github.com/hanagantig/cron (go): Package cron implements a cron spec parser and job runner with the ability to distributed lock.  To download the specific tagged release, run:  Import it in your program as:  It requires Go 1.13 or later due to usage of redsync.  Callers may register Funcs to be invoked on a given schedule.  Cron will run them in their own goroutines. JobKey is a unique key for a job, that used for distributed job locks.  A cron expression represents a set of times, using 5 space-separated fields.  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 Cron expression formats support other fields like seconds. You can implement that by creating a custom Parser as follows.  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 custom parser you saw earlier, except that its seconds field is REQUIRED:  That emulates Quartz, the most popular alternative Cron schedule format: http://www.quartz-scheduler.org/documentation/quartz-2.x/tutorials/crontrigger.html  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.  You may use one of several pre-defined schedules in place of a cron expression.  You may also schedule a job to execute at fixed intervals, starting at the time it's added or cron is run. This is supported by formatting the cron spec like this:  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 after 1 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.  By default, all interpretation and scheduling is done in the machine's local time zone (time.Local). You can specify a different time zone on construction:  Individual cron schedules may also override the time zone they are to be interpreted in by providing an additional space-separated field at the beginning of the cron spec, of the form "CRON_TZ=Asia/Tokyo".  For example:  The prefix "TZ=(TIME ZONE)" is also supported for legacy compatibility.  Be aware that jobs scheduled during daylight-savings leap-ahead transitions will not be run!  A Cron runner may be configured with a chain of job wrappers to add cross-cutting functionality to all submitted jobs. For example, they may be used to achieve the following effects:  Install wrappers for all jobs added to a cron using the `cron.WithChain` option:  Install wrappers for individual jobs by explicitly wrapping them:  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.  Cron defines a Logger interface that is a subset of the one defined in github.com/go-logr/logr. It has two logging levels (Info and Error), and parameters are key/value pairs. This makes it possible for cron logging to plug into structured logging systems. An adapter, [Verbose]PrintfLogger, is provided to wrap the standard library *log.Logger.  For additional insight into Cron operations, verbose logging may be activated which will record job runs, scheduling decisions, and added or removed jobs. Activate it with a one-off logger as follows:  Cron defines a JobLocker interface to manage job locking. It has the built-in JobLocker implementation based on redsync (Redis-based distributed mutual exclusion lock) https://github.com/go-redsync/redsync. Before running a job func Cron will lock given job by the key. By default it locks for 10 seconds. If job locked successfully Cron will also runs continuousLock method in separate goroutine and try to extend the lock each 5 second till job done. At the job end lock will remove.  If Cron runs a job, while other Cron instance has locked the same job key, late job will skip his turn.  There is capability to implement your own JobLocker as well, if you're not compatible with redsync lock.  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:
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