HDFS

HDFS Component

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

URI format

hdfs://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:

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.
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 seg0, seg1, seg2, etc.

Options

Name	Default Value	Description
`overwrite`	`true`	The file can be overwritten
`bufferSize`	`4096`	The buffer size used by HDFS
`replication`	`3`	The HDFS replication factor
`blockSize`	`67108864`	The size of the HDFS blocks
`fileType`	`NORMAL_FILE`	It can be SEQUENCE_FILE, MAP_FILE, ARRAY_FILE, or BLOOMMAP_FILE, see Hadoop
`fileSystemType`	`HDFS`	It can be LOCAL for local filesystem
`keyType`	`NULL`	The type for the key in case of sequence or map files. See below.
`valueType`	`TEXT`	The type for the key in case of sequence or map files. See below.
`splitStrategy`		A string describing the strategy on how to split the file based on different criteria. See below.
`openedSuffix`	`opened`	When a file is opened for reading/ writing the file is renamed with this suffix to avoid to read it during the writing phase.
`readSuffix`	`read`	Once the file has been read is renamed with this suffix to avoid to read it again.
`initialDelay`	`0`	For the consumer, how much to wait (milliseconds) before to start scanning the directory.
`delay`	`0`	The interval (milliseconds) between the directory scans.
`pattern`	`*`	The pattern used for scanning the directory
`chunkSize`	`4096`	When reading a normal file, this is split into chunks producing a message per chunk

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 (0.20.1) opening a file in append mode is disabled since it's not enough 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 actual file name will become a directory name and a <file name>/seg0 will be initially created.
Every time a splitting condition is met a new file is created with name <original file name>/segN where N is 1, 2, 3, etc.
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:
hdfs://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 find the following files seg0, seg1, seg2, etc

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