HBase: use MapReduce to update rows?

Question

I have a table in HBase that I'd like to perform an update on. For example, I'd like to update a column to a value if pred(row) == true ( pred is a function written in Java).

Can I use MapReduce for this? Initially I thought I could but now I see that MapReduce is used to read from one table and write to another (or to disk). I then considered implementing a parallel scan which will iterate over the entire table using multiple threads, but it seems as though I'm reinventing the wheel.

Answer 1

For this task, MapReduce is not needed. You can connect to HBASE and get the work done from java application itself. A little help with the code below

HTable table = new HTable(HBaseConfiguration.create(), "MYTABLE");
Scan scan = new Scan();

scan.addFamily(Bytes.toBytes("myfamily"));
ResultScanner scanner = table.getScanner(scan);

for (Result result = scanner.next(); (result != null); result = scanner.next()) {
    for(KeyValue keyValue : result.list()) {
    // Make use of keyValue.getKeyString() and keyValue.getValue() here
    }
}

Code snippet to update a particular row is as below

  Put p = new Put(Bytes.toBytes("row1"));
  p.add(Bytes.toBytes("myfamily"),
  Bytes.toBytes("fieldname"),Bytes.toBytes("NEWVALUE"));
  table.put(p);