How to temporarily connect to MySQL in MapReduce - hadoop

Question

I want to connect to MySQL in MapReduce temporarily. In other words, I want to bring table in MySQL to Map function temporarily to change InputData(Text) but, the Result was empty.
Below is my code:
public class Map extends Mapper{
private Text outputKey=new Text();
private final static IntWritable outputValue=new IntWritable(1);
public void map(LongWritable key,Text value,Context context)
throws IOException,InterruptedException
{
int i=0;
try{
Connection con=DriverManager.getConnection("jdbc:sqlserver://locallhost:3306/test_db",
"user","user");
PreparedStatement ps=con.prepareStatement("select * from studentinfo");
ResultSet rs=ps.executeQuery();
while(rs.next())
i++;
AirlinePerformanceParser parser=new AirlinePerformanceParser(value);
outputKey.set(parser.getMonth_day()+","+i);
context.write(outputKey, outputValue);
}catch(SQLException e)
{
e.printStackTrace();
}
}
}
Map and reduce work well (there are no exceptions), but Result was empty.
I don't want to use DBInputFormat. MySQL table must not be used by DBInputFormat.
What is the problem?

Answer 1

If your query is static, i.e.it does not depend on map input (as is your current query), you should move your sql code into the setup function, to query your DB only once.
// set global the data buffer from your sql results (here a counter)
int nbResults;
#Override
protected void setup(Context context) throws IOException, InterruptedException {
// query your DB
Connection con [...]
}
public void map(LongWritable key,Text value,Context context)throws IOException,InterruptedException {
AirlinePerformanceParser parser=new AirlinePerformanceParser(value);
outputKey.set(parser.getMonth_day()+","+nbResults);
context.write(outputKey, outputValue);
}
As pointed by #Ramzy, check the connexion status.
In addition, do not perform a SELECT * if what you need is a COUNT(*).