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From "Ming Ma (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HDFS-7521) Refactor DN state management
Date Tue, 16 Dec 2014 20:13:14 GMT

     [ https://issues.apache.org/jira/browse/HDFS-7521?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Ming Ma updated HDFS-7521:
--------------------------
    Attachment: HDFS-7521.patch
                DNStateMachines.png

We will limit this jira to the first aspect, the DN state machine refactoring. For the state
machine consistency among NNs, we can open another jira.

Here are the state machines and initial patch. There is still some amount of work that needs
to be done such as definition of "stale" state. But want to send it out to get comments from
others first. Here are the high level changes.

* Define two state machines for {{DataNodeDescriptor}}. One is called liveness state. Another
one is called admin state.
* Define two type of events, one is for {{DataNodeDescriptor}} where both state machines will
take action on. Another one is for {{DataNodeManager}} which is used to handle the actual
datanode operations.
* Datanode operations such as removeNode require FSN lock. They are handled by DataNodeManager
on the event dispatcher thread.

Appreciate any input. Is the work overall useful? Any opinion on reusing YARN state machine
management? Any suggestions on the correctness of actual state machine definition?

> 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
case.
> ** 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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