Deterministic order when cron schedules line up

[tardisx]

Hello,

I'm implementing a system where people supply multiple scripts to run, with a cron specification for scheduling.

To ensure the scripts can operate on some shared data, I would like to specify an ordering for job tasks to execute in, sequentially. For example, given:

*   * * * * * task_one
*/2 * * * * * task_two
*/3 * * * * * task_three

Every 3rd second, all three tasks will run, but task_one must run (to completion) before task_two, which must finish before task_three.

Using gocron.NewScheduler(gocron.WithLimitConcurrentJobs(1, gocron.LimitModeWait) seems to ensure that the tasks will not run simultaneously, but testing indicates the ordering is not deterministic:

{"time":"2025-03-13T16:55:06.001191+10:30","level":"INFO","msg":"executing one"}
{"time":"2025-03-13T16:55:06.102424+10:30","level":"INFO","msg":"executing two"}
{"time":"2025-03-13T16:55:06.203622+10:30","level":"INFO","msg":"executing three"}

{"time":"2025-03-13T16:55:12.001243+10:30","level":"INFO","msg":"executing three"}
{"time":"2025-03-13T16:55:12.102427+10:30","level":"INFO","msg":"executing two"}
{"time":"2025-03-13T16:55:12.203608+10:30","level":"INFO","msg":"executing one"}

(each of these test tasks has a 100ms sleep to test this).

Is there any way I can get the execution ordering at a particular point in time to be deterministic?

[JohnRoesler]

There is no guaranteed way within gocron to get deterministic order of the jobs. The closest way I can think of with the current capabilities would be to use a duration job and offset the starts using the WithStartAt option -> https://pkg.go.dev/github.com/go-co-op/gocron/v2#example-WithStartAt

For example:

package main

import (
	"log"
	"time"

	"github.com/go-co-op/gocron/v2"
)

func main() {

	s, err := gocron.NewScheduler()
	if err != nil {
		log.Fatal(err)
	}

	now := time.Now()

	_, err = s.NewJob(
		gocron.DurationJob(1*time.Second),
		gocron.NewTask(func() {
			log.Println("job 1 ran")
		}),
		gocron.WithStartAt(
			gocron.WithStartDateTime(
				now.Add(100*time.Millisecond),
			),
		),
	)

	_, err = s.NewJob(
		gocron.DurationJob(2*time.Second),
		gocron.NewTask(func() {
			log.Println("job 2 ran")
		}),
		gocron.WithStartAt(
			gocron.WithStartDateTime(
				now.Add(2*time.Second+200*time.Millisecond),
			),
		),
	)

	_, err = s.NewJob(
		gocron.DurationJob(3*time.Second),
		gocron.NewTask(func() {
			log.Println("job 3 ran")
		}),
		gocron.WithStartAt(
			gocron.WithStartDateTime(
				now.Add(3*time.Second+300*time.Millisecond),
			),
		),
	)

	if err != nil {
		log.Fatal(err)
	}

	log.Println("starting")
	s.Start()

	select {}
}

Output:

2025/03/13 11:39:50 starting
2025/03/13 11:39:50 job 1 ran

2025/03/13 11:39:51 job 1 ran

2025/03/13 11:39:52 job 1 ran
2025/03/13 11:39:52 job 2 ran

2025/03/13 11:39:53 job 1 ran
2025/03/13 11:39:53 job 3 ran

2025/03/13 11:39:54 job 1 ran
2025/03/13 11:39:54 job 2 ran

2025/03/13 11:39:55 job 1 ran

2025/03/13 11:39:56 job 1 ran
2025/03/13 11:39:56 job 2 ran
2025/03/13 11:39:56 job 3 ran