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From "Paulo Motta (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-7431) Hadoop integration does not perform reverse DNS lookup correctly on EC2
Date Sat, 21 Jun 2014 04:26:24 GMT
Paulo Motta created CASSANDRA-7431:
--------------------------------------

             Summary: Hadoop integration does not perform reverse DNS lookup correctly on
EC2
                 Key: CASSANDRA-7431
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7431
             Project: Cassandra
          Issue Type: Bug
          Components: Hadoop
            Reporter: Paulo Motta
            Assignee: Paulo Motta


The split assignment on AbstractColumnFamilyInputFormat:247 peforms a reverse DNS lookup of
a Cassandra endpoint in order to preserve locality in Hadoop (task trackers are identified
by hostnames, not IPs).

However, the reverse DNS lookup of an EC2 endpoint does not yield the EC2 hostname of that
endpoint when running from an EC2 instance due to the use of InetAddress.getHostname().

In order to show this, consider the following piece of code:

{code:title=DnsResolver.java|borderStyle=solid}
public class DnsResolver {
    public static void main(String[] args) throws Exception {
        InetAddress namenodePublicAddress = InetAddress.getByName(args[0]);
        System.out.println("getHostAddress: " + namenodePublicAddress.getHostAddress());
        System.out.println("getHostName: " + namenodePublicAddress.getHostName());
    }
}
{code}

When this code is run from my machine to perform reverse lookup of an EC2 IP, the output is:
{code:none}
➜  java DnsResolver 54.201.254.99
getHostAddress: 54.201.254.99
getHostName: ec2-54-201-254-99.compute-1.amazonaws.com
{code}

When this code is executed from inside an EC2 machine, the output is:
{code:none}
➜  java DnsResolver 54.201.254.99
getHostAddress: 54.201.254.99
getHostName: 54.201.254.99
{code}

However, when using linux tools such as "host" or "dig", the EC2 hostname is properly resolved
from the EC2 instance, so there's some problem with Java's InetAddress.getHostname() and EC2.

Two consequences of this bug during AbstractColumnFamilyInputFormat split definition are:
1) If the Hadoop cluster is configured to use EC2 public DNS, the locality will be lost, because
Hadoop will try to match the CFIF split location (public IP) with the task tracker location
(public DNS), so no matches will be found.
2) If the Cassandra nodes' broadcast_address is set to public IPs, all hadoop communication
will be done via the public IP, what will incurr additional transference charges. If the public
IP is mapped to the EC2 DNS during split definition, when the task is executed, ColumnFamilyRecordReader
will resolve the public DNS to the private IP of the instance, so there will be not additional
charges.

A similar bug was filed in the WHIRR project: 
https://issues.apache.org/jira/browse/WHIRR-128



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