Creating a Table
Function Description
In HBase, a table is created using the createTable method of the org.apache.hadoop.hbase.client.Admin object. You need to specify a table name and a column family name. You can create a table by using either of the following methods, but the latter one is recommended:
- Quickly create a table. A newly created table contains only one region, which will be automatically split into multiple new regions as data increases.
- Create a table using pre-assigned regions. You can pre-assign multiple regions before creating a table. This mode accelerates data write at the beginning of massive data write.
![](https://support.huaweicloud.com/intl/en-us/devg-cloudtable/public_sys-resources/note_3.0-en-us.png)
The table name and column family name of a table consist of letters, digits, and underscores (_) but cannot contain any special characters.
Sample Code
public void testCreateTable() { LOG.info("Entering testCreateTable."); // Specify the table descriptor. HTableDescriptor htd = new HTableDescriptor(tableName); // (1) // Set the column family name to info. HColumnDescriptor hcd = new HColumnDescriptor("info"); // (2) // Set data encoding methods. HBase provides DIFF,FAST_DIFF,PREFIX // and PREFIX_TREE hcd.setDataBlockEncoding(DataBlockEncoding.FAST_DIFF); // Note [1] // Set compression methods, HBase provides two default compression // methods:GZ and SNAPPY // GZ has the highest compression rate,but low compression and // decompression efficiency,fit for cold data // SNAPPY has low compression rate, but high compression and // decompression efficiency,fit for hot data. // it is advised to use SANPPY hcd.setCompressionType(Compression.Algorithm.SNAPPY); htd.addFamily(hcd); // (3) Admin admin = null; try { // Instantiate an Admin object. admin = conn.getAdmin(); // (4) if (!admin.tableExists(tableName)) { LOG.info("Creating table..."); admin.createTable(htd); // Note [2] (5) LOG.info(admin.getClusterStatus()); LOG.info(admin.listNamespaceDescriptors()); LOG.info("Table created successfully."); } else { LOG.warn("table already exists"); } } catch (IOException e) { LOG.error("Create table failed.", e); } finally { if (admin != null) { try { // Close the Admin object. admin.close(); } catch (IOException e) { LOG.error("Failed to close admin ", e); } } } LOG.info("Exiting testCreateTable."); }
Explanation
(1) Create a table descriptor.
(2) Create a column family descriptor.
(3) Add the column family descriptor to the table descriptor.
(4) Obtain the Admin object. You use the Admin object to create a table and a column family, check whether the table exists, modify the table structure and column family structure, and delete the table.
(5) Invoke the Admin object to create a table.
Precautions
- Note [1] Use the following code to set the compression mode for a column family:
// Set an encoding algorithm. HBase provides four encoding algorithms: DIFF, FAST_DIFF, PREFIX, and PREFIX_TREE. hcd.setDataBlockEncoding(DataBlockEncoding.FAST_DIFF); // Set a file compression mode. By default, HBase provides two compression algorithms: GZ and SNAPPY. // GZ has a high compression rate but low compression and decompression performance. It is applicable to cold data. // SNAPPY has a low compression rate but high compression and decompression performance. It is applicable to hot data. // It is recommended that SNAPPY compression be enabled by default. hcd.setCompressionType(Compression.Algorithm.SNAPPY);
- Note [2] Create a table by specifying the start and end RowKeys or pre-assigning regions using RowKey arrays. The code snippet is as follows:
// Create a table with pre-split regions. byte[][] splits = new byte[4][]; splits[0] = Bytes.toBytes("A"); splits[1] = Bytes.toBytes("H"); splits[2] = Bytes.toBytes("O"); splits[3] = Bytes.toBytes("U"); admin.createTable(htd, splits);
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