Build job schedules with kotlinx-datetime and kotlinx-coroutines.

val tz = TimeZone.of("America/New_York")
Clock.System.schedulePulse(atSecond = 0).beat { instant ->
    println("The time in $tz is ${instant.toLocalDateTime(tz)}.")
}

The time in America/New_York is 2023-10-07T19:49:00. The time in America/New_York is 2023-10-07T19:50:00. The time in America/New_York is 2023-10-07T19:51:00. The time in America/New_York is 2023-10-07T19:52:00. The time in America/New_York is 2023-10-07T19:53:00. 

Introduction

Cardiologist integrates with kotlinx-datetime to provide you scheduling based on Instant, LocalDateTime, and LocalTime, and integrates with kotlinx-coroutines to provide a suspending API to trigger pending jobs.

Simple suspending functions are available to delay until a given moment in time.

Clock.System.delayUntil(instant = Instant.DISTANT_FUTURE)
Clock.System.delayUntil(dateTime = LocalDateTime.MAX, timeZone = TimeZone.UTC)

...or suspending for a given period of time.

Clock.System.delayFor(period = DateTimePeriod(months = 1, days = 2), timeZone = TimeZone.UTC)

...or suspending until the next time a LocalTime occurs in a certain time zone.

val midnight = LocalTime(hour = 0, minute = 0)
Clock.System.delayUntilNext(time = midnight, timeZone = TimeZone.UTC)

Repeating intervals are provided as a Pulse, a type which holds a reccurence cadence.

val hourlyPulse: Pulse = Clock.System.intervalPulse(interval = 1.hours)
val dailyPulse: Pulse = Clock.System.intervalPulse(
    period = DateTimePeriod(days = 1),
    timeZone = TimeZone.UTC,
)

Pulse schedules can be built with a cron like API.

// Schedules a pulse to occur on the 5th of every month at 12:30 in UTC. 
val scheduledPulse = Clock.System.schedulePulse(
    timeZone = TimeZone.UTC,
    atSecond = 0,
    atMinute = 30,
    atHour = 12,
    onDayOfMonth = 5, 
)

...and can be invoked by calling Pulse.beat.

scheduledPulse.beat { instant -> println("$instant") }

Beating a pulse is a backpressure sensitive operation. If your job has not completed when the next pulse is scheduled to occur, it will by default be cancelled. Cardiologist provides two other modes to beat a pulse which allow jobs to run concurrently or apply backpressure.

import io.github.kevincianfarini.cardiologist.RecurringJobMode.*

// Cancels previous job when next job is scheduled to occur. 
hourlyPulse.beat(mode = CancellingSequential) { instant -> longRunningOperation(instant) }

// Allows jobs to run concurrently if previous job is still active. 
hourlyPulse.beat(mode = Concurrent) { instant -> longRunningOperation(instant) }

// Will not emit again until previous job finishes, applying backpressure.  
hourlyPulse.beat(mode = DelayBetweenSequential) { instant -> longRunningOperation(instant) }

Cardiologist is not a replacement for crontab(5) or Android WorkManager.

Cardiologist is an in process job scheduling library. It is meant to run concurrently to the rest of your process, not as a separate process. Use cases include job scheduling within a long-lived process like a server or a daemon.

// With ktor, for example. 
fun main() = runBlocking { // this: CoroutineScope
    this.embeddedServer(Netty, port = 8080) { /* omitted */ }.start(wait = false)
    launch { recurringJob() }
}

private suspend fun recurringJob() {
    Clock.System.schedulePulse(
        onDayOfMonth = 1, 
        atHour = 0, 
        atMinute = 0, 
        atSecond = 0
    ).beat { instant -> someWork(instant) }
}

Cron jobs and Android's WorkManager leverage standalone processes and will launch your program as a separate processes. This is outside the scope of Cadiologist's goals.