Theory of operation
Tuesday, February 23, 2010
8:26 PM
Theory of operation of MapReduce
Three kinds of nodes
Trackers: initiate a MapReduce, gather results. 
Mappers: perform the map part of an operation, contain data. 
Reducers: perform the reduce part of an operation, don't contain data. 
How a MapReduce is implemented: 
Select a Tracker by flowless switching; send it the query. 
Tracker contacts its mappers (mechanisms differ; google uses UDP broadcast; hadoop uses tree propogation). 
Answer flows back from mappers to reducers to tracker to client, in a tree ("funnel") shape. 