At time of this writing, when source records are produced to their respective topic, the message key is given by default in the format

{  "_id": { <bson resume token> }} 

Instead, it would be useful if there was an option to configure the message key so that, for example, you could choose a different field from the change-stream document to be converted to a source record.

One particularly useful case would be to choose the "_id" of the actual collection document, if present in the "fullDocument" "documentKey" field. This would allow the corresponding kafka topic to be partitioned on that collection's "_id" field.