Viewing Log statements in Hadoop Map-reduce jobs

August 23, 2014


Anyone who is new to the hadoop world often ends up frantically searching for the debug log statements that he might have added in mapper or reducer function. At least this has been the case with me when I started working on Hadoop. Hence I thought it might be a good idea to post this particular entry.

The mapper and reducer are not executed locally and hence you can not find the logs for those on local file system. The mapper/reducer are run on the hadoop cluster and hence cluster is the place where you should look for them. However you do need to know the “JobTracker” URL for your cluster.

  1. Access the “JobTracker.  The default URL for JobTracker is “http://{hostname}:50030/jobtracker.jsp”.
  2. This simple UI lists down the different Map-reduce jobs and their states ( namely “running”,”completed”, “failed” and “retired”).
  3. You need to locate the Map-reduce job that you started (Please Refer the attached screenshot.)jobtracker
  4. There are various ways to identify your Map-reduce job.
    1. If you run a Pig script, the Pig client will log the job id which you can search in jobtracker. The screen shot shows the map-reduce job corresponding to a Pig script.
    2. If you are running a plain map-reduce application then once you submit the job, you can search the same either by using the userid used to submit the job or the application name.
  5. Clicking on the job number reveals the job details as shown below –map reduce job details
  6. Following screenshot shows way to navigate to the log statements. Please note that the screenshot comprises of three steps.Map Reduce Log Statements

Hope you find this post useful while learning Hadoop.