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## Using Arrow's HDFS (Apache Hadoop Distributed File System) interface

### Build requirements

To build the integration, pass the following option to CMake

```shell
-DARROW_HDFS=on
```

For convenience, we have bundled `hdfs.h` for libhdfs from Apache Hadoop in
Arrow's thirdparty. If you wish to build against the `hdfs.h` in your installed
Hadoop distribution, set the `$HADOOP_HOME` environment variable.

### Runtime requirements

By default, the HDFS client C++ class in `libarrow_io` uses the libhdfs JNI
interface to the Java Hadoop client. This library is loaded **at runtime**
(rather than at link / library load time, since the library may not be in your
LD_LIBRARY_PATH), and relies on some environment variables.

* `HADOOP_HOME`: the root of your installed Hadoop distribution. Often has
`lib/native/libhdfs.so`.
* `JAVA_HOME`: the location of your Java SDK installation.
* `CLASSPATH`: must contain the Hadoop jars. You can set these using:

```shell
export CLASSPATH=`$HADOOP_HOME/bin/hadoop classpath --glob`
```

* `ARROW_LIBHDFS_DIR` (optional): explicit location of `libhdfs.so` if it is
installed somewhere other than `$HADOOP_HOME/lib/native`.

To accommodate distribution-specific nuances, the `JAVA_HOME` variable may be
set to the root path for the Java SDK, the JRE path itself, or to the directory
containing the `libjvm` library.

### Mac Specifics

The installed location of Java on OS X can vary, however the following snippet
will set it automatically for you:

```shell
export JAVA_HOME=$(/usr/libexec/java_home)
```

Homebrew's Hadoop does not have native libs. Apache doesn't build these, so
users must build Hadoop to get the native libs. See this Stack Overflow
answer for details:

http://stackoverflow.com/a/40051353/478288

Be sure to include the path to the native libs in `JAVA_LIBRARY_PATH`:

```shell
export JAVA_LIBRARY_PATH=$HADOOP_HOME/lib/native:$JAVA_LIBRARY_PATH
```

If you get an error about needing to install Java 6, then add *BundledApp* and
*JNI* to the `JVMCapabilities` in `$JAVA_HOME/../Info.plist`. See

https://oliverdowling.com.au/2015/10/09/oracles-jre-8-on-mac-os-x-el-capitan/

https://derflounder.wordpress.com/2015/08/08/modifying-oracles-java-sdk-to-run-java-applications-on-os-x/