You might be wondering whether you still need backups if you have replication. The answer is yes, because they have different purposes: replicas quickly reflect writes from one node on other nodes, but backups store old snapshots of the data so that you can go back in time. If you accidentally delete some data, replication doesn’t help since the deletion will also have been propagated to the replicas; you need a backup if you want to restore the deleted data.
In fact, replication and backups are often complementary. Backups are sometimes part of the process of setting up replication, as we shall see in “Setting Up New Followers”. Conversely, archiving replication logs can be part of a backup process.
Some databases internally maintain immutable snapshots of past states, which serve as a kind of internal backup. However, this means keeping old versions of the data on the same storage medium as the current state. If you have a large amount of data, it can be cheaper to keep the backups of old data in an object store that is optimized for infrequently accessed data and to store only the current state of the database in primary storage.