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https://issues.apache.org/jira/browse/HDFS-9924?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15332191#comment-15332191
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Xiaobing Zhou commented on HDFS-9924:
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{quote}
With your solution, you will block on request 1 for a long time before resubmit
the failed 2-99 request.
This is a inherent defect of lacking the support of callback.
And a better solution is, sorry, but again, using multiple threads
With a thread pool and CompletionService, you can (sometimes) get the failed
request first.
{quote}
You just had an extreme example trying to establish cause and effect. If it's
really the case, why not to resort to Future#IsDone() or call Future#get(long
timeout, TimeUnit unit) with neglectable timeout? You don't have to be blocked
to send failed requests earlier.
In addition, the RPC layer is async as is, Connection#receiveRpcResponse is run
by a thread (i.e. Connection extends Thread) to actively buffer the final
result into Client#Call as long as it's available on socket. As a result, The
final result is already available in Client#Call most often. You will not be
experiencing the block subject to Future#get unless the result is not returned
by server.
Thank you anyway for the comments.
> [umbrella] Asynchronous HDFS Access
> -----------------------------------
>
> Key: HDFS-9924
> URL: https://issues.apache.org/jira/browse/HDFS-9924
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: fs
> Reporter: Tsz Wo Nicholas Sze
> Assignee: Xiaobing Zhou
> Attachments: AsyncHdfs20160510.pdf
>
>
> This is an umbrella JIRA for supporting Asynchronous HDFS Access.
> Currently, all the API methods are blocking calls -- the caller is blocked
> until the method returns. It is very slow if a client makes a large number
> of independent calls in a single thread since each call has to wait until the
> previous call is finished. It is inefficient if a client needs to create a
> large number of threads to invoke the calls.
> We propose adding a new API to support asynchronous calls, i.e. the caller is
> not blocked. The methods in the new API immediately return a Java Future
> object. The return value can be obtained by the usual Future.get() method.
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