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https://issues.apache.org/jira/browse/HDFS-2141?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13068068#comment-13068068
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Sanjay Radia commented on HDFS-2141:
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Konstantine, what is the difference between backup and standby from your
perspective.
>We used to say that we start NameNode in active, backup, or checkpoint role.
The original NN was not designed with HA in mind. They were roles and once the
process/daemon is started its role does not change.
In HA howerver, the process/daemon will change its state from standby to active
and from active to standby.
Further, in standby state, the NN can use one of two techniques to keep in sync
with the active NN: via shared storage (as in Avatar) or via the reading from
an edits stream directly form the active. Both are valid approaches.
Hence:
# A NN running as active
# A NN running as standby and it keeps in sync with the active using one of two
methods
** a) syncing from the active via shared storage
** b) syncing from the active via direct edits stream (as done in the backup NN
code)
You seem to suggest that backup NN is the only way to be in standby mode i.e.
2b; but 2a is an equally valid
way to perfrom the standby function.
The HA community calls "active' and 'standby" as states not as roles because
there is state machine with
state transitions from active to standby and standby to active. But this is a
terminology issue of whether or not to call these things roles, states or
modes.
The checkpointer is a different beast - it is not really a NN - current
checkpointer uses the NN code
but can never become a NN. But backup NN code can perform the checkpointing
function as side benefit.
> Remove NameNode roles Active and Standby (they become states)
> -------------------------------------------------------------
>
> Key: HDFS-2141
> URL: https://issues.apache.org/jira/browse/HDFS-2141
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: name-node
> Reporter: Suresh Srinivas
> Assignee: Suresh Srinivas
> Fix For: 0.23.0
>
> Attachments: HDFS-2141.1.patch, HDFS-2141.patch
>
>
> In HDFS, following roles are supported in NameNodeRole: ACTIVE, BACKUP,
> CHECKPOINT and STANDBY.
> Active and Standby are the state of the NameNode. While Backup and CheckPoint
> are the name/role of the daemons that are started. This mixes up the run time
> state of NameNode with the daemon role. I propose changing the NameNodeRole
> to: NAMENODE, BACKUP, CHECKPOINT. HDFS-1974 will introduce the states active
> and standby to the daemon that is running in the role NAMENODE.
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