SequenceFile (Hadoop 1.2.1 API)
Date: 2013-07-22

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## org.apache.hadoop.io
Class SequenceFile

    java.lang.Object
      org.apache.hadoop.io.SequenceFile

    public class SequenceFile

extends Object

`SequenceFile`s are flat files consisting of binary key/value pairs.

`SequenceFile` provides `Writer`, `Reader` and `SequenceFile.Sorter` classes for writing, reading and sorting respectively.

There are three `SequenceFile` `Writer`s based on the `SequenceFile.CompressionType` used to compress key/value pairs:

1. `Writer` : Uncompressed records.
2. `RecordCompressWriter` : Record-compressed files, only compress values.
3. `BlockCompressWriter` : Block-compressed files, both keys & values are collected in 'blocks' separately and compressed. The size of the 'block' is configurable.

The actual compression algorithm used to compress key and/or values can be specified by using the appropriate `CompressionCodec`.

The recommended way is to use the static `createWriter` methods provided by the `SequenceFile` to chose the preferred format.

The `Reader` acts as the bridge and can read any of the above `SequenceFile` formats.

#### SequenceFile Formats

Essentially there are 3 different formats for `SequenceFile`s depending on the `CompressionType` specified. All of them share a common header described below.

##### SequenceFile Header

- version - 3 bytes of magic header SEQ, followed by 1 byte of actual version number (e.g. SEQ4 or SEQ6)
- keyClassName -key class
- valueClassName - value class
- compression - A boolean which specifies if compression is turned on for keys/values in this file.
- blockCompression - A boolean which specifies if block-compression is turned on for keys/values in this file.
- compression codec - `CompressionCodec` class which is used for compression of keys and/or values (if compression is enabled).
- metadata - `SequenceFile.Metadata` for this file.
- sync - A sync marker to denote end of the header.

##### Uncompressed SequenceFile Format

- Header
- Record
  - Record length
  - Key length
  - Key
  - Value
- A sync-marker every few `100` bytes or so.

##### Record-Compressed SequenceFile Format

- Header
- Record
  - Record length
  - Key length
  - Key
  - Compressed Value
- A sync-marker every few `100` bytes or so.

##### Block-Compressed SequenceFile Format

- Header
- Record Block
  - Compressed key-lengths block-size
  - Compressed key-lengths block
  - Compressed keys block-size
  - Compressed keys block
  - Compressed value-lengths block-size
  - Compressed value-lengths block
  - Compressed values block-size
  - Compressed values block
- A sync-marker every few `100` bytes or so.

The compressed blocks of key lengths and value lengths consist of the actual lengths of individual keys/values encoded in ZeroCompressedInteger format.

See Also:
  `CompressionCodec`

Nested Class Summary
`static class` | `SequenceFile.CompressionType` The compression type used to compress key/value pairs in the `SequenceFile`.
`static class` | `SequenceFile.Metadata` The class encapsulating with the metadata of a file.
`static class` | `SequenceFile.Reader` Reads key/value pairs from a sequence-format file.
`static class` | `SequenceFile.Sorter` Sorts key/value pairs in a sequence-format file.
`static interface` | `SequenceFile.ValueBytes` The interface to 'raw' values of SequenceFiles.
`static class` | `SequenceFile.Writer` Write key/value pairs to a sequence-format file.

Field Summary
`static int` | `SYNC_INTERVAL` The number of bytes between sync points.

Method Summary
`static SequenceFile.Writer` | `createWriter(Configuration conf, FSDataOutputStream out, Class keyClass, Class valClass, SequenceFile.CompressionType compressionType, CompressionCodec codec)` Construct the preferred type of 'raw' SequenceFile Writer.
`static SequenceFile.Writer` | `createWriter(Configuration conf, FSDataOutputStream out, Class keyClass, Class valClass, SequenceFile.CompressionType compressionType, CompressionCodec codec, SequenceFile.Metadata metadata)` Construct the preferred type of 'raw' SequenceFile Writer.
`static SequenceFile.Writer` | `createWriter(FileSystem fs, Configuration conf, Path name, Class keyClass, Class valClass)` Construct the preferred type of SequenceFile Writer.
`static SequenceFile.Writer` | `createWriter(FileSystem fs, Configuration conf, Path name, Class keyClass, Class valClass, int bufferSize, short replication, long blockSize, boolean createParent, SequenceFile.CompressionType compressionType, CompressionCodec codec, SequenceFile.Metadata metadata)` Construct the preferred type of SequenceFile Writer.
`static SequenceFile.Writer` | `createWriter(FileSystem fs, Configuration conf, Path name, Class keyClass, Class valClass, int bufferSize, short replication, long blockSize, SequenceFile.CompressionType compressionType, CompressionCodec codec, Progressable progress, SequenceFile.Metadata metadata)` Construct the preferred type of SequenceFile Writer.
`static SequenceFile.Writer` | `createWriter(FileSystem fs, Configuration conf, Path name, Class keyClass, Class valClass, SequenceFile.CompressionType compressionType)` Construct the preferred type of SequenceFile Writer.
`static SequenceFile.Writer` | `createWriter(FileSystem fs, Configuration conf, Path name, Class keyClass, Class valClass, SequenceFile.CompressionType compressionType, CompressionCodec codec)` Construct the preferred type of SequenceFile Writer.
`static SequenceFile.Writer` | `createWriter(FileSystem fs, Configuration conf, Path name, Class keyClass, Class valClass, SequenceFile.CompressionType compressionType, CompressionCodec codec, Progressable progress)` Construct the preferred type of SequenceFile Writer.
`static SequenceFile.Writer` | `createWriter(FileSystem fs, Configuration conf, Path name, Class keyClass, Class valClass, SequenceFile.CompressionType compressionType, CompressionCodec codec, Progressable progress, SequenceFile.Metadata metadata)` Construct the preferred type of SequenceFile Writer.
`static SequenceFile.Writer` | `createWriter(FileSystem fs, Configuration conf, Path name, Class keyClass, Class valClass, SequenceFile.CompressionType compressionType, Progressable progress)` Construct the preferred type of SequenceFile Writer.
`static SequenceFile.CompressionType` | `getCompressionType(Configuration job)` Deprecated. Use `SequenceFileOutputFormat.getOutputCompressionType(org.apache.hadoop.mapred.JobConf)` to get `SequenceFile.CompressionType` for job-outputs.
`static void` | `setCompressionType(Configuration job, SequenceFile.CompressionType val)` Deprecated. Use the one of the many SequenceFile.createWriter methods to specify the `SequenceFile.CompressionType` while creating the `SequenceFile` or `SequenceFileOutputFormat.setOutputCompressionType(org.apache.hadoop.mapred.JobConf, org.apache.hadoop.io.SequenceFile.CompressionType)` to specify the `SequenceFile.CompressionType` for job-outputs. or

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Field Detail

### SYNC_INTERVAL

    public static final int SYNC_INTERVAL

  The number of bytes between sync points.

  See Also:
    Constant Field Values

Method Detail

### getCompressionType

    @Deprecated
    public static SequenceFile.CompressionType getCompressionType(Configuration job)

  Deprecated. Use `SequenceFileOutputFormat.getOutputCompressionType(org.apache.hadoop.mapred.JobConf)` to get `SequenceFile.CompressionType` for job-outputs.

  Get the compression type for the reduce outputs

  Parameters:
    `job` - the job config to look in
  Returns:
    the kind of compression to use

### setCompressionType

    @Deprecated
    public static void setCompressionType(Configuration job,
                                                     SequenceFile.CompressionType val)

  Deprecated. Use the one of the many SequenceFile.createWriter methods to specify the `SequenceFile.CompressionType` while creating the `SequenceFile` or `SequenceFileOutputFormat.setOutputCompressionType(org.apache.hadoop.mapred.JobConf, org.apache.hadoop.io.SequenceFile.CompressionType)` to specify the `SequenceFile.CompressionType` for job-outputs. or

  Set the compression type for sequence files.

  Parameters:
    `job` - the configuration to modify
    `val` - the new compression type (none, block, record)

### createWriter

    public static SequenceFile.Writer createWriter(FileSystem fs,
                                                   Configuration conf,
                                                   Path name,
                                                   Class keyClass,
                                                   Class valClass)
                                            throws IOException

  Construct the preferred type of SequenceFile Writer.

  Parameters:
    `fs` - The configured filesystem.
    `conf` - The configuration.
    `name` - The name of the file.
    `keyClass` - The 'key' type.
    `valClass` - The 'value' type.
  Returns:
    Returns the handle to the constructed SequenceFile Writer.
  Throws:
    `IOException`

### createWriter

    public static SequenceFile.Writer createWriter(FileSystem fs,
                                                   Configuration conf,
                                                   Path name,
                                                   Class keyClass,
                                                   Class valClass,
                                                   SequenceFile.CompressionType compressionType)
                                            throws IOException

  Construct the preferred type of SequenceFile Writer.

  Parameters:
    `fs` - The configured filesystem.
    `conf` - The configuration.
    `name` - The name of the file.
    `keyClass` - The 'key' type.
    `valClass` - The 'value' type.
    `compressionType` - The compression type.
  Returns:
    Returns the handle to the constructed SequenceFile Writer.
  Throws:
    `IOException`

### createWriter

    public static SequenceFile.Writer createWriter(FileSystem fs,
                                                   Configuration conf,
                                                   Path name,
                                                   Class keyClass,
                                                   Class valClass,
                                                   SequenceFile.CompressionType compressionType,
                                                   Progressable progress)
                                            throws IOException

  Construct the preferred type of SequenceFile Writer.

  Parameters:
    `fs` - The configured filesystem.
    `conf` - The configuration.
    `name` - The name of the file.
    `keyClass` - The 'key' type.
    `valClass` - The 'value' type.
    `compressionType` - The compression type.
    `progress` - The Progressable object to track progress.
  Returns:
    Returns the handle to the constructed SequenceFile Writer.
  Throws:
    `IOException`

### createWriter

    public static SequenceFile.Writer createWriter(FileSystem fs,
                                                   Configuration conf,
                                                   Path name,
                                                   Class keyClass,
                                                   Class valClass,
                                                   SequenceFile.CompressionType compressionType,
                                                   CompressionCodec codec)
                                            throws IOException

  Construct the preferred type of SequenceFile Writer.

  Parameters:
    `fs` - The configured filesystem.
    `conf` - The configuration.
    `name` - The name of the file.
    `keyClass` - The 'key' type.
    `valClass` - The 'value' type.
    `compressionType` - The compression type.
    `codec` - The compression codec.
  Returns:
    Returns the handle to the constructed SequenceFile Writer.
  Throws:
    `IOException`

### createWriter

    public static SequenceFile.Writer createWriter(FileSystem fs,
                                                   Configuration conf,
                                                   Path name,
                                                   Class keyClass,
                                                   Class valClass,
                                                   SequenceFile.CompressionType compressionType,
                                                   CompressionCodec codec,
                                                   Progressable progress,
                                                   SequenceFile.Metadata metadata)
                                            throws IOException

  Construct the preferred type of SequenceFile Writer.

  Parameters:
    `fs` - The configured filesystem.
    `conf` - The configuration.
    `name` - The name of the file.
    `keyClass` - The 'key' type.
    `valClass` - The 'value' type.
    `compressionType` - The compression type.
    `codec` - The compression codec.
    `progress` - The Progressable object to track progress.
    `metadata` - The metadata of the file.
  Returns:
    Returns the handle to the constructed SequenceFile Writer.
  Throws:
    `IOException`

### createWriter

    public static SequenceFile.Writer createWriter(FileSystem fs,
                                                   Configuration conf,
                                                   Path name,
                                                   Class keyClass,
                                                   Class valClass,
                                                   int bufferSize,
                                                   short replication,
                                                   long blockSize,
                                                   SequenceFile.CompressionType compressionType,
                                                   CompressionCodec codec,
                                                   Progressable progress,
                                                   SequenceFile.Metadata metadata)
                                            throws IOException

  Construct the preferred type of SequenceFile Writer.

  Parameters:
    `fs` - The configured filesystem.
    `conf` - The configuration.
    `name` - The name of the file.
    `keyClass` - The 'key' type.
    `valClass` - The 'value' type.
    `bufferSize` - buffer size for the underlaying outputstream.
    `replication` - replication factor for the file.
    `blockSize` - block size for the file.
    `compressionType` - The compression type.
    `codec` - The compression codec.
    `progress` - The Progressable object to track progress.
    `metadata` - The metadata of the file.
  Returns:
    Returns the handle to the constructed SequenceFile Writer.
  Throws:
    `IOException`

### createWriter

    public static SequenceFile.Writer createWriter(FileSystem fs,
                                                   Configuration conf,
                                                   Path name,
                                                   Class keyClass,
                                                   Class valClass,
                                                   int bufferSize,
                                                   short replication,
                                                   long blockSize,
                                                   boolean createParent,
                                                   SequenceFile.CompressionType compressionType,
                                                   CompressionCodec codec,
                                                   SequenceFile.Metadata metadata)
                                            throws IOException

  Construct the preferred type of SequenceFile Writer.

  Parameters:
    `fs` - The configured filesystem.
    `conf` - The configuration.
    `name` - The name of the file.
    `keyClass` - The 'key' type.
    `valClass` - The 'value' type.
    `bufferSize` - buffer size for the underlaying outputstream.
    `replication` - replication factor for the file.
    `blockSize` - block size for the file.
    `createParent` - create parent directory if non-existent
    `compressionType` - The compression type.
    `codec` - The compression codec.
    `metadata` - The metadata of the file.
  Returns:
    Returns the handle to the constructed SequenceFile Writer.
  Throws:
    `IOException`

### createWriter

    public static SequenceFile.Writer createWriter(FileSystem fs,
                                                   Configuration conf,
                                                   Path name,
                                                   Class keyClass,
                                                   Class valClass,
                                                   SequenceFile.CompressionType compressionType,
                                                   CompressionCodec codec,
                                                   Progressable progress)
                                            throws IOException

  Construct the preferred type of SequenceFile Writer.

  Parameters:
    `fs` - The configured filesystem.
    `conf` - The configuration.
    `name` - The name of the file.
    `keyClass` - The 'key' type.
    `valClass` - The 'value' type.
    `compressionType` - The compression type.
    `codec` - The compression codec.
    `progress` - The Progressable object to track progress.
  Returns:
    Returns the handle to the constructed SequenceFile Writer.
  Throws:
    `IOException`

### createWriter

    public static SequenceFile.Writer createWriter(Configuration conf,
                                                   FSDataOutputStream out,
                                                   Class keyClass,
                                                   Class valClass,
                                                   SequenceFile.CompressionType compressionType,
                                                   CompressionCodec codec,
                                                   SequenceFile.Metadata metadata)
                                            throws IOException

  Construct the preferred type of 'raw' SequenceFile Writer.

  Parameters:
    `conf` - The configuration.
    `out` - The stream on top which the writer is to be constructed.
    `keyClass` - The 'key' type.
    `valClass` - The 'value' type.
    `compressionType` - The compression type.
    `codec` - The compression codec.
    `metadata` - The metadata of the file.
  Returns:
    Returns the handle to the constructed SequenceFile Writer.
  Throws:
    `IOException`

### createWriter

    public static SequenceFile.Writer createWriter(Configuration conf,
                                                   FSDataOutputStream out,
                                                   Class keyClass,
                                                   Class valClass,
                                                   SequenceFile.CompressionType compressionType,
                                                   CompressionCodec codec)
                                            throws IOException

  Construct the preferred type of 'raw' SequenceFile Writer.

  Parameters:
    `conf` - The configuration.
    `out` - The stream on top which the writer is to be constructed.
    `keyClass` - The 'key' type.
    `valClass` - The 'value' type.
    `compressionType` - The compression type.
    `codec` - The compression codec.
  Returns:
    Returns the handle to the constructed SequenceFile Writer.
  Throws:
    `IOException`

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