cron

A cron library for Go. See the godoc.

Usage

Callers may register Funcs to be invoked on a given schedule. Cron will run them in their own goroutines. A name must be provided.

c := cron.New()
c.AddFunc("0 5 * * * *",  func() { fmt.Println("Every 5 minutes") }, "Often")
c.AddFunc("@hourly",      func() { fmt.Println("Every hour") }, "Frequent")
c.AddFunc("@every 1h30m", func() { fmt.Println("Every hour thirty") }, "Less Frequent")
c.Start()
..
// Funcs are invoked in their own goroutine, asynchronously.
...
// Funcs may also be added to a running Cron
c.AddFunc("@daily", func() { fmt.Println("Every day") }, "My Job")
..
// Inspect the cron job entries' next and previous run times.
inspect(c.Entries())
..
// Remove an entry from the cron by name.
c.RemoveJob("My Job")
..
c.Stop()  // Stop the scheduler (does not stop any jobs already running).

CRON Expression

This section describes the specific format accepted by this cron. Some snippets are taken from the wikipedia article.

Format

A cron expression represents a set of times, using 6 space-separated fields.

Field name	Mandatory?	Allowed values	Allowed special characters
Seconds	Yes	0-59	* / , -
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	* / , - ?

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

Special Characters

Asterisk ( * )

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

Slash ( / )

Slashes are used to describe increments of ranges. For example 3-59/15 in the 1st field (minutes) would indicate the 3rd minute of the hour and every 15 minutes 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 the increment 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" in the 5th field (day of week) would mean Mondays, Wednesdays and Fridays.

Hyphen ( - )

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

Question mark ( ? )

Question mark may be used instead of '*' for leaving either day-of-month or day-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 0 1 1 *`
@monthly	Run once a month, midnight, first of month	`0 0 0 1 * *`
@weekly	Run once a week, midnight on Sunday	`0 0 0 * * 0`
@daily (or @midnight)	Run once a day, midnight	`0 0 0 * * *`
@hourly	Run once an hour, beginning of hour	`0 0 * * * *`

Intervals

You may also schedule a job to execute at fixed intervals. This is supported by formatting the cron spec like this:

@every <duration>

where <duration> is a string accepted by time.ParseDuration.

For example, @every 1h30m10s would indicate a schedule that activates every 1 hour, 30 minutes, 10 seconds.

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

All interpretation and scheduling is done in the machine's local time zone (as provided by the Go time package).

Be aware that jobs scheduled during daylight-savings leap-ahead transitions will not be run!

Thread safety

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

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

Implementation

Cron entries are stored in an array, sorted by their next activation time. Cron sleeps 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.

dancebear / cron