Friday, December 23, 2011

Communicate with HBase: Linux Shell, Java API for Client access

  • HBase : Introduction
HBase is a type of "NoSQL" database which runs on top of Hadoop/HDFS clusters. It stores data in form of key-value pair on Hadoop distributed file system. Table schema contains number of column-family which contain  a few to hundreds of thousand or millions of column. HBase is well suited for huge data ( Tera bytes or Peta bytes or whatever you can generate today) with random read/write access. Each row contains a rowkey as identifier and can be accessed as version# when inserted multiple times. Column values are stored with TIMESTAMP, and TTL can also be set for any column-family.
  • Linux Shell
Linux Shell allows an easy access to HBase. Open a Shell/Terminal  in  your Linux system (assuming HBase is already configured on your machine), and reach to your HBase installation folder. 
Try out these commands : 
  • Start HBase
user@ubuntu~$bin/start-hbase.sh // to start hbase/HMaster 
user@ubuntu~$bin/hbase shell // to enter into HBase shell
  • Create
hbase(main):003:0> create 'htable', 'cf' // table name:'htable' & column-family:'cf' - it is advised to have small column-family names usually 1 or 2 letter!!
0 row(s) in 1.2200 seconds.
  • Put
hbase(main):004:0> put 'htable', 'rowkey1', 'cf:qualifier1', 'value1' // rowkey:'rowkey1' & column name:'qualifier1' & column value:'value1'
0 row(s) in 1.2200 seconds.
hbase(main):005:0> put 'htable', 'rowkey2', 'cf:qualifier1', 'value2' // inserted as next row.
hbase(main):006:0> put 'htable', 'rowkey1', 'cf:qualifier1', 'value3' // inserted as next version for rowkey:'rowkey1'
any number of column/qualifier can be created while inserting data, only column-family(s) are required at the time of Table schema design.
  • Scan
hbase(main):007:0> scan 'htable' // list all Rows & Columns.
ROW        COLUMN+CELL
rowkey1       column=cf:qualifier1, timestamp=1288380747188, value=value3
rowkey2       column=cf:qualifier1, timestamp=1288380738440, value=value2
2 row(s) in 0.0590 seconds
Scan only fetches latest version for any row.
  • Get
hbase(main):008:0> get 'htable', 'rowkey2' // get accepts table-name & rowkey
COLUMN      CELL
cf:qualifier1        timestamp=1288380738440, value=value2
1 row(s) in 0.0400 seconds
Number of shell commands are available to work with it. But stored procedure, cursor, triggers, functions are not available here as you can find in RDBMS.
  • Java API for client
HBase API provide outside applications(desktop, web etc) to access HBase table using org.apache.hadoop.hbase.client package. 
==================================================================
import java.io.IOException;

import org.apache.hadoop.hbase.HBaseConfiguration;
import org.apache.hadoop.hbase.client.Get;
import org.apache.hadoop.hbase.client.HTable;
import org.apache.hadoop.hbase.client.Put;
import org.apache.hadoop.hbase.client.Result;
import org.apache.hadoop.hbase.client.ResultScanner;
import org.apache.hadoop.hbase.client.Scan;
import org.apache.hadoop.hbase.util.Bytes;


// Class that has nothing but a main.
// Does a Put, Get and a Scan against an hbase table.
public class MyLittleHBaseClient {
  public static void main(String[] args) throws IOException {
    // You need a configuration object to tell the client where to connect.
    // When you create a HBaseConfiguration, it reads in whatever you've set
    // into your hbase-site.xml and in hbase-default.xml, as long as these can
    // be found on the CLASSPATH
    Configuration config = HBaseConfiguration.create();

    // This instantiates an HTable object that connects you to
    // the "myLittleHBaseTable" table.
    HTable table = new HTable(config, "myLittleHBaseTable");

    // To add to a row, use Put.  A Put constructor takes the name of the row
    // you want to insert into as a byte array.  In HBase, the Bytes class has
    // utility for converting all kinds of java types to byte arrays.  In the
    // below, we are converting the String "myLittleRow" into a byte array to
    // use as a row key for our update. Once you have a Put instance, you can
    // adorn it by setting the names of columns you want to update on the row,
    // the timestamp to use in your update, etc.If no timestamp, the server
    // applies current time to the edits.
    Put p = new Put(Bytes.toBytes("myLittleRow"));

    // To set the value you'd like to update in the row 'myLittleRow', specify
    // the column family, column qualifier, and value of the table cell you'd
    // like to update.  The column family must already exist in your table
    // schema.  The qualifier can be anything.  All must be specified as byte
    // arrays as hbase is all about byte arrays.  Lets pretend the table
    // 'myLittleHBaseTable' was created with a family 'myLittleFamily'.
    p.add(Bytes.toBytes("myLittleFamily"), Bytes.toBytes("someQualifier"),
      Bytes.toBytes("Some Value"));

    // Once you've adorned your Put instance with all the updates you want to
    // make, to commit it do the following (The HTable#put method takes the
    // Put instance you've been building and pushes the changes you made into
    // hbase)
    table.put(p);

    // Now, to retrieve the data we just wrote. The values that come back are
    // Result instances. Generally, a Result is an object that will package up
    // the hbase return into the form you find most palatable.
    Get g = new Get(Bytes.toBytes("myLittleRow"));
    Result r = table.get(g);
    byte [] value = r.getValue(Bytes.toBytes("myLittleFamily"),
      Bytes.toBytes("someQualifier"));
    // If we convert the value bytes, we should get back 'Some Value', the
    // value we inserted at this location.
    String valueStr = Bytes.toString(value);
    System.out.println("GET: " + valueStr);

    // Sometimes, you won't know the row you're looking for. In this case, you
    // use a Scanner. This will give you cursor-like interface to the contents
    // of the table.  To set up a Scanner, do like you did above making a Put
    // and a Get, create a Scan.  Adorn it with column names, etc.
    Scan s = new Scan();
    s.addColumn(Bytes.toBytes("myLittleFamily"), Bytes.toBytes("someQualifier"));
    ResultScanner scanner = table.getScanner(s);
    try {
      // Scanners return Result instances.
      // Now, for the actual iteration. One way is to use a while loop like so:
      for (Result rr = scanner.next(); rr != null; rr = scanner.next()) {
        // print out the row we found and the columns we were looking for
        System.out.println("Found row: " + rr);
      }

      // The other approach is to use a foreach loop. Scanners are iterable!
      // for (Result rr : scanner) {
      //   System.out.println("Found row: " + rr);
      // }
    } finally {
      // Make sure you close your scanners when you are done!
      // Thats why we have it inside a try/finally clause
      scanner.close();
    }
  }
}

=================================================================
Above program automatically connects with local HBase configuration. To connect with  remote HBase machine, it requires path to 'hbase-site.xml' on Configuration.Path(path);

Apart from these two methods, HBase provides Thrift & REST gateways. Map Reduce programs can also read/write data to HBase clusters. 
will come with more in next post :)

18 comments:

  1. Update :
    There is nothing like schema design in HBase.
    You have to pass a Key ie: 'Column Name' and Value ie: 'Column Value' when you want to store data, with a unique row-key (if row-key is existing, it will insert data as next version).
    This way you can have any number of columns (ie: also different number of columns for different row) in a table, and you can keep adding!!

    ReplyDelete
  2. get command doesn't work with multiple column qualifier.

    ReplyDelete
  3. It should work -
    On hbase shell : get 'table', 'rowkey', ['cf:col1','cf:col2']
    & with Get java api : use addColumn (family, qualifier) with get object for all the column want to retrieve.
    http://hbase.apache.org/0.94/apidocs/org/apache/hadoop/hbase/client/Get.html#addColumn(byte[],%20byte[])

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