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From "Ming Ma (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7521) Refactor DN state management
Date Fri, 19 Dec 2014 23:08:15 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7521?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14254194#comment-14254194

Ming Ma commented on HDFS-7521:

[~zhz], good point. Yes, that seems to be the only case so far.

With https://issues.apache.org/jira/browse/HDFS-7411, all blocks of any Decommission_In_Progress
node will be tracked even after the node becomes dead. With that, the admin state machine
can always transition to Decommission_In_Progress state upon DECOMISSION_REQUESTED. If the
node is already dead, it will then transition from Decommission_In_Progress to Decommissioned.
In that way, we can make two state machines independent.

Still, we might introduce new state that requires state dependency. Alternatively, we can

> Refactor DN state management
> ----------------------------
>                 Key: HDFS-7521
>                 URL: https://issues.apache.org/jira/browse/HDFS-7521
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: Ming Ma
>         Attachments: DNStateMachines.png, HDFS-7521.patch
> There are two aspects w.r.t. DN state management in NN.
> * State machine management within active NN
> NN maintains states of each data node regarding whether it is running or being decommissioned.
But the state machine isn’t well defined. We have dealt with some corner case bug in this
area. It will be useful if we can refactor the code to use clear state machine definition
that define events, available states and actions for state transitions. It has these benefits.
> ** Make it easy to define correctness of DN state management. Currently some of the state
transitions aren't defined in the code. For example, if admins remove a node from include
host file while the node is being decommissioned, it will be transitioned to DEAD and DECOMM_IN_PROGRESS.
That might not be the intention. If we have state machine definition, we can identify this
> ** Make it easy to add new state for DN later. For example, people discussed about new
“maintenance” state for DN to support the scenario where admins need to take the machine/rack
down for 30 minutes for repair.
> We can refactor DN with clear state machine definition based on YARN state related components.
> * State machine consistency between active and standby NN
> Another dimension of state machine management is consistency across NN pairs. We have
dealt with bugs due to different live nodes between active NN and standby NN. Current design
is to have each NN manage its own state based on the events it receives. For example, DNs
will send heartbeat to both NNs; admins will issue decommission commands to both NNs. Alternative
design approach could be to have ZK manage the state.
> Thoughts?

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