[jira] Updated: (HADOOP-5708) Configuration should provide a way to write only properties that have been set

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

     [ https://issues.apache.org/jira/browse/HADOOP-5708?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Shevek updated HADOOP-5708:
---------------------------

    Attachment: 01_configuration.patch

Here's a fairly good stab. It passes all tests in conf/*. I started documenting the structure more clearly, I'll finish when I get back to the office. It has some advantages:

* There is no longer a REGISTRY of Configurations - memory management is better.
* Configurations may be arbitrarily layered.
* Adding a global default does not cause everyone to reload themselves.

The semantics of overlays are a bit weird, and may be unnecessary now that we have layers, but it made sense to conform exactly to the existing APIs.

I know the Java memory model extremely well, and in my opinion, if this object _is_ accessed from more than one thread, you're up the creek without the paddle. If you need it thread-safe without losing performance, please say so and I will do an optimal implementation.

> Configuration should provide a way to write only properties that have been set
> ------------------------------------------------------------------------------
>
>                 Key: HADOOP-5708
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5708
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: conf
>    Affects Versions: 0.19.1
>            Reporter: Topher ZiCornell
>            Priority: Minor
>         Attachments: 01_configuration.patch
>
>
> The Configuration.write and .writeXml methods always output all properties, whether they came from a default source, a loaded resource file, or an "overlay" set call.  There should be a way to write only the properties that were set, leaving out the properties that came from a default source.
> Why?  Suppose I build a configuration on a machine that is not associated with a grid, write it out to XML, then try to load it on a grid gateway.  The configuration would contain all of the defaults picked up from my non-grid machine, and would completely overwrite all the defaults on that grid.
> I propose to add methods to write out only the overlay values in Object and XML formats.
> I see two options for implementing this:
> 1) Either completely new methods could be crafted (writeOverlay(DataOutput) and writeOverlayXml(OutputStream), or 
> 2) The existing write() and writeXml() methods could be adjusted to take an additional parameter indicating whether the full properties or overlay properties should be written.  (Of course, the existing write() and writeXml() methods would remain, defaulting to the current behavior.)
> Option 1 has less impact to existing code.  Option 2 is a cleaner implementation with less code-duplication involved.  I would much prefer to do option 2.
> Oh, and in case it's not clear, I'm offering to make this change and submit it.
> Thoughts?
> .  Topher

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