Question

I am very new for Hbase and Map Reduce API.
I am very confused with Map Reduce concepts. I need to Load text file into Hbase table using MAPReduce API. I googled some Examples but in that I can find MAPPER () not reducer method. I am confused with when to use mapper and when to use Reducer ().

I am thinking in the way like :

1. TO write data to a Hbase we use mapper
2. TO read data from
   HBASE we use mapper and reducer(). please can any one clear me with
   detail explanation.
3. I am trying to load data from text file into
   HBASE table. I googled and tried some code but i dont know, how to
   load the text file and read in HBASE mapreduce API.

I really thank full for certain help

Answer 1

With regard to your questions:

• The Mapper receives splits of data and returns a pair key, set<values>
• The Reducer receives the output of from the Mapper and generates a pair <key, value>

Generally, will be your Reducer task which will write results (to the filesystem or to HBase), but the Mapper can do that too. There are MapReduce jobs which don’t require a Reducer. With regard to reading from HBase, it’s the Mapper class that has the configuration from which table to read from. But there’s nothing related a Mapper is a reader and Reducer a writer. This article “HBase MapReduce Examples” provides good examples about how to read from and write into HBase using MapReduce.

In any case, if what you need is to bulk import some .csv files into HBase, you don’t really need to do it with a MapReduce job. You can do it directly with the HBase API. In pseudocode:

table = hbase.createTable(tablename, fields); 
foreach (File file: dir) {
   content = readfile(file);    
   hbase.insert(table, content); 
}

I wrote an importer of .mbox files into HBase. Take a look at the code, it may give you some ideas.

Once your data is imported into HBase, then you do need to code a MapReduce job to operate with that data.