HDFS2 Component

Available as of Camel 2.13

The hdfs2 component enables you to read and write messages from/to an HDFS file system using Hadoop 2.x. HDFS is the distributed file system at the heart of Hadoop.

Maven users will need to add the following dependency to their pom.xml for this component:

<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-hdfs2</artifactId>
    <version>x.x.x</version>
    <!-- use the same version as your Camel core version -->
</dependency>

URI format

hdfs2://hostname[:port][/path][?options]

You can append query options to the URI in the following format, ?option=value&option=value&...
The path is treated in the following way:

1. as a consumer, if it's a file, it just reads the file, otherwise if it represents a directory it scans all the file under the path satisfying the configured pattern. All the files under that directory must be of the same type.
2. as a producer, if at least one split strategy is defined, the path is considered a directory and under that directory the producer creates a different file per split named using the configured UuidGenerator.

Options

KeyType and ValueType

• NULL it means that the key or the value is absent
• BYTE for writing a byte, the java Byte class is mapped into a BYTE
• BYTES for writing a sequence of bytes. It maps the java ByteBuffer class
• INT for writing java integer
• FLOAT for writing java float
• LONG for writing java long
• DOUBLE for writing java double
• TEXT for writing java strings

BYTES is also used with everything else, for example, in Camel a file is sent around as an InputStream, int this case is written in a sequence file or a map file as a sequence of bytes.

Splitting Strategy

In the current version of Hadoop opening a file in append mode is disabled since it's not very reliable. So, for the moment, it's only possible to create new files. The Camel HDFS endpoint tries to solve this problem in this way:

• If the split strategy option has been defined, the hdfs path will be used as a directory and files will be created using the configured UuidGenerator
• Every time a splitting condition is met, a new file is created.
  The splitStrategy option is defined as a string with the following syntax:
  splitStrategy=<ST>:<value>,<ST>:<value>,*

where <ST> can be:

• BYTES a new file is created, and the old is closed when the number of written bytes is more than <value>
• MESSAGES a new file is created, and the old is closed when the number of written messages is more than <value>
• IDLE a new file is created, and the old is closed when no writing happened in the last <value> milliseconds

for example:

hdfs2://localhost/tmp/simple-file?splitStrategy=IDLE:1000,BYTES:5

it means: a new file is created either when it has been idle for more than 1 second or if more than 5 bytes have been written. So, running hadoop fs -ls /tmp/simple-file you'll see that multiple files have been created.

Message Headers

The following headers are supported by this component:

Producer only

Controlling to close file stream

When using the HDFS2 producer without a split strategy, then the file output stream is by default closed after the write. However you may want to keep the stream open, and only explicitly close the stream later. For that you can use the header HdfsConstants.HDFS_CLOSE (value = "CamelHdfsClose") to control this. Setting this value to a boolean allows you to explicit control whether the stream should be closed or not.

Notice this does not apply if you use a split strategy, as there are various strategies that can control when the stream is closed.

Using this component in OSGi

There are some quirks when running this component in an OSGi environment related to the mechanism Hadoop 2.x uses to discover different org.apache.hadoop.fs.FileSystem implementations. Hadoop 2.x uses java.util.ServiceLoader which looks for /META-INF/services/org.apache.hadoop.fs.FileSystem files defining available filesystem types and implementations. These resources are not available when running inside OSGi.

As with camel-hdfs component, the default configuration files need to be visible from the bundle class loader. A typical way to deal with it is to keep a copy of core-default.xml (and e.g., hdfs-default.xml) in your bundle root.

Using this component with manually defined routes

There are two options:

1. Package /META-INF/services/org.apache.hadoop.fs.FileSystem resource with bundle that defines the routes. This resource should list all the required Hadoop 2.x filesystem implementations.
2. Provide boilerplate initialization code which populates internal, static cache inside org.apache.hadoop.fs.FileSystem class:

org.apache.hadoop.conf.Configuration conf = new org.apache.hadoop.conf.Configuration();
conf.setClass("fs.file.impl", org.apache.hadoop.fs.LocalFileSystem.class, FileSystem.class);
conf.setClass("fs.hdfs.impl", org.apache.hadoop.hdfs.DistributedFileSystem.class, FileSystem.class);
...
FileSystem.get("file:///", conf);
FileSystem.get("hdfs://localhost:9000/", conf);
...

Using this component with Blueprint container

Two options:

1. Package /META-INF/services/org.apache.hadoop.fs.FileSystem resource with bundle that contains blueprint definition.
2. Add the following to the blueprint definition file:

<bean id="hdfsOsgiHelper" class="org.apache.camel.component.hdfs2.HdfsOsgiHelper">
   <argument>
      <map>
         <entry key="file:///" value="org.apache.hadoop.fs.LocalFileSystem"  />
         <entry key="hdfs://localhost:9000/" value="org.apache.hadoop.hdfs.DistributedFileSystem" />
         ...
      </map>
   </argument>
</bean>

<bean id="hdfs2" class="org.apache.camel.component.hdfs2.HdfsComponent" depends-on="hdfsOsgiHelper" />

This way Hadoop 2.x will have correct mapping of URI schemes to filesystem implementations.