Standard practices for logging in MapReduce jobs

You could use log4j which is the default logging framework that hadoop uses. So, from your MapReduce application you could do something like this:

import org.apache.log4j.Logger;// other imports omittedpublic class SampleMapper extends Mapper<LongWritable, Text, Text, Text> {    private Logger logger = Logger.getLogger(SampleMapper.class);    @Override    protected void setup(Context context) {        logger.info("Initializing NoSQL Connection.")        try {            // logic for connecting to NoSQL - ommitted        } catch (Exception ex) {            logger.error(ex.getMessage());        }    }    @Override    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {        // mapper code ommitted    }}        

This sample code will user log4j logger to log events to the inherited Mapper logger. All the log events will be logged to their respective task log's. You could visit the task logs from either JobTracker(MRv1)/ResourceManager(MRv2) webpage.

If you are using yarn you could access the application logs from command line using the following command:

yarn logs -applicationId <application_id>

While if you are using mapreduce v1, there is no single point of access from command line; hence you have to log into each TaskTracker and look in the configured path generally /var/log/hadoop/userlogs/attempt_<job_id>/syslog specified in ${hadoop.log.dir}/userlogs contains log4j output.

To add to @Ashrith 's answer: you can view the individual task tracker logs via the JobTracker GUI. The running task attempts are visible by the JT Gui and you can click on any of the following: stderr, stdout, and system logs. The system logs are where you find your log4j outputs.