interface NameNodeCallback {
> public void onFileCreate(SomeFileInformation f);
> public void onFileClose(SomeFileInformation f);
> public void onFileDelete(SomeFileInformation f);
> ...
> }
> {code}
> A user creates a class that implements this method and loads it somehow (for
> example, using an extra jar in classpath) in NameNode's JVM. NameNode
> includes a configuration option that specifies names of such class(es) - then
> NameNode instantiates them and calls methods from them (in a separate thread)
> on every valid event happening.
> This would allow systems such as I've described in the beginning to be
> implemented without polling.
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