We use oplog collection(that is present in local database) in our code to check if an event happens on a specific db and fire an event in our code.But,as aws document db doesn't support oplog as mentioned in "https://docs.aws.amazon.com/documentdb/latest/developerguide/developerguide.pdf",how can we do that?Is there an alternative approach?
You shouldn't use oplog even with your local mongodb. This is a deprecated approach. The modern way is to use Change Streams, which is supported by Amazon DocumentDB: https://docs.aws.amazon.com/documentdb/latest/developerguide/change_streams.html