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From "Benoit Sigoure (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-12910) Add new FileSystem API to support asynchronous method calls
Date Tue, 07 Jun 2016 04:52:21 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-12910?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15317847#comment-15317847

Benoit Sigoure commented on HADOOP-12910:

One of the key things that {{Deferred}} and [Finagle's {{Future}}|https://twitter.github.io/finagle/guide/Futures.html#sequential-composition]
both enable is composition.  This allows you to chain multiple asynchronous operations in
a type-safe fashion.

For example, when making an RPC call to HBase, a client might first need to do a lookup in
ZooKeeper, which asynchronously returns a byte array, then parse a RegionInfo out of the byte
array and make an RPC call to an HBase server, which returns some response of type {{FooResponse}}.
 In this semi-hypothetical pipeline, {{Deferred}} and {{Future}} allow you to build a string
of asynchronous operations, in which the (successful) output of one operation is the input
of the next operation.  I don't see how the proposed {{FutureWithCallback}} interface can
cater to such use cases, which will lead to asynchronous APIs that are difficult to compose.

In the proposal for trunk at least with {{CompletionStage}} you get that with {{thenApply}},
but I hope this means that not too many APIs will be created with {{FutureWithCallback}},
as they'll probably stick around for a while and be hard to get rid of.

> Add new FileSystem API to support asynchronous method calls
> -----------------------------------------------------------
>                 Key: HADOOP-12910
>                 URL: https://issues.apache.org/jira/browse/HADOOP-12910
>             Project: Hadoop Common
>          Issue Type: New Feature
>          Components: fs
>            Reporter: Tsz Wo Nicholas Sze
>            Assignee: Xiaobing Zhou
>         Attachments: HADOOP-12910-HDFS-9924.000.patch, HADOOP-12910-HDFS-9924.001.patch,
> Add a new API, namely FutureFileSystem (or AsynchronousFileSystem, if it is a better
name).  All the APIs in FutureFileSystem are the same as FileSystem except that the return
type is wrapped by Future, e.g.
> {code}
>   //FileSystem
>   public boolean rename(Path src, Path dst) throws IOException;
>   //FutureFileSystem
>   public Future<Boolean> rename(Path src, Path dst) throws IOException;
> {code}
> Note that FutureFileSystem does not extend FileSystem.

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