A trigger in a relational database is a code that gets invoked when an insert, update, or a delete operation is executed on a table in the database. A trigger can be invoked either before or after the operation. Triggers are not implemented in MongoDB out of the box, and in case you need some sort of notification for your application whenever any insert, update, and delete operations are executed, you are left to manage them by yourself in the application. One approach is to have some sort of data access layer in the application that is the only place to query, insert, update, or delete documents from the collections. However, there are a few challenges to this. First, you need to explicitly code the logic to accommodate this requirement in the application, which may or may not be feasible. If the database is shared and multiple applications access it, things become even more difficult. Second, the access needs to be strictly regulated and no...