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From: Harsh J <harsh@cloudera.com>
Date: Tue, 9 Apr 2013 09:16:49 +0530
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 <CAOcnVr2H0f+4-2AA-CMF8m4i8fXuhxcX4ytq7CSi792+rfReeQ@mail.gmail.com>
Subject: Re: mr default=local?
To: "<user@hadoop.apache.org>" <user@hadoop.apache.org>
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Hey John,

Sorta unclear on what is prompting this question (to answer it more
specifically) but my response below:

On Tue, Apr 9, 2013 at 9:05 AM, John Meza <j_mezazap@hotmail.com> wrote:
> The default mode for hadoop is Standalone, PsuedoDistributed and Fully
> Distributed modes. It is configured for Psuedo and Fully Distributed via
> configuration file, but defaults to Standalone otherwise (correct?).

The mapred-default.xml we ship, has "mapred.job.tracker"
(0.20.x/1.x/0.22.x) set to local, or "mapreduce.framework.name"
(0.23.x, 2.x, trunk) set to local. This is why, without reconfiguring
an installation to point to a proper cluster (JT or YARN), you will
get local job runner activated.

> Question about the -defaulting- mechanism:
>     -Does it get the -default- configuration via one of the config files?

For any Configuration type of invocation:
1. First level of defaults come from *-default.xml embedded inside the
various relevant jar files.
2. Configurations further found in a classpath resource XML
(core,mapred,hdfs,yarn, *-site.xmls) are applied on top of the
defaults.
3. User applications' code may then override this set, with any
settings of their own, if needed.

>     -Or does it get the -default- configuration via hard-coded values?

There may be a few cases of hardcodes, missing documentation and
presence in *-default.xml, but they should still be configurable via
(2) and (3).

>     -Or another mechanism?

--
Harsh J