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Diffstat (limited to 'modules/cron/doc.go')
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diff --git a/modules/cron/doc.go b/modules/cron/doc.go new file mode 100644 index 0000000000..dbdf50127a --- /dev/null +++ b/modules/cron/doc.go @@ -0,0 +1,129 @@ +/* +Package cron implements a cron spec parser and job runner. + +Usage + +Callers may register Funcs to be invoked on a given schedule. Cron will run +them in their own goroutines. + + c := cron.New() + c.AddFunc("0 30 * * * *", func() { fmt.Println("Every hour on the half hour") }) + c.AddFunc("@hourly", func() { fmt.Println("Every hour") }) + c.AddFunc("@every 1h30m", func() { fmt.Println("Every hour thirty") }) + 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") }) + .. + // 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 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 +(http://golang.org/pkg/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 (http://www.golang.org/pkg/time). + +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. +*/ +package cron |