The DatanodeID "name" field is overloaded
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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
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 hostname,
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 return 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 part of the new
DatanodeID "name" field it creates (and sets the DatanodeID "hostName" field to
the reported "name"). The DN gets the DatanodeID back from the NN and clobbers
its "hostName" member with the "name" field of the returned DatanodeID. This
makes the code hard to reason about eg DN#getMachine name sometimes returns a
hostname and sometimes not, depending on when it's called in sequence with the
registration. Ditto for uses of the "name" field. I think these contortions
were originally performed because the DatanodeID didn't have a hostName field
(it was part of DatanodeInfo) and so there 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 that the "name" field always and only has an IP
address and the "hostName" field always and only has a hostname.
In HDFS-3144 I'm going to rename the "name" field so its clear that it contains
an IP address. The above is enough scope for one change.
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