In this recipe, we introduced some debugging and testing techniques you can use while implementing the MapReduce program. First, we introduced the technique to test a MapReduce program in a local mode. If you would like to run the MapReduce program in a pseudo-distributed or fully-distributed mode, it would take you a few minutes to several hours to complete the task, which would involve a lot of wastage of time while troubleshooting your MapReduce program. Therefore, you can set the backend to the local mode in rmr.options so that the program will be executed in the local mode, which takes lesser time to execute.

Another debugging technique is to list the content of the variable within the map or reduce function. In an R program, you can use the str function to display the compact structure of a single variable. In rmr2, the package also provides a function named rmr.str, which allows you to print out the content of a single variable onto the console. In this example, we use rmr.str to print the content of variables within a MapReduce program.