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From "Andrzej Bialecki (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-127) Unclear precedence of config files and property definitions
Date Mon, 10 Apr 2006 09:47:58 GMT
Unclear precedence of config files and property definitions
-----------------------------------------------------------

         Key: HADOOP-127
         URL: http://issues.apache.org/jira/browse/HADOOP-127
     Project: Hadoop
        Type: Bug

  Components: conf  
 Environment: Hadoop 0.1.1, Nutch 0.8-dev
    Reporter: Andrzej Bialecki 


The order in which configuration resources are read is not sufficiently documented, and also
there are no mechanisms preventing harmful re-definition of certain properties, if they are
put in wrong config files.

>From reading the code in Hadoop Configuration.java, JobConf.java and Nutch NutchConfiguration.java
I _think_ this is what's happening.

There are two groups of resources: default resources, loaded first, and final resources, loaded
at the end. All properties (re)-defined in files loaded later will override any previous definitions:

* default resources: loaded in the order as they are added. The following files are added
here, in order:

    1. hadoop-default.xml (Configuration)
    2. nutch-default.xml  (NutchConfiguration)
    3. mapred-default.xml (JobConf)
    4. job_xx_xxx.xml       (JobConf, in JobConf(File config))

* final resource: which always come after default resources, i.e. if any value is defined
here it will always override those set in default resources (NOTE: including per job settings!!!).
The following files are added here, in reversed order:

    2. hadoop-site.xml (Configuration)
    1. nutch-site.xml    (NutchConfiguration)

(i.e. hadoop-site.xml will take precedence over anything else defined in any other config
file).

I would appreciate checking that this is indeed the case, and suggestions how to ensure that
you cannot so easily shoot yourself in the foot if you define wrong properties in hadoop-site
or nutch-site ...

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