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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3856) Asynchronous IO Handling in Hadoop and HDFS
Date Thu, 09 Oct 2008 17:52:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3856?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12638346#action_12638346

Doug Cutting commented on HADOOP-3856:

I think before we can commit to an async strategy for HDFS we'll need to perform some experiments.
 And before we commit async RPCs to trunk they ought to be used, to test their adequacy. 
So, it could be done as two separate issues, but they're best developed in parallel.  For
example, there may be no point in committing async RPC extensions if buffer-by-buffer access
proves impractical in HDFS.  We don't need features that are not used.  So I think this could
be done in a single Jira, or two that are closely coordinated.

> async RPC calls where the caller does not need to wait for a response

Yes, I agree.  My suggestion above was that we might model this in an interface by declaring
methods with a particular return type.  On further thought, that wouldn't work, since that
method could not be implemented server-side.  But it would sure be nice if one didn't have
to use meta-programming (e.g., RPC.asyncCall(Class.getMethod(...))) but could instead directly
invoke async methods.  So perhaps one could declare pairs of methods, like:

interface FooProtocol {
  Foo getFoo();
  SelectionKey getFooAsync();

The RPC runtime would match methods whose name ends with "Async" and whose return type is
SelectionKey with a method of the same name w/o "Async" and a different return type.  The
client could call getFooAsync() to get a SelectionKey, the server would call getFoo(), and
the client would cast the result from the server to a Bar.  The ugly part is that the implementation
on the server would have to provide some definition of fooAsync() in order to compile, but
it would never actually be called.  Perhaps to avoid this we could add a client-specific interface:

interface FooProtocol {
  Foo getFoo();
interface FooClient extends FooProtocol {
  SelectionKey getFooAsync();
Then pass both classes to RPC#getProxy().  It would return a FooClient, but use FooProtocol
to talk to the server.  The server would only implement FooProtocol.  Could that work?

> 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?

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