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https://issues.apache.org/jira/browse/HADOOP-3856?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12618302#action_12618302
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Ankur commented on HADOOP-3856:
-------------------------------
You can extend NIOSession to wrap the non-blocking sockets. The wrapper can
then be added to a NIOProcessor that can be run via an executor service.
However taking a look at the trunk code, NIOProcessor seems to be marked final
so it looks like it can't be extended to modify/add behaviour. Instead one can
extend AbstractPollingIOProcessor to implement a custom NIOProcessor that takes
care of doing non-blocking work on NIOSession.
It doesn't look like there are other such libraries having Apache compatible
license. So we are left with little choice.
Apache Mina looks more complex to me than using Java NIO and we might end up
writing more code than ideal to workaround its various limitations which would
ultimately make our code unnecessarily more complex. This would offset the
benefit of using an external NIO library.
Unless we have a better option, I would suggest have something custom written
that leaves all the flexibility and control with us.
> Asynchronous IO Handling in Hadoop and HDFS
> -------------------------------------------
>
> Key: HADOOP-3856
> URL: https://issues.apache.org/jira/browse/HADOOP-3856
> Project: Hadoop Core
> Issue Type: New Feature
> Components: dfs, io
> Reporter: Raghu Angadi
>
> I think Hadoop needs utilities or framework to make it simpler to deal with
> generic asynchronous IO in Hadoop.
> Example use case :
> Its been a long standing problem that DataNode takes too many threads for
> data transfers. Each write operation takes up 2 threads at each of the
> datanodes and each read operation takes one irrespective of how much activity
> is on the sockets. The kinds of load that HDFS serves has been expanding
> quite fast and HDFS should handle these varied loads better. If there is a
> framework for non-blocking IO, read and write pipeline state machines could
> be implemented with async events on a fixed number of threads.
> A generic utility is better since it could be used in other places like
> DFSClient. DFSClient currently creates 2 extra threads for each file it has
> open for writing.
> Initially I started writing a primitive "selector", then tried to see if such
> facility already exists. [Apache MINA|http://mina.apache.org] seemed to do
> exactly this. My impression after looking the the interface and examples is
> that it does not give kind control we might prefer or need. First use case I
> was thinking of implementing using MINA was to replace "response handlers" in
> DataNode. The response handlers are simpler since they don't involve disk
> I/O. I [asked on MINA user
> list|http://www.nabble.com/Async-events-with-existing-NIO-sockets.-td18640767.html],
> but looks like it can not be done, I think mainly because the sockets are
> already created.
> Essentially what I have in mind is similar to MINA, except that read and
> write of the sockets is done by the event handlers. The lowest layer
> essentially invokes selectors, invokes event handlers on single or on
> multiple threads. Each event handler is is expected to do some non-blocking
> work. We would of course have utility handler implementations that do read,
> write, accept etc, that are useful for simple processing.
> Sam Pullara mentioned that [xSockets|http://xsocket.sourceforge.net/] is more
> flexible. It is under GPL.
> Are there other such implementations we should look at?
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