Why do we need custom reduce functions? Sometimes, the built-in reduce function doesn't meet our requirements, although it will suffice most of the time.

Custom reduce functions allow you to create your own reduce function. In such a reduce function, output of map function goes to the corresponding reduce function group as per the key of the map output and the group level parameter. Couchbase ensures that output from the map will be grouped by key and supplied to reduce. Then it's developer's role to define logic in reduce, what to perform on the data such as aggregating, addition etc.

To handle the incremental MapReduce functionality (that is, updating an existing view), each function must also be able to handle and consume it's own output. In an incremental situation, the function must handle both new records and previously computed reductions.

The input to reduce function can be not only raw data from the map phase but also output of a previous reduce phase. This...