This is a Map-Reduce implementation for LevelUp.
I really like the map-reduce pattern for querying keystores.
I'm a big fan of Riak even though the Map part of Map-Reduce is database key specific.
I wanted to build a Riak-like embedded database using LevelDB, but instead have a proper Map-Reduce.
My work in progress is DeputyDB. npm install deputydb
This is a simplified version of what is in DeputyDB. DeputyDB's MapReduce can take a pre-filtered list of keys, does everything atomically despite Node.js's event loop, and has an optional batch step at the end.
What makes this different from Riak's Map-Reduce is the ability to return any number of key/value pairs from the map step (0+). You could return key/values that have nothing to do with the key itself.
For example, it could return word-counts or any derived pairs. Or you could simple return a single [{dbkey: value}] for every key that matched a pattern. Maps could also convert formats. Reduce is then called to reduce together any duplicate keys emitted from mapping.
Map-Reduce is an incredibly flexible pattern.