crontab: cron-like scheduling🔗 i

Bogdan Popa <bogdan@defn.io>

(require crontab ) package: crontab-lib

This package provides functionality for scheduling work inside of Racket using cron-like syntax.

1Reference🔗 i

1.1Crontabs🔗 i

syntax
( crontab
[schedule-exprhandler-expr]...+)

schedule-expr : string?
handler-expr : (-> exact-integer? any )

Spawns a thread for every entry in the table, where each thread synchronizes on the schedule and executes its associated handler procedure when the schedule is ready for synchronization. Returns a procedure that stops all the threads when applied.

Handlers are executed synchronously within each thread, meaning procedures that take a long time to execute may cause runs to be skipped.

Examples:

> (define stop
(crontab
["* * * * * *"println ]))

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> (sleep 5)

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> (stop)

The spawned threads log information about what is currently being run to the “crontab” topic.

Changed in version 0.2 of package crontab-lib: Extended logging.

1.2Schedules🔗 i

Schedules constrain the components of a timestamp to match specific moments in time.

Schedules can be used as synchronizable events. A schedule is ready for synchronization when the value of (current-seconds ) is less than or equal to the value of (schedule-next s) as of the moment the event is synchronized on. The synchronization result of a schedule is the schedule itself and the timestamp (in seconds) it became ready for synchronization at.

Examples:

> (require racket/date)

> (define (~datets)
(parameterize ([date-display-format 'iso-8601])
(date->string (seconds->date ts)#t)))
>

> (define now
(find-seconds 3021101982022))
>
> (~datenow)

"2022-08-19T10:21:30"
> (~date(schedule-next (parse-schedule "* * * * * *")now))

"2022-08-19T10:21:30"
> (~date(schedule-next (parse-schedule "* * * * *")now))

"2022-08-19T10:22:00"
> (~date(schedule-next (parse-schedule "0 5-23/5 * * *")now))

"2022-08-19T15:00:00"
> (~date(schedule-next (parse-schedule "0 12 * * 5")now))

"2022-08-19T12:00:00"
> (~date(schedule-next (parse-schedule "* * 29 2 *")now))

"2024-02-29T00:00:00"
>

> (let ([s(parse-schedule "0 * * * *")])
(for/fold ([tsnow])
([_ (in-range 10)])
(define new-ts
(schedule-next sts))
(displayln (~datenew-ts))
(add1 new-ts)))

2022年08月19日T11:00:00

2022年08月19日T12:00:00

2022年08月19日T13:00:00

2022年08月19日T14:00:00

2022年08月19日T15:00:00

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2022年08月19日T20:00:00

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>
> (sync (parse-schedule "* * * * * *"))

#<schedule>

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procedure
( schedule? v)→boolean?
v:any/c

Returns #t when v is a schedule.

procedure
( parse-schedule s[local-time?])→schedule?
s:string?
local-time?:boolean? =#t

Parses the cron schedule represented by s and returns a schedule. Supports the same syntax as BSD cron, with the following differences:

“@” commands are not supported,
“0” is not a valid weekday, and
month and weekday names are not supported.

When a schedule contains 6 fields instead of 5, the first field is treated as a constraint on the seconds component of a timestamp.

Timestamps are processed by the schedule according to the local time zone if local-time? is #t and UTC otherwise.

procedure
( schedule-matches? stimestamp)→boolean?
s:schedule?
timestamp:exact-integer?

Returns #t when timestamp matches the schedule.

procedure
( schedule-next s[start-timestamp])→exact-integer?
s:schedule?
start-timestamp:exact-integer? =(current-seconds )

Returns a timestamp representing the first moment that matches the schedule including and after start-timestamp.

procedure
( schedule->string s)→string?
s:schedule?

Returns a representation of the schedule in cron format.

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