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From "Patrick Kling (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-7001) Allow configuration changes without restarting configured nodes
Date Thu, 14 Oct 2010 20:03:34 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-7001?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12921102#action_12921102
] 

Patrick Kling commented on HADOOP-7001:
---------------------------------------

I agree, Reconfigurable sounds better. Also, we would be extending the interface Configurable
(not the class Configured, I got confused there). I also wanted to point out that Reconfigurable
would introduce the new method changeConf in addition to the method setConf from Configurable
because there is a difference in semantics:
changeConf would make sure that all changes immediately affect the behaviour of the node (or
failing that an exception would be thrown), whereas setConf as it is used now does not give
any such guarantee (since some configuration values might be cached in other variables and
some behaviour might not be changeable without a restart).


> 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
>
> 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:
> 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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