Rotation

The rules

RotationPolicy(last, daily, weekly, monthly) keeps:

A set kept by any rule is kept; the others have expired. This is the grandfather-father-son scheme: with last: 3, daily: 7, weekly: 4, monthly: 12 and a backup every night, the directory holds about two dozen sets reaching a year back.

Each count is zero or more, and one at least is above zero; a rotation that would keep nothing is refused with InvalidBackupValue. The most recent set is always kept, since every rule keeps at least one.

Periods

Days, weeks and months are those of UTC, the time of the file names, so a server's change of time zone or of summer time moves no backup from one day to another. Weeks are ISO weeks, Monday to Sunday: a Sunday backup belongs to the week of the Monday before it. A period without a backup is not counted: after a week of outage, daily: 7 still keeps seven sets.

What is rotated

expired($sets) gives the expired sets, oldest first; Backups::prune() removes them and gives them back. Only sets without a label are rotated: the copy taken before a restore, an anonymized copy or one taken before a migration stays until it is removed by hand, or by Backups::keepLatest() for one label. Files of the directory that are not sets are never touched.