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From "Kihwal Lee (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-7472) RPC client should deal with the IP address changes
Date Wed, 03 Aug 2011 19:18:27 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-7472?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13078937#comment-13078937

Kihwal Lee commented on HADOOP-7472:

If you call InetAddress.getByAddress(String hostname, byte[] addr), the hostname is saved
in the new InetAddress, so getHostName() will return the hostname used to instantiate this
InetAddress object.  But if InetAddress.getByName() is used, only the resolved address is
saved. The hostname is saved in {{h_name}} of {{struct hostent}}, but it is never looked at.

So, {{new InetSocketAddress(name, port)}} will not store {{name}] in its hostName field nor
in InetAddress.hostname.

I need to do lookup with the old name, and then construct an InetAddress with the old name
and the new address. Only then I can create a new InetSocketAddress with it and InetSocketAddress.getHostName()
will get the name from the InetAddress I created.

This assumes that the InetSocketAddress passed to getProxy() contains the name or the alias
that stays the same after IP address changes. My very first patch tried to address this in
DFSClient, but it was limited in scope.  MAPREDUCE-2764 will have more comprehensive solution.

> RPC client should deal with the IP address changes
> --------------------------------------------------
>                 Key: HADOOP-7472
>                 URL: https://issues.apache.org/jira/browse/HADOOP-7472
>             Project: Hadoop Common
>          Issue Type: Improvement
>          Components: ipc
>    Affects Versions:
>            Reporter: Kihwal Lee
>            Assignee: Kihwal Lee
>            Priority: Minor
>             Fix For:
>         Attachments: addr_change_dfs-1.patch.txt, addr_change_dfs-2.patch.txt, addr_change_dfs-3.patch.txt,
addr_change_dfs.patch.txt, addr_change_dfs_0_20s.patch.txt, addr_change_dfs_trunk-1.patch.txt,
> The current RPC client implementation and the client-side callers assume that the hostname-address
mappings of servers never change. The resolved address is stored in an immutable InetSocketAddress
object above/outside RPC, and the reconnect logic in the RPC Connection implementation also
trusts the resolved address that was passed down.
> If the NN suffers a failure that requires migration, it may be started on a different
node with a different IP address. In this case, even if the name-address mapping is updated
in DNS, the cluster is stuck trying old address until the whole cluster is restarted.
> The RPC client-side should detect this situation and exit or try to recover.
> Updating ConnectionId within the Client implementation may get the system work for the
moment, there always is a risk of the cached address:port become connectable again unintentionally.
The real solution will be notifying upper layer of the address change so that they can re-resolve
and retry or re-architecture the system as discussed in HDFS-34. 
> For 0.20 lines, some type of compromise may be acceptable. For example, raise a custom
exception for some well-defined high-impact upper layer to do re-resolve/retry, while other
will have to restart.  For TRUNK, the HA work will most likely determine what needs to be
done.  So this Jira won't cover the solutions for TRUNK.

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