Release dates can be global, continent-wide or country-wide.
Each of these is a node. We represent the relations as a directed graph.
Global
|
|----------------|----------------...
v v
Asia Europe
| |
|-----|-----... |-----|-----...
v v v v
India China Britain France
Each node contains either a release date if available or null, the 'weight' of
the node with global being the highest and a set of all its child node ids.
The mapper emits a key value for each of that nodes children with the key being
the id of the child and the value being the parent's value (release date).
The reducer takes all the messages for a particular node and rewrites the
node's release date with the max of these after filtering out the nulls,
sorting by weight.
The process is repeated until there are no changes.