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Chapter 136. HDFS2 Component

Available as of Camel version 2.14

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>

136.1. 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.

When consuming from hdfs2 then in normal mode, a file is split into chunks, producing a message per chunk. You can configure the size of the chunk using the chunkSize option. If you want to read from hdfs and write to a regular file using the file component, then you can use the fileMode=Append to append each of the chunks together.

136.2. Options

The HDFS2 component supports 2 options which are listed below.

NameDescriptionDefaultType

jAASConfiguration (common)

To use the given configuration for security with JAAS.

 

Configuration

resolveProperty Placeholders (advanced)

Whether the component should resolve property placeholders on itself when starting. Only properties which are of String type can use property placeholders.

true

boolean

The HDFS2 endpoint is configured using URI syntax: 

hdfs2:hostName:port/path

with the following path and query parameters:

136.2.1. Path Parameters (3 parameters):

NameDescriptionDefaultType

hostName

Required HDFS host to use

 

String

port

HDFS port to use

8020

int

path

Required The directory path to use

 

String

136.2.2. Query Parameters (38 parameters):

NameDescriptionDefaultType

connectOnStartup (common)

Whether to connect to the HDFS file system on starting the producer/consumer. If false then the connection is created on-demand. Notice that HDFS may take up till 15 minutes to establish a connection, as it has hardcoded 45 x 20 sec redelivery. By setting this option to false allows your application to startup, and not block for up till 15 minutes.

true

boolean

fileSystemType (common)

Set to LOCAL to not use HDFS but local java.io.File instead.

HDFS

HdfsFileSystemType

fileType (common)

The file type to use. For more details see Hadoop HDFS documentation about the various files types.

NORMAL_FILE

HdfsFileType

keyType (common)

The type for the key in case of sequence or map files.

NULL

WritableType

owner (common)

The file owner must match this owner for the consumer to pickup the file. Otherwise the file is skipped.

 

String

valueType (common)

The type for the key in case of sequence or map files

BYTES

WritableType

bridgeErrorHandler (consumer)

Allows for bridging the consumer to the Camel routing Error Handler, which mean any exceptions occurred while the consumer is trying to pickup incoming messages, or the likes, will now be processed as a message and handled by the routing Error Handler. By default the consumer will use the org.apache.camel.spi.ExceptionHandler to deal with exceptions, that will be logged at WARN or ERROR level and ignored.

false

boolean

pattern (consumer)

The pattern used for scanning the directory

*

String

sendEmptyMessageWhenIdle (consumer)

If the polling consumer did not poll any files, you can enable this option to send an empty message (no body) instead.

false

boolean

exceptionHandler (consumer)

To let the consumer use a custom ExceptionHandler. Notice if the option bridgeErrorHandler is enabled then this options is not in use. By default the consumer will deal with exceptions, that will be logged at WARN or ERROR level and ignored.

 

ExceptionHandler

exchangePattern (consumer)

Sets the exchange pattern when the consumer creates an exchange.

 

ExchangePattern

pollStrategy (consumer)

A pluggable org.apache.camel.PollingConsumerPollingStrategy allowing you to provide your custom implementation to control error handling usually occurred during the poll operation before an Exchange have been created and being routed in Camel.

 

PollingConsumerPoll Strategy

append (producer)

Append to existing file. Notice that not all HDFS file systems support the append option.

false

boolean

overwrite (producer)

Whether to overwrite existing files with the same name

true

boolean

blockSize (advanced)

The size of the HDFS blocks

67108864

long

bufferSize (advanced)

The buffer size used by HDFS

4096

int

checkIdleInterval (advanced)

How often (time in millis) in to run the idle checker background task. This option is only in use if the splitter strategy is IDLE.

500

int

chunkSize (advanced)

When reading a normal file, this is split into chunks producing a message per chunk.

4096

int

compressionCodec (advanced)

The compression codec to use

DEFAULT

HdfsCompressionCodec

compressionType (advanced)

The compression type to use (is default not in use)

NONE

CompressionType

openedSuffix (advanced)

When a file is opened for reading/writing the file is renamed with this suffix to avoid to read it during the writing phase.

opened

String

readSuffix (advanced)

Once the file has been read is renamed with this suffix to avoid to read it again.

read

String

replication (advanced)

The HDFS replication factor

3

short

splitStrategy (advanced)

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

 

String

synchronous (advanced)

Sets whether synchronous processing should be strictly used, or Camel is allowed to use asynchronous processing (if supported).

false

boolean

backoffErrorThreshold (scheduler)

The number of subsequent error polls (failed due some error) that should happen before the backoffMultipler should kick-in.

 

int

backoffIdleThreshold (scheduler)

The number of subsequent idle polls that should happen before the backoffMultipler should kick-in.

 

int

backoffMultiplier (scheduler)

To let the scheduled polling consumer backoff if there has been a number of subsequent idles/errors in a row. The multiplier is then the number of polls that will be skipped before the next actual attempt is happening again. When this option is in use then backoffIdleThreshold and/or backoffErrorThreshold must also be configured.

 

int

delay (scheduler)

Milliseconds before the next poll. You can also specify time values using units, such as 60s (60 seconds), 5m30s (5 minutes and 30 seconds), and 1h (1 hour).

500

long

greedy (scheduler)

If greedy is enabled, then the ScheduledPollConsumer will run immediately again, if the previous run polled 1 or more messages.

false

boolean

initialDelay (scheduler)

Milliseconds before the first poll starts. You can also specify time values using units, such as 60s (60 seconds), 5m30s (5 minutes and 30 seconds), and 1h (1 hour).

1000

long

runLoggingLevel (scheduler)

The consumer logs a start/complete log line when it polls. This option allows you to configure the logging level for that.

TRACE

LoggingLevel

scheduledExecutorService (scheduler)

Allows for configuring a custom/shared thread pool to use for the consumer. By default each consumer has its own single threaded thread pool.

 

ScheduledExecutor Service

scheduler (scheduler)

To use a cron scheduler from either camel-spring or camel-quartz2 component

none

ScheduledPollConsumer Scheduler

schedulerProperties (scheduler)

To configure additional properties when using a custom scheduler or any of the Quartz2, Spring based scheduler.

 

Map

startScheduler (scheduler)

Whether the scheduler should be auto started.

true

boolean

timeUnit (scheduler)

Time unit for initialDelay and delay options.

MILLISECONDS

TimeUnit

useFixedDelay (scheduler)

Controls if fixed delay or fixed rate is used. See ScheduledExecutorService in JDK for details.

true

boolean

136.2.3. 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.

136.3. 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

note that this strategy currently requires either setting an IDLE value or setting the HdfsConstants.HDFS_CLOSE header to false to use the BYTES/MESSAGES configuration…​otherwise, the file will be closed with each message

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.

136.4. Message Headers

The following headers are supported by this component:

136.4.1. Producer only

HeaderDescription

CamelFileName

Camel 2.13: Specifies the name of the file to write (relative to the endpoint path). The name can be a String or an Expression object. Only relevant when not using a split strategy.

136.5. 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.

136.6. 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.

136.6.1. 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);
...

136.6.2. 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.

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