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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4952) Improved files system interface for the application writer.
Date Wed, 25 Mar 2009 09:26:54 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4952?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12689045#action_12689045

Doug Cutting commented on HADOOP-4952:

> In the NIO.2 API there is no concept of a working directory.

Rather I suspect the working directory concept in NIO.2 is specific to the URI scheme, as
in Hadoop, that for file: uri's the java processes working directory would apply, so that
file:foo refers to foo in the connected directory in both APIs.

A material difference (that Sanjay alluded to above) is that the default filesystem (uri scheme
& authority) in NIO.2 is fixed to be the local filesystem (file:), while in Hadoop it
is configurable.  So "foo" in NIO.2 always refers to "file:foo", while in Hadoop it might
refer to "hdfs:foo" if HDFS is configured as the default filesystem.

> Improved files system interface for the application writer.
> -----------------------------------------------------------
>                 Key: HADOOP-4952
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4952
>             Project: Hadoop Core
>          Issue Type: Improvement
>    Affects Versions: 0.21.0
>            Reporter: Sanjay Radia
>            Assignee: Sanjay Radia
>         Attachments: Files.java
> Currently the FIleSystem interface serves two purposes:
> - an application writer's interface for using the Hadoop file system
> - a file system implementer's interface (e.g. hdfs, local file system, kfs, etc)
> This Jira proposes that we provide a simpler interfaces for the application writer and
leave the FilsSystem  interface for the implementer of a filesystem.
> - Filesystem interface  has a  confusing set of methods for the application writer
> - We could make it easier to take advantage of the URI file naming
> ** Current approach is to get FileSystem instance by supplying the URI and then access
that name space. It is consistent for the FileSystem instance to not accept URIs for other
schemes, but we can do better.
> ** The special copyFromLocalFIle can be generalized as a  copyFile where the src or target
can be generalized to any URI, including the local one.
> ** The proposed scheme (below) simplifies this.
> -	The client side config can be simplified. 
> ** New config() by default uses the default config. Since this is the common usage pattern,
one should not need to always pass the config as a parameter when accessing the file system.
> -	
> ** It does not handle multiple file systems too well. Today a site.xml is derived from
a single Hadoop cluster. This does not make sense for multiple Hadoop clusters which may have
different defaults.
> ** Further one should need very little to configure the client side:
> *** Default files system.
> *** Block size 
> *** Replication factor
> *** Scheme to class mapping
> ** It should be possible to take Blocksize and replication factors defaults from the
target file system, rather then the client size config.  I am not suggesting we don't allow
setting client side defaults, but most clients do not care and would find it simpler to take
the defaults for their systems  from the target file system. 

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