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package cron
import (
// Cron keeps track of any number of entries, invoking the associated func as
// specified by the schedule. It may be started, stopped, and the entries may
// be inspected while running.
type Cron struct {
entries []*Entry
stop chan struct{}
add chan *Entry
snapshot chan []*Entry
running bool
ErrorLog *log.Logger
location *time.Location
// Job is an interface for submitted cron jobs.
type Job interface {
// The Schedule describes a job's duty cycle.
type Schedule interface {
// Return the next activation time, later than the given time.
// Next is invoked initially, and then each time the job is run.
Next(time.Time) time.Time
// Entry consists of a schedule and the func to execute on that schedule.
type Entry struct {
// The schedule on which this job should be run.
Schedule Schedule
// The next time the job will run. This is the zero time if Cron has not been
// started or this entry's schedule is unsatisfiable
Next time.Time
// The last time this job was run. This is the zero time if the job has never
// been run.
Prev time.Time
// The Job to run.
Job Job
// byTime is a wrapper for sorting the entry array by time
// (with zero time at the end).
type byTime []*Entry
func (s byTime) Len() int { return len(s) }
func (s byTime) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
func (s byTime) Less(i, j int) bool {
// Two zero times should return false.
// Otherwise, zero is "greater" than any other time.
// (To sort it at the end of the list.)
if s[i].Next.IsZero() {
return false
if s[j].Next.IsZero() {
return true
return s[i].Next.Before(s[j].Next)
// New returns a new Cron job runner, in the Local time zone.
func New() *Cron {
return NewWithLocation(time.Now().Location())
// NewWithLocation returns a new Cron job runner.
func NewWithLocation(location *time.Location) *Cron {
return &Cron{
entries: nil,
add: make(chan *Entry),
stop: make(chan struct{}),
snapshot: make(chan []*Entry),
running: false,
ErrorLog: nil,
location: location,
// A wrapper that turns a func() into a cron.Job
type FuncJob func()
func (f FuncJob) Run() { f() }
// AddFunc adds a func to the Cron to be run on the given schedule.
func (c *Cron) AddFunc(spec string, cmd func()) error {
return c.AddJob(spec, FuncJob(cmd))
// AddJob adds a Job to the Cron to be run on the given schedule.
func (c *Cron) AddJob(spec string, cmd Job) error {
schedule, err := Parse(spec)
if err != nil {
return err
c.Schedule(schedule, cmd)
return nil
// Schedule adds a Job to the Cron to be run on the given schedule.
func (c *Cron) Schedule(schedule Schedule, cmd Job) {
entry := &Entry{
Schedule: schedule,
Job: cmd,
if !c.running {
c.entries = append(c.entries, entry)
c.add <- entry
// Entries returns a snapshot of the cron entries.
func (c *Cron) Entries() []*Entry {
if c.running {
c.snapshot <- nil
x := <-c.snapshot
return x
return c.entrySnapshot()
// Location gets the time zone location
func (c *Cron) Location() *time.Location {
return c.location
// Start the cron scheduler in its own go-routine, or no-op if already started.
func (c *Cron) Start() {
if c.running {
c.running = true
// Run the cron scheduler, or no-op if already running.
func (c *Cron) Run() {
if c.running {
c.running = true
func (c *Cron) runWithRecovery(j Job) {
defer func() {
if r := recover(); r != nil {
const size = 64 << 10
buf := make([]byte, size)
buf = buf[:runtime.Stack(buf, false)]
c.logf("cron: panic running job: %v\n%s", r, buf)
// Run the scheduler. this is private just due to the need to synchronize
// access to the 'running' state variable.
func (c *Cron) run() {
// Figure out the next activation times for each entry.
now :=
for _, entry := range c.entries {
entry.Next = entry.Schedule.Next(now)
for {
// Determine the next entry to run.
var timer *time.Timer
if len(c.entries) == 0 || c.entries[0].Next.IsZero() {
// If there are no entries yet, just sleep - it still handles new entries
// and stop requests.
timer = time.NewTimer(100000 * time.Hour)
} else {
timer = time.NewTimer(c.entries[0].Next.Sub(now))
for {
select {
case now = <-timer.C:
now = now.In(c.location)
// Run every entry whose next time was less than now
for _, e := range c.entries {
if e.Next.After(now) || e.Next.IsZero() {
go c.runWithRecovery(e.Job)
e.Prev = e.Next
e.Next = e.Schedule.Next(now)
case newEntry := <-c.add:
now =
newEntry.Next = newEntry.Schedule.Next(now)
c.entries = append(c.entries, newEntry)
case <-c.snapshot:
c.snapshot <- c.entrySnapshot()
case <-c.stop:
// Logs an error to stderr or to the configured error log
func (c *Cron) logf(format string, args ...interface{}) {
if c.ErrorLog != nil {
c.ErrorLog.Printf(format, args...)
} else {
log.Printf(format, args...)
// Stop stops the cron scheduler if it is running; otherwise it does nothing.
func (c *Cron) Stop() {
if !c.running {
c.stop <- struct{}{}
c.running = false
// entrySnapshot returns a copy of the current cron entry list.
func (c *Cron) entrySnapshot() []*Entry {
entries := []*Entry{}
for _, e := range c.entries {
entries = append(entries, &Entry{
Schedule: e.Schedule,
Next: e.Next,
Prev: e.Prev,
Job: e.Job,
return entries
// now returns current time in c location
func (c *Cron) now() time.Time {
return time.Now().In(c.location)