Time for action – the fourth and last run

Let's perform the fourth execution to validate that the output has now reached its final stable state.

1. Execute the MapReduce job:

   $ hadoop jar graph.jarGraphPathgraphout3graphout4
   

2. Examine the output file:

   $ hadoop fs -cat /user/hadoop/graphout4/part-r-00000
   12,3,40D
   21,41D
   31,5,61D
   41,21D
   53,62D
   63,52D
   76-1P
   

What just happened?

The output is as expected; since node 7 is not reachable by node 1 or any of its neighbors, it will remain Pending and never be processed further. Consequently, our graph is unchanged as shown in the following figure:

The one thing we did not build into our algorithm was an understanding of a terminating condition; the process is complete if a run does not create any new D or C nodes.

The mechanism we use here is manual, that is, we knew by examination that the graph representation had reached its final stable state. There are ways of doing this programmatically, however. In a later chapter, we will discuss custom job counters...