Package org.apache.hadoop.hbase.client

Provides HBase Client

See: Description

Package org.apache.hadoop.hbase.client Description

Provides HBase Client

Table of Contents

Overview

To administer HBase, create and drop tables, list and alter tables, use HBaseAdmin. Once created, table access is via an instance of HTable. You add content to a table a row at a time. To insert, create an instance of a Put object. Specify value, target column and optionally a timestamp. Commit your update using HTable.put(Put). To fetch your inserted value, use Get. The Get can be specified to be broad -- get all on a particular row -- or narrow; i.e. return only a single cell value. After creating an instance of Get, invoke HTable.get(Get). Use Scan to set up a scanner -- a Cursor- like access. After creating and configuring your Scan instance, call HTable.getScanner(Scan) and then invoke next on the returned object. Both HTable.get(Get) and HTable.getScanner(Scan) return a Result. A Result is a List of KeyValues. It has facility for packaging the return in different formats. Use Delete to remove content. You can remove individual cells or entire families, etc. Pass it to HTable.delete(Delete) to execute.

Puts, Gets and Deletes take out a lock on the target row for the duration of their operation. Concurrent modifications to a single row are serialized. Gets and scans run concurrently without interference of the row locks and are guaranteed to not to return half written rows.

Client code accessing a cluster finds the cluster by querying ZooKeeper. This means that the ZooKeeper quorum to use must be on the client CLASSPATH. Usually this means make sure the client can find your hbase-site.xml.

Example API Usage

Once you have a running HBase, you probably want a way to hook your application up to it. If your application is in Java, then you should use the Java API. Here's an example of what a simple client might look like. This example assumes that you've created a table called "myTable" with a column family called "myColumnFamily". 

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();
    }
  }
}

There are many other methods for putting data into and getting data out of HBase, but these examples should get you started. See the HTable javadoc for more methods. Additionally, there are methods for managing tables in the HBaseAdmin class.

If your client is NOT Java, then you should consider the Thrift or REST libraries.

Related Documentation

There are many other methods for putting data into and getting data out of HBase, but these examples should get you started. See the HTable javadoc for more methods. Additionally, there are methods for managing tables in the HBaseAdmin class.

See also the section in the HBase Reference Guide where it discusses HBase Client. It has section on how to access HBase from inside your multithreaded environtment how to control resources consumed client-side, etc.

Copyright © 2013 The Apache Software Foundation. All Rights Reserved.