Sean Gallardy checks out the past:
Around 2005, mirroring was born. It was an evolution on log shipping, which is taking log backups, moving them around, and restoring them all in an automated fashion to different servers. Mirroring upped that game and created a dedicated network channel between servers (you could only have 1 principle and 1 mirror, so 2 total) so that there wasn’t this funny business of copying and restoring, additionally it allowed the mirror server to be a highly available copy with automatic failover. Since Microsoft marketing is terrible at naming things, it was originally called, “Real Time Log Shipping” which was then changed to “Mirroring” and in typical fashion you can find the unofficial “Real Time Log Shipping” name all over the place where it was never updated. (I can’t really blame them here, though, it’s hard to find all the little places you’re putting this moniker in and then having some other team tell you to change it all at some way later point)
Read the whole thing. It’s a fun read, a little sad, and helps us understand a bit of availability group behavior which might bite the unaware. I will definitely defend Microsoft’s backward-compatibility emphasis. This makes life so much easier for developers than a lot of other languages and environments. In the R and Python worlds, breaking changes are the norm, meaning that when you update packages, you can expect something to break and now that “20-minute” package upgrade ticket becomes 3 days of trying to sort out what went wrong.