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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-7001) Allow configuration changes without restarting configured nodes
Date Mon, 25 Oct 2010 22:14:25 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-7001?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12924755#action_12924755

Doug Cutting commented on HADOOP-7001:

It doesn't seem unreasonable to me for some limited changes might be permitted without restarting.
 Restarting a namenode is not a lightweight operation.

If the interface is 'Reconfigurable' then the method might be named 'reconfigureProp(String,String)',

I don't like adding new interfaces that aren't yet used.  So it might be good to file issues
for at least one the daemons that you expect to implement this and at least draft those patches
before this is committed.  If we can't agree on some concrete usages of this interface within
Hadoop, then there's no point in adding it.

The servlet should probably return an HTTP error code when reconfiguration fails.

A command-line client might be useful.  Perhaps this will be added to, e.g.. the existing
HDFS admin commands.  But each command-line tool that uses this will share client-side HTTP
call, handling of error codes, etc.  So perhaps that should be encapsulated in a shared utility.

> Allow configuration changes without restarting configured nodes
> ---------------------------------------------------------------
>                 Key: HADOOP-7001
>                 URL: https://issues.apache.org/jira/browse/HADOOP-7001
>             Project: Hadoop Common
>          Issue Type: Task
>            Reporter: Patrick Kling
>            Assignee: Patrick Kling
>         Attachments: HADOOP-7001.patch, reconfigurable.patch
> Currently, changing the configuration on a node (e.g., the name node) requires that we
restart the node. We propose a change that would allow us to make configuration changes without
restarting. Nodes that support configuration changes at run time should implement the following
> interface ChangeableConfigured extends Configured {
>    void changeConfiguration(Configuration newConf) throws ConfigurationChangeException;
> }
> The contract of changeConfiguration is as follows:
> The node will compare newConf to the existing configuration. For each configuration property
that is set to a different value than in the current configuration, the node will either adjust
its behaviour to conform to the new configuration or throw a ConfigurationChangeException
if this change is not possible at run time. If a configuration property is set in the current
configuration but is unset in newConf, the node should use its default value for this property.
After a successful invocation of changeConfiguration, the behaviour of the configured node
should be indistinguishable from the behaviour of a node that was configured with newConf
at creation.
> It should be easy to change existing nodes to implement this interface. We can start
by throwing the exception for all changes and then gradually start supporting more and more
changes at run time. (We might even consider replacing Configured with ChangeableConfigured
entirely, but I think the proposal above afford greater flexibility).

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