A copy elsewhere

Think of it when data would be gone for good if someone deleted it today.

In everyday life

Like a printed photo album kept at your parents’ house. A second phone that syncs with yours deletes a photo when you do; the album still has it.

One case

A booking site copies its database to a second server every second, and the team calls that copy the backup. At 10:00 a bad script deletes all 3,000 bookings. One second later the second server deletes them too. No copy from before 10:00 exists anywhere.

The second server copied the delete one second later
The case, cell by cell1 cell = 1 minute
Bookings that should exist
300030003000300030003000
Main database
300030000000
Second server, the "backup"
300030003000000
Bookings left anywhere
0
Seconds the copy lagged
1
Each value is the count at the start of the minute. The delete reached the second server at 10:00:01.

Every case

A mirror protects you from a broken machine: one server dies, and the other has everything. It does not protect you from a wrong change, because it copies the change faithfully. Data is often lost to a wrong command, not a broken machine. Only a copy from the past, kept where the mistake cannot reach it, can undo that. And a backup that nobody has ever brought back is a hope: you learn it was broken on the day you need it.

The word

Engineers call this

backupsA separate copy of data from the past that you can bring back. A sync or a mirrorA second copy kept identical to the first, second by second. A change to one is copied to the other at once. is not a backup: it copies mistakes too, including deletes.

Keep copies from the past, somewhere a mistake cannot reach. RestorePut saved data back in place of the current data. one on a schedule to prove it works.

Where this idea leads