The HBase client HTable
is responsible for finding RegionServers that are serving the particular row range of
interest. It does this by querying the hbase:meta table. See Section 9.2.2, “hbase:meta” for details. After locating the required region(s), the
client contacts the RegionServer serving that region, rather than going through the master,
and issues the read or write request. This information is cached in the client so that
subsequent requests need not go through the lookup process. Should a region be reassigned
either by the master load balancer or because a RegionServer has died, the client will
requery the catalog tables to determine the new location of the user region.
See Section 9.5.2, “Runtime Impact” for more information about the impact of the Master on HBase Client communication.
Administrative functions are handled through HBaseAdmin
For connection configuration information, see Section 2.4.4, “Client configuration and dependencies connecting to an HBase cluster”.
HTable instances are not thread-safe. Only one thread use an instance of HTable at any given time. When creating HTable instances, it is advisable to use the same HBaseConfiguration instance. This will ensure sharing of ZooKeeper and socket instances to the RegionServers which is usually what you want. For example, this is preferred:
HBaseConfiguration conf = HBaseConfiguration.create(); HTable table1 = new HTable(conf, "myTable"); HTable table2 = new HTable(conf, "myTable");
as opposed to this:
HBaseConfiguration conf1 = HBaseConfiguration.create(); HTable table1 = new HTable(conf1, "myTable"); HBaseConfiguration conf2 = HBaseConfiguration.create(); HTable table2 = new HTable(conf2, "myTable");
For more information about how connections are handled in the HBase client, see HConnectionManager.
For applications which require high-end multithreaded access (e.g., web-servers or application servers that may serve many application threads
in a single JVM), you can pre-create an HConnection, as shown in
the following example:
Example 9.1. Pre-Creating a HConnection
// Create a connection to the cluster.
HConnection connection = HConnectionManager.createConnection(Configuration);
HTableInterface table = connection.getTable("myTable");
// use table as needed, the table returned is lightweight
table.close();
// use the connection for other access to the cluster
connection.close();Constructing HTableInterface implementation is very lightweight and resources are controlled.
HTablePool is DeprecatedPrevious versions of this guide discussed HTablePool, which was
deprecated in HBase 0.94, 0.95, and 0.96, and removed in 0.98.1, by HBASE-6500.
Please use HConnection instead.
If Section 14.8.4, “HBase Client: AutoFlush” is turned off on
HTable,
Puts are sent to RegionServers when the writebuffer
is filled. The writebuffer is 2MB by default. Before an HTable instance is
discarded, either close() or
flushCommits() should be invoked so Puts
will not be lost.
Note: htable.delete(Delete); does not go in the writebuffer! This only applies to Puts.
For additional information on write durability, review the ACID semantics page.
For fine-grained control of batching of
Puts or Deletes,
see the batch methods on HTable.
Information on non-Java clients and custom protocols is covered in Chapter 11, Apache HBase External APIs