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From "Eli Collins (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-3150) Add option for clients to contact DNs via hostname in branch-1
Date Sun, 25 Mar 2012 21:22:28 GMT
Add option for clients to contact DNs via hostname in branch-1
--------------------------------------------------------------

                 Key: HDFS-3150
                 URL: https://issues.apache.org/jira/browse/HDFS-3150
             Project: Hadoop HDFS
          Issue Type: Sub-task
          Components: data-node, hdfs client
            Reporter: Eli Collins
            Assignee: Eli Collins


Per the document attached to HADOOP-8198, this is just for branch-1, and unbreaks DN multihoming.
The datanode can be configured to listen on a bond, or all interfaces by specifying the wildcard
in the dfs.datanode.*.address configuration options, however per HADOOP-6867 only the source
address of the registration is exposed to clients. HADOOP-985 made clients access datanodes
by IP primarily to avoid the latency of a DNS lookup, this had the side effect of breaking
DN multihoming. In order to fix it let's add back the option for Datanodes to be accessed
by hostname. This can be done by:
# Modifying the primary field of the Datanode descriptor to be the hostname, or 
# Modifying Client/Datanode <-> Datanode access use the hostname field instead of the
IP

I'd like to go with approach #2 as it does not require making an incompatible change to the
client protocol, and is much less invasive. It minimizes the scope of modification to just
places where clients and Datanodes connect, vs changing all uses of Datanode identifiers.

New client and Datanode configuration options are introduced:
- {{dfs.client.use.datanode.hostname}} indicates all client to datanode connections should
use the datanode hostname (as clients outside cluster may not be able to route the IP)
- {{dfs.datanode.use.datanode.hostname}} indicates whether Datanodes should use hostnames
when connecting to other Datanodes for data transfer

If the configuration options are not used, there is no change in the current behavior.

I'm doing something similar to #1 btw in trunk in HDFS-3144 - refactoring the use of DatanodeID
to use the right field (IP, IP:xferPort, hostname, etc) based on the context the ID is being
used in, vs always using the IP:xferPort as the Datanode's name, and using the name everywhere.

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