Read Data from HBase

问题:

I'm new to HBase, what's the best way to retrieve results from a table, row by row? I would like to read the entire data in the table. My table has two column families say col1 and col2.

回答1:

From Hbase shell, you can use scan command to list data in table, or get to retrieve a record. Reference here

回答2:

I think here is what you need: both through HBase shell and Java API: http://cook.coredump.me/post/19672191046/hbase-client-example

However you should understand hbase shell 'scan' is very slow (it is not cached). But it is intended only for debug purpose.

Another useful part of information for you is here: http://hbase.apache.org/book/perf.reading.html This chapter is right about reading from HBase but is is somewhat harder to understand because it assumes some level of familiarity and contains more advanced advices. I'd recommend to you this guide starting from the beginning.

回答3:

USe Scan api of Hbase , there you can specify start row and end row and can retrive data frm the table .

Here is an example:

http://eternaltechnology.blogspot.in/2013/05/hbase-scanner-example-scanning.html

回答4:

I was looking for something like this!

Map function

public void map(ImmutableBytesWritable row, Result value, Context context) throws InterruptedException, IOException {

            String x1 = new String(value.getValue(Bytes.toBytes("ColumnFamily"), Bytes.toBytes("X1")));
            String x2 = new String(value.getValue(Bytes.toBytes("ColumnFamily"), Bytes.toBytes("X2")));


}

Driver file:

Configuration config2 = new Configuration();
            Job job2 = new Job(config1, "kmeans2");
            //Configuration for job2

            job2.setJarByClass(Converge.class);
            job2.setMapperClass(Converge.Map.class);
            job2.setReducerClass(Converge.Reduce.class);
            job2.setInputFormatClass(TableInputFormat.class);
            job2.setOutputFormatClass(NullOutputFormat.class);
            job2.setOutputKeyClass(Text.class);
            job2.setOutputValueClass(Text.class);
            job2.getConfiguration().set(TableInputFormat.INPUT_TABLE, "tablename");