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From "Zesheng Wu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6507) Improve DFSAdmin to support HA cluster better
Date Thu, 12 Jun 2014 10:11:02 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6507?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14028986#comment-14028986
] 

Zesheng Wu commented on HDFS-6507:
----------------------------------

[~vinayrpet], Thanks for your feedback.
bq. Only worry is, if commands pass on one namenode and fails on another namenode, how to
handle the failures.? whether just log the errors or rollback?
>From what I know so far, all the operations mentioned in the description are idempotent,
so when a command fails on one(some) of the NNs, users can just retry the command. What we
need to do is report the error clearly to users, either by logs or exit codes. Rollback is
too complex.

bq. If manual operations required before command execution, then its user's responsibility
to make sure that configurations updated at both namenodes before command execution.
The manual operations required before command execution can be done by tools in batches, for
example, if users want to do refreshNodes, they can upload new excludes/includes file to all
of the NNs without caring which NN is Active or Standby. After uploading the files, they just
run refreshNodes command, it's enough.

bq. Only Active leave safemode, which on failover again puts the cluster into safemode.
If we decide to send set safemode for both NN, we should let the user know whether there's
failure clearly. If there's failure, users should do retry until all succeed.


> Improve DFSAdmin to support HA cluster better
> ---------------------------------------------
>
>                 Key: HDFS-6507
>                 URL: https://issues.apache.org/jira/browse/HDFS-6507
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: tools
>    Affects Versions: 2.4.0
>            Reporter: Zesheng Wu
>            Assignee: Zesheng Wu
>
> Currently, the commands supported in DFSAdmin can be classified into three categories
according to the protocol used:
> 1. ClientProtocol
> Commands in this category generally implement by calling the corresponding function of
the DFSClient class, and will call the corresponding remote implementation function at the
NN side finally. At the NN side, all these operations are classified into five categories:
UNCHECKED, READ, WRITE, CHECKPOINT, JOURNAL. Active NN will allow all operations, and Standby
NN only allows UNCHECKED operations. In the current implementation of DFSClient, it will connect
one NN first, if the first NN is not Active and the operation is not allowed, it will failover
to the second NN. So here comes the problem, some of the commands(setSafeMode, saveNameSpace,
restoreFailedStorage, refreshNodes, setBalancerBandwidth, metaSave) in DFSAdmin are classified
as UNCHECKED operations, and when executing these commands in the DFSAdmin command line, they
will be sent to a definite NN, no matter it is Active or Standby. This may result in two problems:

> a. If the first tried NN is standby, and the operation takes effect only on Standby NN,
which is not the expected result.
> b. If the operation needs to take effect on both NN, but it takes effect on only one
NN. In the future, when there is a NN failover, there may have problems.
> Here I propose the following improvements:
> a. If the command can be classified as one of READ/WRITE/CHECKPOINT/JOURNAL operations,
we should classify it clearly.
> b. If the command can not be classified as one of the above four operations, or if the
command needs to take effect on both NN, we should send the request to both Active and Standby
NNs.
> 2. Refresh protocols: RefreshAuthorizationPolicyProtocol, RefreshUserMappingsProtocol,
RefreshUserMappingsProtocol, RefreshCallQueueProtocol
> Commands in this category, including refreshServiceAcl, refreshUserToGroupMapping, refreshSuperUserGroupsConfiguration
and refreshCallQueue, are implemented by creating a corresponding RPC proxy and sending the
request to remote NN. In the current implementation, these requests will be sent to a definite
NN, no matter it is Active or Standby. Here I propose that we sent these requests to both
NNs.
> 3. ClientDatanodeProtocol
> Commands in this category are handled correctly, no need to improve.



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