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Date: Thu, 3 May 2012 01:39:49 +0000 (UTC)
From: "Tsz Wo (Nicholas), SZE (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] [Commented] (HDFS-3171) The DatanodeID "name" field is
 overloaded
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    [ https://issues.apache.org/jira/browse/HDFS-3171?page=3Dcom.atlassian.=
jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=3D13267=
089#comment-13267089 ]=20

Tsz Wo (Nicholas), SZE commented on HDFS-3171:
----------------------------------------------

Hi Eli, the new DataNode.getDisplayName() method may throw NPE; see HADOOP-=
8348.
               =20
> The DatanodeID "name" field is overloaded=20
> ------------------------------------------
>
>                 Key: HDFS-3171
>                 URL: https://issues.apache.org/jira/browse/HDFS-3171
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: data-node
>            Reporter: Eli Collins
>            Assignee: Eli Collins
>             Fix For: 2.0.0
>
>         Attachments: hdfs-3171.txt
>
>
> The DatanodeID "name" field is currently overloaded, when the DN creates =
a DatanodeID to register with the NN it sets "name" to be the datanode host=
name, which is the DN's "hostName" member. This isnot necesarily a FQDN, it=
 is either set explicitly or determined by the DNS class, which could retur=
n the machine's hostname or the result of a DNS lookup, if configured to do=
 so. The NN then clobbers the "name" field of the DatanodeID with the IP pa=
rt of the new DatanodeID "name" field it creates (and sets the DatanodeID "=
hostName" field to the reported "name"). The DN gets the DatanodeID back fr=
om the NN and clobbers its "hostName" member with the "name" field of the r=
eturned DatanodeID. This makes the code hard to reason about eg DN#getMachi=
ne name sometimes returns a hostname and sometimes not, depending on when i=
t's called in sequence with the registration. Ditto for uses of the "name" =
field. I think these contortions were originally performed because the Data=
nodeID didn't have a hostName field (it was part of DatanodeInfo) and so th=
ere was no way to communicate both at the same time. Now that the hostName =
field is in DatanodeID (as of HDFS-3164) we can establish the invariant tha=
t the "name" field always and only has an IP address and the "hostName" fie=
ld always and only has a hostname.
> In HDFS-3144 I'm going to rename the "name" field so its clear that it co=
ntains an IP address. The above is enough scope for one change.

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