Overview¶

Package cron implements a cron spec parser and job runner.

Installation¶

To download the specific tagged release, run:

go get github.com/robfig/cron/v3@v3.0.0

Import it in your program as:

import "github.com/robfig/cron/v3"

It requires Go 1.11 or later due to usage of Go Modules.

Usage¶

Callers may register Funcs to be invoked on a given schedule. Cron will runthem in their own goroutines.

c := cron.New()c.AddFunc("30 * * * *", func() { fmt.Println("Every hour on the half hour") })c.AddFunc("30 3-6,20-23 * * *", func() { fmt.Println(".. in the range 3-6am, 8-11pm") })c.AddFunc("CRON_TZ=Asia/Tokyo 30 04 * * *", func() { fmt.Println("Runs at 04:30 Tokyo time every day") })c.AddFunc("@hourly",      func() { fmt.Println("Every hour, starting an hour from now") })c.AddFunc("@every 1h30m", func() { fmt.Println("Every hour thirty, starting an hour thirty from now") })c.Start()..// Funcs are invoked in their own goroutine, asynchronously....// Funcs may also be added to a running Cronc.AddFunc("@daily", func() { fmt.Println("Every day") })..// Inspect the cron job entries' next and previous run times.inspect(c.Entries())..c.Stop()  // Stop the scheduler (does not stop any jobs already running).

CRON Expression Format¶

A cron expression represents a set of times, using 5 space-separated fields.

Field name   | Mandatory? | Allowed values  | Allowed special characters----------   | ---------- | --------------  | --------------------------Minutes      | Yes        | 0-59            | * / , -Hours        | Yes        | 0-23            | * / , -Day of month | Yes        | 1-31            | * / , - ?Month        | Yes        | 1-12 or JAN-DEC | * / , -Day of week  | Yes        | 0-6 or SUN-SAT  | * / , - ?

Month and Day-of-week field values are case insensitive. "SUN", "Sun", and"sun" are equally accepted.

The specific interpretation of the format is based on the Cron Wikipedia page:https://en.wikipedia.org/wiki/Cron

Alternative Formats¶

Alternative Cron expression formats support other fields like seconds. You canimplement that by creating a custom Parser as follows.

cron.New(cron.WithParser(cron.NewParser(cron.SecondOptional | cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor)))

Since adding Seconds is the most common modification to the standard cron spec,cron provides a builtin function to do that, which is equivalent to the customparser you saw earlier, except that its seconds field is REQUIRED:

cron.New(cron.WithSeconds())

That emulates Quartz, the most popular alternative Cron schedule format:http://www.quartz-scheduler.org/documentation/quartz-2.x/tutorials/crontrigger.html

Special Characters¶

Asterisk ( * )

The asterisk indicates that the cron expression will match for all values of thefield; e.g., using an asterisk in the 5th field (month) would indicate everymonth.

Slash ( / )

Slashes are used to describe increments of ranges. For example 3-59/15 in the1st field (minutes) would indicate the 3rd minute of the hour and every 15minutes thereafter. The form "*\/..." is equivalent to the form "first-last/...",that is, an increment over the largest possible range of the field. The form"N/..." is accepted as meaning "N-MAX/...", that is, starting at N, use theincrement until the end of that specific range. It does not wrap around.

Comma ( , )

Commas are used to separate items of a list. For example, using "MON,WED,FRI" inthe 5th field (day of week) would mean Mondays, Wednesdays and Fridays.

Hyphen ( - )

Hyphens are used to define ranges. For example, 9-17 would indicate everyhour between 9am and 5pm inclusive.

Question mark ( ? )

Question mark may be used instead of '*' for leaving either day-of-month orday-of-week blank.

Predefined schedules¶

You may use one of several pre-defined schedules in place of a cron expression.

Entry                  | Description                                | Equivalent To-----                  | -----------                                | -------------@yearly (or @annually) | Run once a year, midnight, Jan. 1st        | 0 0 1 1 *@monthly               | Run once a month, midnight, first of month | 0 0 1 * *@weekly                | Run once a week, midnight between Sat/Sun  | 0 0 * * 0@daily (or @midnight)  | Run once a day, midnight                   | 0 0 * * *@hourly                | Run once an hour, beginning of hour        | 0 * * * *

Intervals¶

You may also schedule a job to execute at fixed intervals, starting at the time it's addedor cron is run. This is supported by formatting the cron spec like this:

@every <duration>

where "duration" is a string accepted by time.ParseDuration(http://golang.org/pkg/time/#ParseDuration).

For example, "@every 1h30m10s" would indicate a schedule that activates after1 hour, 30 minutes, 10 seconds, and then every interval after that.

Note: The interval does not take the job runtime into account. For example,if a job takes 3 minutes to run, and it is scheduled to run every 5 minutes,it will have only 2 minutes of idle time between each run.

Time zones¶

By default, all interpretation and scheduling is done in the machine's localtime zone (time.Local). You can specify a different time zone on construction:

cron.New(    cron.WithLocation(time.UTC))

Individual cron schedules may also override the time zone they are to beinterpreted in by providing an additional space-separated field at the beginningof the cron spec, of the form "CRON_TZ=Asia/Tokyo".

For example:

# Runs at 6am in time.Localcron.New().AddFunc("0 6 * * ?", ...)# Runs at 6am in America/New_Yorknyc, _ := time.LoadLocation("America/New_York")c := cron.New(cron.WithLocation(nyc))c.AddFunc("0 6 * * ?", ...)# Runs at 6am in Asia/Tokyocron.New().AddFunc("CRON_TZ=Asia/Tokyo 0 6 * * ?", ...)# Runs at 6am in Asia/Tokyoc := cron.New(cron.WithLocation(nyc))c.SetLocation("America/New_York")c.AddFunc("CRON_TZ=Asia/Tokyo 0 6 * * ?", ...)

The prefix "TZ=(TIME ZONE)" is also supported for legacy compatibility.

Be aware that jobs scheduled during daylight-savings leap-ahead transitions willnot be run!

Job Wrappers¶

A Cron runner may be configured with a chain of job wrappers to addcross-cutting functionality to all submitted jobs. For example, they may be usedto achieve the following effects:

• Recover any panics from jobs (activated by default)
• Delay a job's execution if the previous run hasn't completed yet
• Skip a job's execution if the previous run hasn't completed yet
• Log each job's invocations

Install wrappers for all jobs added to a cron using the `cron.WithChain` option:

cron.New(cron.WithChain(cron.SkipIfStillRunning(logger),))

Install wrappers for individual jobs by explicitly wrapping them:

job = cron.NewChain(cron.SkipIfStillRunning(logger),).Then(job)

Thread safety¶

Since the Cron service runs concurrently with the calling code, some amount ofcare must be taken to ensure proper synchronization.

All cron methods are designed to be correctly synchronized as long as the callerensures that invocations have a clear happens-before ordering between them.

Logging¶

Cron defines a Logger interface that is a subset of the one defined ingithub.com/go-logr/logr. It has two logging levels (Info and Error), andparameters are key/value pairs. This makes it possible for cron logging to pluginto structured logging systems. An adapter, [Verbose]PrintfLogger, is providedto wrap the standard library *log.Logger.

For additional insight into Cron operations, verbose logging may be activatedwhich will record job runs, scheduling decisions, and added or removed jobs.Activate it with a one-off logger as follows:

cron.New(cron.WithLogger(cron.VerbosePrintfLogger(log.New(os.Stdout, "cron: ", log.LstdFlags))))

Implementation¶

Cron entries are stored in an array, sorted by their next activation time. Cronsleeps until the next job is due to be run.

Upon waking:

• it runs each entry that is active on that second
• it calculates the next run times for the jobs that were run
• it re-sorts the array of entries by next activation time.
• it goes to sleep until the soonest job.

Index¶

• type Chain
• • func NewChain(c ...JobWrapper) Chain
• • func (c Chain) Then(j Job) Job
• type ConstantDelaySchedule
• • func Every(duration time.Duration) ConstantDelaySchedule
• • func (schedule ConstantDelaySchedule) Next(t time.Time) time.Time
• type Cron
• • func New(opts ...Option) *Cron
• • func (c *Cron) AddFunc(spec string, cmd func()) (EntryID, error)
  • func (c *Cron) AddJob(spec string, cmd Job) (EntryID, error)
  • func (c *Cron) Entries() []Entry
  • func (c *Cron) Entry(id EntryID) Entry
  • func (c *Cron) Location() *time.Location
  • func (c *Cron) Remove(id EntryID)
  • func (c *Cron) Run()
  • func (c *Cron) Schedule(schedule Schedule, cmd Job) EntryID
  • func (c *Cron) Start()
  • func (c *Cron) Stop() context.Context
• type Entry
• • func (e Entry) Valid() bool
• type EntryID
• type FuncJob
• • func (f FuncJob) Run()
• type Job
• type JobWrapper
• • func DelayIfStillRunning(logger Logger) JobWrapper
  • func Recover(logger Logger) JobWrapper
  • func SkipIfStillRunning(logger Logger) JobWrapper
• type Logger
• • func PrintfLogger(l interface{ ... }) Logger
  • func VerbosePrintfLogger(l interface{ ... }) Logger
• type Option
• • func WithChain(wrappers ...JobWrapper) Option
  • func WithLocation(loc *time.Location) Option
  • func WithLogger(logger Logger) Option
  • func WithParser(p ScheduleParser) Option
  • func WithSeconds() Option
• type ParseOption
• type Parser
• • func NewParser(options ParseOption) Parser
• • func (p Parser) Parse(spec string) (Schedule, error)
• type Schedule
• • func ParseStandard(standardSpec string) (Schedule, error)
• type ScheduleParser
• type SpecSchedule
• • func (s *SpecSchedule) Next(t time.Time) time.Time

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