HBase includes several methods of loading data into tables.
The most straightforward method is to either use the TableOutputFormat
class from a MapReduce job, or use the normal client APIs; however,
these are not always the most efficient methods.
The bulk load feature uses a MapReduce job to output table data in HBase's internal data format, and then directly loads the generated StoreFiles into a running cluster. Using bulk load will use less CPU and network resources than simply using the HBase API.
As bulk loading bypasses the write path, the WAL doesn’t get written to as part of the process. Replication works by reading the WAL files so it won’t see the bulk loaded data – and the same goes for the edits that use Put.setWriteToWAL(true). One way to handle that is to ship the raw files or the HFiles to the other cluster and do the other processing there.
The HBase bulk load process consists of two main steps.
The first step of a bulk load is to generate HBase data files (StoreFiles) from
a MapReduce job using HFileOutputFormat. This output format writes
out data in HBase's internal storage format so that they can be
later loaded very efficiently into the cluster.
In order to function efficiently, HFileOutputFormat must be
configured such that each output HFile fits within a single region.
In order to do this, jobs whose output will be bulk loaded into HBase
use Hadoop's TotalOrderPartitioner class to partition the map output
into disjoint ranges of the key space, corresponding to the key
ranges of the regions in the table.
HFileOutputFormat includes a convenience function,
configureIncrementalLoad(), which automatically sets up
a TotalOrderPartitioner based on the current region boundaries of a
table.
After the data has been prepared using
HFileOutputFormat, it is loaded into the cluster using
completebulkload. This command line tool iterates
through the prepared data files, and for each one determines the
region the file belongs to. It then contacts the appropriate Region
Server which adopts the HFile, moving it into its storage directory
and making the data available to clients.
If the region boundaries have changed during the course of bulk load
preparation, or between the preparation and completion steps, the
completebulkloads utility will automatically split the
data files into pieces corresponding to the new boundaries. This
process is not optimally efficient, so users should take care to
minimize the delay between preparing a bulk load and importing it
into the cluster, especially if other clients are simultaneously
loading data through other means.
After a data import has been prepared, either by using the
importtsv tool with the
"importtsv.bulk.output" option or by some other MapReduce
job using the HFileOutputFormat, the
completebulkload tool is used to import the data into the
running cluster.
The completebulkload tool simply takes the output path
where importtsv or your MapReduce job put its results, and
the table name to import into. For example:
$ hadoop jar hbase-server-VERSION.jar completebulkload [-c /path/to/hbase/config/hbase-site.xml] /user/todd/myoutput mytable
The -c config-file option can be used to specify a file
containing the appropriate hbase parameters (e.g., hbase-site.xml) if
not supplied already on the CLASSPATH (In addition, the CLASSPATH must
contain the directory that has the zookeeper configuration file if
zookeeper is NOT managed by HBase).
Note: If the target table does not already exist in HBase, this tool will create the table automatically.
This tool will run quickly, after which point the new data will be visible in the cluster.
For more information about the referenced utilities, see Section 17.1.11, “ImportTsv” and Section 17.1.12, “CompleteBulkLoad”.
See How-to: Use HBase Bulk Loading, and Why for a recent blog on current state of bulk loading.
Although the importtsv tool is useful in many cases, advanced users may
want to generate data programatically, or import data from other formats. To get
started doing so, dig into ImportTsv.java and check the JavaDoc for
HFileOutputFormat.
The import step of the bulk load can also be done programatically. See the
LoadIncrementalHFiles class for more information.