Comment A and B systems (Score 5, Interesting) 55
Back in the last quarter of the 20th century, many POTS telephone switches had an A side and a B side.
During software upgrades - and yes those were a thing - they would route all new calls to the A side and wait until the last call on the B side ended before they would take the B side down for the upgrade.
Then they would do some testing before bringing up the B side.
Only after they were confident that the B side was working would they upgrade the A side.
This worked in part because they could schedule upgrades when the load was low enough to be handled only by half of the switch's capacity.