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Debug MapReduce Algorithms

This example shows how to debug your mapreduce algorithms in MATLAB® using a simple example file, MaxMapReduceExample.m. Debugging enables you to follow the movement of data between the different phases of mapreduce execution and inspect the state of all intermediate variables.

To begin, save the contents of the following functions as separate files in your current directory.




Set Breakpoint

Set one or more breakpoints in your map or reduce function files so you can examine the variable values where you think the problem is. For more information, see Set Breakpoints.

Open the file maxArrivalDelayMapper.m.

edit maxArrivalDelayMapper.m

Set a breakpoint on line 9. This breakpoint causes execution of mapreduce to pause right before each call to the map function adds a key-value pair to the intermediate KeyValueStore object, named intermKVStore.

Screenshot of map function file with breakpoint added on line that calls the add method.

Execute mapreduce

Run the mapreduce example file MaxMapReduceExample.m. Specify mapreducer(0) to ensure that the algorithm does not run in parallel, since parallel execution of mapreduce using Parallel Computing Toolbox™ ignores breakpoints.


MATLAB stops execution of the file when it encounters the breakpoint in the map function. During the pause in execution, you can hover over the different variable names in the map function, or type one of the variable names at the command line to inspect the values.

In this case, the display indicates that, as yet, there are no key-value pairs in intermKVStore.

Screenshot of hover text showing KeyValueStore with no key-value pairs.

Step Through Map Function

  1. Continue past the breakpoint. You can use dbstep to execute a single line, or dbcont to continue execution until MATLAB encounters another breakpoint. Alternatively, you can click Step or Continue in the Editor tab. For more information about all the available options, see Debug MATLAB Code Files.

    In this case, use dbstep (or click Step) to execute only line 9, which adds a key-value pair to intermKVStore. Inspect the new display for intermKVStore.

    Screenshot of hover text showing KeyValueStore with one key-value pair.

  2. Now, use dbcont (or click Continue) to continue execution of mapreduce. During the next call to the map function, MATLAB halts again on line 9. The new display for intermKVStore indicates that it does not contain any key-value pairs, because the display is meant to show only the most recent key-value pairs that are added in the current call to the map (or reduce) function.

  3. Step past line 9 again using dbstep (or click Step) to add the next key-value pair to intermKVStore, and inspect the new display for the variable. MATLAB displays only the key-value pair added during the current call to the map function.

    Screenshot of hover text showing KeyValueStore with multiple key-value pairs. The display shows the last key-value pair added.

  4. Complete the debugging of the map function by removing the breakpoint and closing the file maxArrivalDelayMapper.m.

Step Through Reduce Function

  1. You can use the same process to set breakpoints and step through execution of a reduce function. The reduce function for this example is maxArrivalDelayReducer.m. Open this file for editing.

    edit maxArrivalDelayReducer.m
  2. Set two breakpoints: one on line 10, and one on line 13. This enables you to inspect the ValueIterator and the final key-value pairs added to the output, outKVStore.

  3. Run the main example file.

  4. The execution of the example will pause when the breakpoint on line 10 is encountered. The debug display for the ValueIterator indicates the active key and whether any values remain to be retrieved.

    Screenshot of hover text showing ValueIterator with an active key and one or more values available.

  5. Now, remove the breakpoint on line 10 and use dbcont (or click Continue) to continue execution of the example until the next breakpoint is reached (on line 13). Since this reduce function continually compares each new value from the ValueIterator to the global maximum, mapreduce execution ends by adding a single key-value pair to outKVStore.

  6. Use dbstep (or click Step) to execute line 13 only. The display for outKVStore shows the global maximum value that mapreduce will return as the final answer.

    Screenshot of hover text showing KeyValueStore with one key-value pair added.

  7. Now use dbcont (or click Continue) to advance execution, enabling the example to finish running. mapreduce returns the final results.

    Map 100% Reduce 100%
    ans = 
               Key           Value 
        _________________    ______
        'MaxArrivalDelay'    [1014]

For a complete guide to debugging in MATLAB, see Debugging and Analysis.

See Also

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