NameNode HA when using hdfs:// URI

With HDFS or HFTP URI scheme (e.g. hdfs://namenode/path/to/file) I can access HDFS clusters without requiring their XML configuration files. It is very handy when running shell commands like hdfs dfs -get, hadoop distcp or reading files from Spark like sc.hadoopFile(), because I don't have to copy and manage xml files for all relevant HDFS clusters to all nodes that those codes might potentially run.

One drawback of this approach is that I have to use the active NameNode's hostname, otherwise Hadoop will throw an exception complaining that the NN is standby.

A usual workaround is to try one and then try another if any exception is caught, or to connect to ZooKeeper directly and parse the binary data using protobuf.

Both of these methods are cumbersome, when compared to (for example) mysql's loadbalance URI or ZooKeeper's connection string where I can just comma-separate all hosts in the URI and the driver automatically finds a node to talk to.

Say I have active and standby namenode hosts nn1 and nn2. What is the simplest way to refer a specific path of the HDFS, which:

can be used in command-line tools like hdfs, hadoop
can be used in Hadoop Java API (and thus tools depending on it like Spark) with minimum configuration
works regardless of which namenode is currently active.

In this scenarion instead of checking for active namenode host and port combination, we should use nameservice as, nameservice will automatically transfer client requests to active namenode.

Name service acts like a proxy among Namenodes, which always divert HDFS request to active namenode

Example: hdfs://nameservice_id/file/path/in/hdfs

Sample steps to create `nameservice`

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>

There are so many properties involved to Configure Namenode HA properly with Nameservice

With this setup the HDFS url for a file will looks like this
hdfs://mycluster/file/location/in/hdfs/wo/namenode/host

Edit:

Applying properties with java code

Configuration conf = new Configuration(false);
conf.set("dfs.nameservices","mycluster");
conf.set("dfs.ha.namenodes.mycluster","nn1,nn2");
conf.set("dfs.namenode.rpc-address.mycluster.nn1","machine1.example.com:8020");
conf.set("dfs.namenode.rpc-address.mycluster.nn2","machine2.example.com:8020");

FileSystem fsObj =  FileSystem.get("relative/path/of/file/or/dir", conf);

// now use fsObj to perform HDFS shell like operations
fsObj ...

来源：https://stackoverflow.com/questions/29634344/namenode-ha-when-using-hdfs-uri

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