Golang: Implementing a cron / executing tasks at a specific time

I have been looking around for examples on how to implement a function that allows you to execute tasks at a certain time in Go, but I couldn't find anything.

I implemented one myself and I am sharing it in the answers, so other people can have a reference for their own implementation.

This is a general implementation, which lets you set:

• interval period
• hour to tick
• minute to tick
• second to tick

UPDATED: (the memory leak was fixed)

import (
"fmt"
"time"
)

const INTERVAL_PERIOD time.Duration = 24 * time.Hour

const HOUR_TO_TICK int = 23
const MINUTE_TO_TICK int = 00
const SECOND_TO_TICK int = 03

type jobTicker struct {
    timer *time.Timer
}

func runningRoutine() {
    jobTicker := &jobTicker{}
    jobTicker.updateTimer()
    for {
        <-jobTicker.timer.C
        fmt.Println(time.Now(), "- just ticked")
        jobTicker.updateTimer()
    }
}

func (t *jobTicker) updateTimer() {
    nextTick := time.Date(time.Now().Year(), time.Now().Month(), 
    time.Now().Day(), HOUR_TO_TICK, MINUTE_TO_TICK, SECOND_TO_TICK, 0, time.Local)
    if !nextTick.After(time.Now()) {
	    nextTick = nextTick.Add(INTERVAL_PERIOD)
    }
    fmt.Println(nextTick, "- next tick")
    diff := nextTick.Sub(time.Now())
    if t.timer == nil {
	    t.timer = time.NewTimer(diff)
    } else {
	    t.timer.Reset(diff)
    }
}

In case someone drops in on this question searching for a quick solution.
I found a neat library that makes it really easy to schedule jobs.

Link: https://github.com/jasonlvhit/gocron

The API is pretty straightforward:

import (
    "fmt"
    "github.com/jasonlvhit/gocron"
)

func task() {
    fmt.Println("Task is being performed.")
}

func main() {
    s := gocron.NewScheduler()
    s.Every(2).Hours().Do(task)
    <- s.Start()
}

the answer provided by @Daniele B is not good enough, as @Caleb says, that implementation leaks memory, because each time we create a new ticker, the old one will never be released.

so I wrap the time.timer, and reset it everytime, a example here:

package main

import (
	"fmt"
	"time"
)

const INTERVAL_PERIOD time.Duration = 24 * time.Hour

const HOUR_TO_TICK int = 23
const MINUTE_TO_TICK int = 21
const SECOND_TO_TICK int = 03

type jobTicker struct {
	t *time.Timer
}

func getNextTickDuration() time.Duration {
	now := time.Now()
	nextTick := time.Date(now.Year(), now.Month(), now.Day(), HOUR_TO_TICK, MINUTE_TO_TICK, SECOND_TO_TICK, 0, time.Local)
	if nextTick.Before(now) {
		nextTick = nextTick.Add(INTERVAL_PERIOD)
	}
	return nextTick.Sub(time.Now())
}

func NewJobTicker() jobTicker {
	fmt.Println("new tick here")
	return jobTicker{time.NewTimer(getNextTickDuration())}
}

func (jt jobTicker) updateJobTicker() {
	fmt.Println("next tick here")
	jt.t.Reset(getNextTickDuration())
}

func main() {
	jt := NewJobTicker()
	for {
		<-jt.t.C
		fmt.Println(time.Now(), "- just ticked")
		jt.updateJobTicker()
	}
}

I have created a package that actually supports crontab syntax if you are familiar with it, for example:

ctab := crontab.New()
ctab.AddJob("*/5 * * * *", myFunc)
ctab.AddJob("0 0 * * *", myFunc2)

Package link: https://github.com/mileusna/crontab

This is another general implementation without need for a third party library.

Disclaimer: This implementation works with UTC. For managing timezones it has to be modified.

Run a func once a day at noon.

• Period: time.Hour * 24
• Offset: time.Hour * 12

Run a func twice a day at 03:40 (00:00 + 03:40) and 15:40 (12:00 + 03:40).

• Period: time.Hour * 12
• Offset: time.Hour * 3 + time.Minute * 40

Updated (2020-01-28):

Changes:

• context.Context can be used for cancellation, makes it testable.
• time.Ticker removes the need for calculating the time of the next execution.

package main

import (
	"context"
	"time"
)

// Schedule calls function `f` with a period `p` offsetted by `o`.
func Schedule(ctx context.Context, p time.Duration, o time.Duration, f func(time.Time)) {
	// Position the first execution
	first := time.Now().Truncate(p).Add(o)
	if first.Before(time.Now()) {
		first = first.Add(p)
	}
	firstC := time.After(first.Sub(time.Now()))

	// Receiving from a nil channel blocks forever
	t := &time.Ticker{C: nil}

	for {
		select {
		case v := <-firstC:
			// The ticker has to be started before f as it can take some time to finish
			t = time.NewTicker(p)
			f(v)
		case v := <-t.C:
			f(v)
		case <-ctx.Done():
			t.Stop()
			return
		}
	}

}

Original:

package main

import (
	"time"
)

// Repeat calls function `f` with a period `d` offsetted by `o`.
func Repeat(d time.Duration, o time.Duration, f func(time.Time)) {
	next := time.Now().Truncate(d).Add(o)
	if next.Before(time.Now()) {
		next = next.Add(d)
	}

	t := time.NewTimer(next.Sub(time.Now()))

	for {
		v := <-t.C
		next = next.Add(d)
		t.Reset(next.Sub(time.Now()))
		f(v)
	}
}

I'm using https://github.com/ehsaniara/gointerlock. It's also supported in distributed systems and has a builtin distributer lock (Redis)

import (
	"context"
    "fmt"
	"github.com/ehsaniara/gointerlock"
    "log"
	"time"
)

var job = gointerlock.GoInterval{
    Interval: 2 * time.Second,
    Arg:      myJob,
}

err := job.Run(ctx)
if err != nil {
    log.Fatalf("Error: %s", err)
}

func myJob() {
    fmt.Println(time.Now(), " - called")
}