[jira] Commented: (HADOOP-3856) Asynchronous IO Handling in Hadoop and HDFS

["Trustin Lee (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-3856?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12699472#action_12699472 ] 

Trustin Lee commented on HADOOP-3856:
-------------------------------------

MINA is the successor of Netty 2.x.  Netty 3.x has been written from scratch based on my experience with MINA, so Netty 3 is *not* the successor of MINA.  They are independent from each other.

Regarding LGPL, the main problem with depending on LGPL library in an ASF project is that an ASF project is required to explicitly state there's a dependency with incompatible license and not to distribute LGPL library JAR.  Given that FSF already clarified the linkage issue in Java, this is not a legal issue but an ASF policy issue IMHO.  There was an in-depth discussion about this at legal-discuss.

To work around this issue, you need to make Hadoop build without LGPL library by default and make it build the component that depends on a LGPL library only when a user specified a certain build flag (e.g. -Dwith-lgpl-dependency).  The technique was used in the MINA project, which optionally depends on RXTX (LGPL'd library.)  Of course, RXTX.jar is fetched only when such a flag is specified to satisfy the ASF policy thanks to Maven.

Or, you can make the LGPL'd library simply a system requirement and explicitly state that somewhere in the download page or NOTICE.txt and you should not include the JAR of the LGPL'd library in the distribution so that a user download it manually.

> 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
>         Attachments: GrizzlyEchoServer.patch, MinaEchoServer.patch
>
>
> 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?

-- 
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.