I ran into some issues while using M/R. The gist of the issue I was seeing is that:
Start a cluster of Embedded Caches, like 4 nodes
Put in 100 elements
Run a simple M/R job to count the number of keys
If I run the M/R job using the node I'm inserting elements into as coordinator - the result is 100
But if I run the M/R job using a different node as coordinator, the result is less than 100
More interestingly, I can pause for 5 seconds and run the M/R jobs again, the results are always less than 100

This behavior doens't occur if I explicitly run cacheManager.getCache() for each of the nodes...