Timers

Internet Computer canisters can set an arbitrary number of single-expiration or recurring timers. See the Timer.mo module in the base library.

A simple, contrived example is a periodic reminder, that logs a new-year's message:

import { print } = "mo:base/Debug";
import { abs } = "mo:base/Int";
import { now } = "mo:base/Time";
import { setTimer; recurringTimer } = "mo:base/Timer";

actor Reminder {

  let solarYearSeconds = 356_925_216;

  private func remind() : async () {
    print("Happy New Year!");
  };

  ignore setTimer(#seconds (solarYearSeconds - abs(now() / 1_000_000_000) % solarYearSeconds),
    func () : async () {
      ignore recurringTimer(#seconds solarYearSeconds, remind);
      await remind();
  });
}

The underlying mechanism is a canister global timer that, by default, is issued with appropriate callbacks from a priority queue maintained by the Motoko runtime.

The timer mechanism can be disabled completely by passing the -no-timer flag to moc.

When lower-level access to the canister global timer is desired, an actor can elect to receive timer expiry messages by declaring a system function, named timer. The function takes one argument (to re-set the global timer), and returns a future of unit type (async ()). If the timer system method is declared, the Timer.mo base library module may not function correctly and should not be used.

The following example of a global timer expiration callback gets called immediately after the canister starts (i.e. after install) and periodically every twenty seconds thereafter:

system func timer(setGlobalTimer : Nat64 -> ()) : async () {
  let next = Nat64.fromIntWrap(Time.now()) + 20_000_000_000;
  setGlobalTimer(next); // absolute time in nanoseconds
  print("Tick!");
}