As the database is updated, changes are recorded in the oplog maintained by the primary server in the replica set, which is then used to replicate changes to the secondaries. Trying to read a list of changes via the oplog is a tedious and resource-intensive process, so many developers choose to use change streams (https://docs.mongodb.com/manual/changeStreams/?jmp=blog&_ga=2.5574835.1698487790.1546401611-137143613.1528093145#change-streams) to subscribe to all changes on a collection. For those of you who are familiar with software design patterns, this is a form of the publish/subscribe pattern.
Aside from their obvious use in troubleshooting and diagnostics, changing streams can also be used to give an indicator of whether or not data changes are durable.
What is new and different in MongoDB 4.x is the introduction of a startAtOperationTime parameter that allows you to specify the timestamp at which you wish to tap into the change stream. This timestamp...
