Namenode high availability client request

Please check Namenode HA architecture with key entities in HDFS client requests handling.

Where this request go first? I mean how would client know that which namenode is active?

For client/driver it doesn't matter which namenode is active. because we query on HDFS with nameservice id rather than hostname of namenode. nameservice will automatically transfer client requests to active namenode.

Example: hdfs://nameservice_id/rest/of/the/hdfs/path

Explanation:

How this hdfs://nameservice_id/ works and what are the confs involved in it?

In hdfs-site.xml file

Create a nameservice by adding an id to it(here nameservice_id is mycluster)

<property>
  <name>dfs.nameservices</name>
  <value>mycluster</value>
  <description>Logical name for this new nameservice</description>
</property>

Now specify namenode ids to determine namenodes in cluster

dfs.ha.namenodes.[$nameservice ID]:

<property>
  <name>dfs.ha.namenodes.mycluster</name>
  <value>nn1,nn2</value>
  <description>Unique identifiers for each NameNode in the nameservice</description>
</property>

Then link namenode ids with namenode hosts

dfs.namenode.rpc-address.[$nameservice ID].[$name node ID]

<property>
  <name>dfs.namenode.rpc-address.mycluster.nn1</name>
  <value>machine1.example.com:8020</value>
</property>
<property>
  <name>dfs.namenode.rpc-address.mycluster.nn2</name>
  <value>machine2.example.com:8020</value>
</property>

After that specify the Java class that HDFS clients use to contact the Active NameNode so that DFS Client uses this class to determine which NameNode is currently serving client requests.

<property>
  <name>dfs.client.failover.proxy.provider.mycluster</name>
  <value>org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider</value>
</property>

Finally HDFS URL will be like this after these configuration changes.

hdfs://mycluster/<file_lication_in_hdfs>

To answer your question I have taken few configuration only. please check the detailed documentation for how does Namenodes, Journalnodes and Zookeeper machines form Namenode HA in HDFS.

If hadoop cluster is configured with HA, then it will have namenode IDs in hdfs-site.xml like this :

<property>
  <name>dfs.ha.namenodes.mycluster</name>
  <value>namenode1,namenode2</value>
</property>

Whichever NameNode is started first will become active. You may choose to start the cluster in a specific order such that your preferred node starts first.

If you want to determine the current status of namenode, you can use getServiceStatus() command :

hdfs haadmin -getServiceState <machine-name>

Well, while writing the driver class, you need to set the following properties in configuration object:

 public static void main(String[] args) throws Exception {
    if (args.length != 2){
        System.out.println("Usage: pgm <hdfs:///path/to/copy> </local/path/to/copy/from>");
        System.exit(1);
    }
    Configuration conf = new Configuration(false);
    conf.set("fs.defaultFS", "hdfs://nameservice1");
    conf.set("fs.default.name", conf.get("fs.defaultFS"));
    conf.set("dfs.nameservices","nameservice1");
    conf.set("dfs.ha.namenodes.nameservice1", "namenode1,namenode2");
    conf.set("dfs.namenode.rpc-address.nameservice1.namenode1","hadoopnamenode01:8020");
    conf.set("dfs.namenode.rpc-address.nameservice1.namenode2", "hadoopnamenode02:8020");
    conf.set("dfs.client.failover.proxy.provider.nameservice1","org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider");

    FileSystem fs =  FileSystem.get(URI.create(args[0]), conf);
    Path srcPath = new Path(args[1]);
    Path dstPath = new Path(args[0]);
    //in case the same file exists on remote location, it will be overwritten
    fs.copyFromLocalFile(false, true, srcPath, dstPath);
}

Request will go to the nameservice1 and further handled by Hadoop cluster as per the namenode status(active/standby).

For more details, please refer the HDFS High availability