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https://issues.apache.org/jira/browse/HADOOP-785?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12519484
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Doug Cutting commented on HADOOP-785:
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A few comments:
1. A job is created by a client, so it gets values from hadoop-client.xml as
well as job-specific things set by the application, right? Then the job is
serialized as a job.xml and sent to a server. When it is read on the server,
should any other configuration files be read at all? I think perhaps not. Job
defaults, site specifics, etc. should all be determined at the client. If a
value from hadoop-server.xml is to be considered, then the parameter is not
client-overrideable. Conversely, if a value is client overrideable, then the
value in hadoop-server.xml will not be consulted, only the value in job.xml
will be seen. A job,xml should contain a complete, standalone set of values,
no? So there are two ways to create a JobConfiguration: one that reads
hadoop-default.xml, hadoop-site,xml and hadoop-client.xml, and one that only
reads job.xml.
2. Many parameters should either be in hadoop-client.xml or hadoop-server.xml,
but not both. Thus we can organize the defaults into separate sections for
client and server. Parameters that are used by both clients and servers can be
in a "universal" section: these may be meaningfully added to the client, server
or site configuration. The top-level organization of hadoop-default.xml can be
by technology (hdfs, mapred, etc.) and within that sub-sections for universal,
client and server parameters. This can provide folks a guide for where things
are intended to be overridden.
> Divide the server and client configurations
> -------------------------------------------
>
> Key: HADOOP-785
> URL: https://issues.apache.org/jira/browse/HADOOP-785
> Project: Hadoop
> Issue Type: Improvement
> Components: conf
> Affects Versions: 0.9.0
> Reporter: Owen O'Malley
> Assignee: Arun C Murthy
> Fix For: 0.15.0
>
>
> The configuration system is easy to misconfigure and I think we need to
> strongly divide the server from client configs.
> An example of the problem was a configuration where the task tracker has a
> hadoop-site.xml that set mapred.reduce.tasks to 1. Therefore, the job tracker
> had the right number of reduces, but the map task thought there was a single
> reduce. This lead to a hard to find diagnose failure.
> Therefore, I propose separating out the configuration types as:
> class Configuration;
> // reads site-default.xml, hadoop-default.xml
> class ServerConf extends Configuration;
> // reads hadoop-server.xml, $super
> class DfsServerConf extends ServerConf;
> // reads dfs-server.xml, $super
> class MapRedServerConf extends ServerConf;
> // reads mapred-server.xml, $super
> class ClientConf extends Configuration;
> // reads hadoop-client.xml, $super
> class JobConf extends ClientConf;
> // reads job.xml, $super
> Note in particular, that nothing corresponds to hadoop-site.xml, which
> overrides both client and server configs. Furthermore, the properties from
> the *-default.xml files should never be saved into the job.xml.
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