Understanding limits of AppEngine MapReduce
Tuesday, February 22, 2011
11:46 AM
You might have noticed that there are severe limits on what you can do with MapReduce directly inside AppEngine: 
No choice on reducer (combine and eliminate duplicates). 
Very limited map capabilities. 
Why? 
If you write a mapper, it has to be propogated to each map node.
This is moving code, not data. 
This is relatively expensive and somewhat dangerous. 
Otherwise, you cannot inject code into the cloud.  
This has security and other implications. 