There are cases when we have had to deprecate configuration keys. Use cases include, changing the names of variables to better match intent, splitting a single parameter into two - for maps, reduces etc.
In such cases, we typically provide a backwards compatible option for the old keys. The handling of such cases might typically be common enough to actually add support for it in a generic fashion in the Configuration class. Some initial discussion around this started in
HADOOP-5919, but since the project split happened in between we decided to open this issue to fix it in common.