vshn/k8up

package schedulecontroller

import (
    "crypto/sha1"
    "encoding/hex"
    "fmt"
    "math/big"

    k8upv1 "github.com/k8up-io/k8up/v2/api/v1"
)

const (
    ScheduleHourlyRandom   = "@hourly-random"
    ScheduleDailyRandom    = "@daily-random"
    ScheduleYearlyRandom   = "@yearly-random"
    ScheduleAnnuallyRandom = "@annually-random"
    ScheduleMonthlyRandom  = "@monthly-random"
    ScheduleWeeklyRandom   = "@weekly-random"
)

func createSeed(schedule *k8upv1.Schedule, jobType k8upv1.JobType) string {
    return schedule.Namespace + "/" + schedule.Name + "@" + jobType.String()
}

// randomizeSchedule randomizes the given originalSchedule with a seed. The originalSchedule has to be one of the supported
// '@x-random' predefined schedules, otherwise it returns an error with the original schedule unmodified.
func randomizeSchedule(seed string, originalSchedule k8upv1.ScheduleDefinition) (k8upv1.ScheduleDefinition, error) {
    checksum := calculateChecksumFromSeed(seed)

    minute := remainderFromModulo(checksum, 60, 0)
    hour := remainderFromModulo(checksum, 24, 0)
    // A month can have between 27 and 31 days. Cron does not automatically schedule at the end of the month if 31 in February.
    // To not cause a skip of a scheduled backup that could potentially raise alerts, we cap the day-of-month to 27 so it fits in all months.
    dayOfMonth := remainderFromModulo(checksum, 27, 1)

    switch originalSchedule {
    case ScheduleHourlyRandom:
        return k8upv1.ScheduleDefinition(fmt.Sprintf("%d * * * *", minute)), nil
    case ScheduleDailyRandom:
        return k8upv1.ScheduleDefinition(fmt.Sprintf("%d %d * * *", minute, hour)), nil
    case ScheduleMonthlyRandom:
        return k8upv1.ScheduleDefinition(fmt.Sprintf("%d %d %d * *", minute, hour, dayOfMonth)), nil
    case ScheduleAnnuallyRandom:
    case ScheduleYearlyRandom:
        month := remainderFromModulo(checksum, 12, 1)
        return k8upv1.ScheduleDefinition(fmt.Sprintf("%d %d %d %d *", minute, hour, dayOfMonth, month)), nil
    case ScheduleWeeklyRandom:
        weekday := remainderFromModulo(checksum, 6, 0)
        return k8upv1.ScheduleDefinition(fmt.Sprintf("%d %d * * %d", minute, hour, weekday)), nil
    default:
        return originalSchedule, fmt.Errorf("unrecognized random schedule: '%s'", originalSchedule)
    }
    return originalSchedule, nil
}

// calculateChecksumFromSeed calculates a SHA1 hexadecimal checksum from the given seed.
func calculateChecksumFromSeed(seed string) *big.Int {
    hash := sha1.New()
    hash.Write([]byte(seed))
    sum := hex.EncodeToString(hash.Sum(nil))
    sumBase16, _ := new(big.Int).SetString(sum, 16)
    return sumBase16
}

// remainderFromModulo calculates the remainder from (dividend % divisor), then adds the offset.
func remainderFromModulo(dividend *big.Int, divisor, offset int64) *big.Int {
    return new(big.Int).Add(big.NewInt(offset), new(big.Int).Mod(dividend, big.NewInt(divisor)))
}