libcron

A C++ scheduling library using cron formatting.

Using the Scheduler

Libcron offers an easy to use API to add callbacks with corresponding cron-formatted strings:

libcron::Cron cron;

cron.add_schedule("Hello from Cron", "* * * * * ?", [=](auto&) {
	std::cout << "Hello from libcron!" std::endl;
});

To trigger the execution of callbacks, one must call libcron::Cron::tick at least once a second to prevent missing schedules:

while(true)
{
	cron.tick();
	std::this_thread::sleep_for(500mS);
}

In case there is a lot of time between you call add_schedule and tick, you can call recalculate_schedule.

The callback must have the following signature:

std::function<void(const libcron::TaskInformation&)>

libcron::Taskinformation offers a convenient API to retrieve further information:

• libcron::TaskInformation::get_delay informs about the delay between planned and actual execution of the callback. Hence, it is possible to ensure that a task was executed within a specific tolerance:

libcron::Cron cron;

cron.add_schedule("Hello from Cron", "* * * * * ?", [=](auto& i) {
	using namespace std::chrono_literals;
	if (i.get_delay() >= 1s)
	{
		std::cout << "The Task was executed too late..." << std::endl;
	}
});

• libcron::TaskInformation::get_name gives you the name of the current Task. This allows to add attach the same callback to multiple schedules:

libcron::Cron cron;

auto f = [](auto& i) {
	if (i.get_name() == "Task 1")
  {
  	do_work_task_1();
  }
  else if (i.get_name() == "Task 2")
  {
  	do_work_task_2();
  }
};

cron.add_schedule("Task 1", "* * * * * ?", f);
cron.add_schedule("Task 2", "* * * * * ?", f);

Adding multiple tasks with individual schedules at once

libcron::cron::add_schedule needs to sort the underlying container each time you add a schedule. To improve performance when adding many tasks by only sorting once, there is a convinient way to pass either a std::map<std::string, std::string>, a std::vector<std::pair<std::string, std::string>>, a std::vector<std::tuple<std::string, std::string>> or a std::unordered_map<std::string, std::string> to add_schedule, where the first element corresponds to the task name and the second element to the task schedule. Only if all schedules in the container are valid, they will be added to libcron::Cron. The return type is a std::tuple<bool, std::string, std::string>, where the boolean is true if the schedules have been added or false otherwise. If the schedules have not been added, the second element in the tuple corresponds to the task-name with the given invalid schedule. If there are multiple invalid schedules in the container, add_schedule will abort at the first invalid element:

std::map<std::string, std::string> name_schedule_map;
for(int i = 1; i <= 1000; i++)
{
	name_schedule_map["Task-" + std::to_string(i)] = "* * * * * ?";
}
name_schedule_map["Task-1000"] = "invalid";
auto res = c1.add_schedule(name_schedule_map, [](auto&) { });
if (std::get<0>(res) == false)
{
	std::cout << "Task " << std::get<1>(res) 
						<< "has an invalid schedule: " 
						<< std::get<2>(res) << std::endl;
}

Removing schedules from libcron::Cron

libcron::Cron offers two convenient functions to remove schedules:

• clear_schedules() will remove all schedules
• remove_schedule(std::string) will remove a specific schedule

For example, cron.remove_schedule("Hello from Cron") will remove the previously added task.

Removing/Adding tasks at runtime in a multithreaded environment

When Calling libcron::Cron::tick from another thread than add_schedule, clear_schedule and remove_schedule, one must take care to protect the internal resources of libcron::Cron so that tasks are not removed or added while libcron::Cron is iterating over the schedules. libcron::Cron can take care of that, you simply have to define your own aliases:

/* The default class uses NullLock, which does not lock the resources at runtime */
template<typename ClockType = libcron::LocalClock, typename LockType = libcron::NullLock>
class Cron
{
	...
}

/* Define an alias for a thread-safe Cron scheduler which automatically locks ressources when needed */ 
using CronMt = libcron::Cron<libcron::LocalClock, libcron::Locker>

CronMt cron;
cron.add_schedule("Hello from Cron", "* * * * * ?", [=]() {
	std::cout << "Hello from CronMt!" std::endl;
});

....

However, this comes with costs: Whenever you call tick, a std::mutex will be locked and unlocked. So only use the libcron::Locker to protect resources when you really need too.

Local time vs UTC

This library uses std::chrono::system_clock::timepoint as its time unit. While that is UTC by default, the Cron-class uses a LocalClock by default which offsets system_clock::now() by the current UTC-offset. If you wish to work in UTC, then construct the Cron instance, passing it a libcron::UTCClock.

Supported formatting

This implementation supports cron format, as specified below.

Each schedule expression conststs of 6 parts, all mandatory. However, if 'day of month' specifies specific days, then 'day of week' is ignored.

┌──────────────seconds (0 - 59)
│ ┌───────────── minute (0 - 59)
│ │ ┌───────────── hour (0 - 23)
│ │ │ ┌───────────── day of month (1 - 31)
│ │ │ │ ┌───────────── month (1 - 12)
│ │ │ │ │ ┌───────────── day of week (0 - 6) (Sunday to Saturday)
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
* * * * * *

• Allowed formats:

  • Special characters: '*', meaning the entire range.

  • '?' used to ignore day of month/day of week as noted below.

  • Ranges: 1,2,4-6

    • Result: 1,2,4,5,6

  • Steps: n/m, where n is the start and m is the step.

    • 1/2 yields 1,3,5,7...
    • 5/3 yields 5,8,11,14...
    • */2 yields Result: 1,3,5,7...

  • Reversed ranges:

    • 0 0 23-2 * * *, meaning top of each minute and hour, of hours, 23, 0, 1 and 2, every day.
      • Compare to 0 0 2-23 * * * which means top of each minute and hour, of hours, 2,3...21,22,23 every day.

For month, these (case insensitive) strings can be used instead of numbers: JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC. Example: JAN,MAR,SEP-NOV

For day of week, these (case insensitive) strings can be used instead of numbers: SUN, MON, TUE, WED, THU, FRI, SAT. Example: MON-THU,SAT

Each part is separated by one or more whitespaces. It is thus important to keep whitespaces out of the respective parts.

• Valid:

  • 0,3,40-50 * * * * ?

• Invalid:

  • 0, 3, 40-50 * * * * ?

Day of month and day of week are mutually exclusive so one of them must at always be ignored using the '?'-character to ensure that it is not possible to specify a statement which results in an impossible mix of these fields.

Examples

Expression	Meaning
* * * * * ?	Every second
0 * * * * ?	Every minute
0 0 12 * * MON-FRI	Every Weekday at noon
0 0 12 1/2 * ?	Every 2 days, starting on the 1st at noon
0 0 */12 ? * *	Every twelve hours
@hourly	Every hour

Note that the expression formatting has a part for seconds and the day of week. For the day of week part, a question mark ? is utilized. This format may not be parsed by all online crontab calculators or expression generators.

Convenience scheduling

These special time specification tokens which replace the 5 initial time and date fields, and are prefixed with the '@' character, are supported:

Token	Meaning
@yearly	Run once a year, ie. "0 0 1 1 *".
@annually	Run once a year, ie. "0 0 1 1 *".
@monthly	Run once a month, ie. "0 0 1 * *".
@weekly	Run once a week, ie. "0 0 * * 0".
@daily	Run once a day, ie. "0 0 * * *".
@hourly	Run once an hour, ie. "0 * * * *".

Randomization

The standard cron format does not allow for randomization, but with the use of CronRandomization you can generate random schedules using the following format: R(range_start-range_end), where range_start and range_end follow the same rules as for a regular cron range (step-syntax is not supported). All the rules for a regular cron expression still applies when using randomization, i.e. mutual exclusiveness and no extra spaces.

Examples

Expression	Meaning
0 0 R(13-20) * * ?	On the hour, on a random hour 13-20, inclusive.
0 0 0 ? * R(0-6)	A random weekday, every week, at midnight.
0 R(45-15) */12 ? * *	A random minute between 45-15, inclusive, every 12 hours.
0 0 0 ? R(DEC-MAR) R(SAT-SUN)	On the hour, on a random month december to march, on a random weekday saturday to sunday.

Used Third party libraries

Howard Hinnant's date libraries