Looking through the supported APIs (https://docs.aws.amazon.com/documentdb/latest/developerguide...), it appears DocumentDB has no support for Mongo's oplog (https://docs.mongodb.com/manual/core/replica-set-oplog/), or change streams (https://docs.mongodb.com/manual/changeStreams), which I guess is no surprise because change streams were introduced in Mongo 4, whereas DocumentDB copied the 3.6 API. So DocumentDB seems much less useful as a reactive data store than MongoDB.

In other words, DocumentDB is only a drop-in replacement for MongoDB if you weren't using any of the features Amazon decided not to support.

Happy to be corrected if I'm misreading the documentation!