Failed to read data from a hadoop URL - hadoop

Question

I am following Tom White's 'Hadoop - The Definitive Guide'.
When I try to use the Java Interface to read data from a hadoop URL I get the following error message:
hadoop#ubuntu:/usr/local/hadoop$ hadoop URLCat hdfs://master/hdfs/data/SampleText.txt
12/11/21 13:46:32 INFO ipc.Client: Retrying connect to server: master/192.168.9.55:8020. Already tried 0 time(s).
12/11/21 13:46:33 INFO ipc.Client: Retrying connect to server: master/192.168.9.55:8020. Already tried 1 time(s).
12/11/21 13:46:34 INFO ipc.Client: Retrying connect to server: master/192.168.9.55:8020. Already tried 2 time(s).
12/11/21 13:46:35 INFO ipc.Client: Retrying connect to server: master/192.168.9.55:8020. Already tried 3 time(s).
12/11/21 13:46:36 INFO ipc.Client: Retrying connect to server: master/192.168.9.55:8020. Already tried 4 time(s).
12/11/21 13:46:37 INFO ipc.Client: Retrying connect to server: master/192.168.9.55:8020. Already tried 5 time(s).
The Contents of the URLCat file are as follows:
import java.net.URL;
import java.io.InputStream;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.fs.FsUrlStreamHandlerFactory;
public class URLCat {
static {
URL.setURLStreamHandlerFactory(new FsUrlStreamHandlerFactory());
}
public static void main(String[] args) throws Exception {
InputStream in = null;
try {
in = new URL(args[0]).openStream();
IOUtils.copyBytes(in, System.out, 4096, false);
} finally {
IOUtils.closeStream(in);
}
}
}
The /etc/hosts file contents are:
127.0.0.1 localhost
127.0.1.1 ubuntu.ubuntu-domain ubuntu
# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
# /ect/hosts Master and slaves
192.168.9.55 master
192.168.9.56 slave1
192.168.9.57 slave2
192.168.9.58 slave3

Answer 1

First I'd check whether the Hadoop daemons are running. A convenient tool is jps . Make sure that (at least) the namenode and the datanodes are running.
If you still can't connect, check whether the url is correct. As you provided hdfs://master/ (without any port number) Hadoop assumes that your namenode listens on port 8020 (default). This is what you see in the logs.
For a quick lookup in core-site.xml (fs.default.name) you can check whether you have a custom port defined for the filesystem URI (in this case 54310).