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https://issues.apache.org/jira/browse/HDFS-1742?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13005217#comment-13005217
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Todd Lipcon commented on HDFS-1742:
-----------------------------------
Hey folks. I think people generally accept that it would be nice to be able to
have an inotify-like interface on top of HDFS. However I don't think the
proposed implementation of doing this inside the NN is a good idea for the
following reasons:
- it adds "less trusted" code running in the same JVM as the NN, which could
crash it, use up memory, etc.
- it adds load to the NN, which is already a scalability limit on large clusters
- it will require a NN restart (or fragile classloader tricks) to reload the
set of hooks
I think the right way forward here is to have some kind of service subscribe to
the NN edit logs and then publish events to subscribers. This would allow the
"pubsub" service to run on a separate machine and not impact the NN in any way.
Monitoring/alerting capability based on lifecycle events in the NN does make
sense to me, though - eg a trigger when the NN enters or exits safemode. These
tend to be lower load infrequent events and pluggable listeners would be plenty
useful. See HADOOP-5640 for an interface like this.
> 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}
> It might be possible to creates a class that implements this method and load
> 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.
> There would be a couple of ready-made pluggable implementations of such a
> class that would be most likely distributed as contrib. Default NameNode's
> process would stay the same without any visible differences.
> Hadoop's JobTracker already extensively uses the same paradigm with pluggable
> Scheduler interfaces, such as [Fair
> Scheduler|https://github.com/apache/hadoop/tree/trunk/src/contrib/fairscheduler],
> [Capacity
> Scheduler|https://github.com/apache/hadoop/tree/trunk/src/contrib/capacity-scheduler],
> [Dynamic
> Scheduler|https://github.com/apache/hadoop/tree/trunk/src/contrib/dynamic-scheduler],
> etc. It also uses a class(es) that loads and runs inside JobTracker's
> context, few relatively trustued varieties exist, they're distributed as
> contrib and purely optional to be enabled by cluster admin.
> This would allow systems such as I've described in the beginning to be
> implemented without polling.
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