Sequence Files in Hadoop

Question

How are these sequence files generated ? I saw a link about sequence file here,

http://wiki.apache.org/hadoop/SequenceFile

Are these written using default Java serializer ? and How do I read a sequence file ?

Answer 1

Sequence files are generated by MapReduce tasks and and can be used as common format to transfer data between MapReduce jobs.

You can read them in the following manner:

Configuration config = new Configuration();
Path path = new Path(PATH_TO_YOUR_FILE);
SequenceFile.Reader reader = new SequenceFile.Reader(FileSystem.get(config), path, config);
WritableComparable key = (WritableComparable) reader.getKeyClass().newInstance();
Writable value = (Writable) reader.getValueClass().newInstance();
while (reader.next(key, value))
  // perform some operating
reader.close();

Also you can generate sequence files by yourself using SequenceFile.Writer.

The classes used in the example are the following:

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.SequenceFile;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.io.WritableComparable;

And are contained within the hadoop-core maven dependency:

<dependency>
    <groupId>org.apache.hadoop</groupId>
    <artifactId>hadoop-core</artifactId>
    <version>1.2.1</version>
</dependency>

Answer 2

Thanks to Lev Khomich's answer, my problem has been solved.

However, the solution has been deprecated for a while and the new API offers more features and also easy to use.

Check out the source code of hadoop.io.SequenceFile, click here:

Configuration config = new Configuration();
Path path = new Path("/Users/myuser/sequencefile");
SequenceFile.Reader reader = new Reader(config, Reader.file(path));
WritableComparable key = (WritableComparable) reader.getKeyClass()
        .newInstance();
Writable value = (Writable) reader.getValueClass().newInstance();

while (reader.next(key, value)) {
    System.out.println(key);
    System.out.println(value);
    System.out.println("------------------------");
}
reader.close();

Extra info, here is the sample output running against the data file generated by Nutch/injector:

------------------------
https://wiki.openoffice.org/wiki/Ru/FAQ
Version: 7
Status: 1 (db_unfetched)
Fetch time: Sun Apr 13 16:12:59 MDT 2014
Modified time: Wed Dec 31 17:00:00 MST 1969
Retries since fetch: 0
Retry interval: 2592000 seconds (30 days)
Score: 1.0
Signature: null
Metadata: 

------------------------
https://www.bankhapoalim.co.il/
Version: 7
Status: 1 (db_unfetched)
Fetch time: Sun Apr 13 16:12:59 MDT 2014
Modified time: Wed Dec 31 17:00:00 MST 1969
Retries since fetch: 0
Retry interval: 2592000 seconds (30 days)
Score: 1.0
Signature: null
Metadata: 

Thanks!