Support the heartbeat

[nomeata]

@roman-kashitsyn leaked on the forum that the canister_heartbeat entry point is now supported by application subnets. The Interface spec doesn’t list it yet, but once it’s there we should probably support it from Motoko.

Unless we take the stance (from the discussion on the feature in the ic-ref at https://github.com/dfinity-lab/ic-ref/issues/245, which I can’t read any more) that canister_heartbeat is a low-level feature meant to be used mainly by canisters providing cron services to other canisters, and that Motoko canisters should not use this (inefficient, inflexible) interface, but should rather use dedicated cron-as-a-service-canisters.

But the pragmatic thing is probably to expose the canister_heartbeat entry point.

[crusso]

Shouldn't be too hard to support.

[nomeata]

Yeah, the straight forward exposition should be almost trivial.

Is our type system strong enough to express a context that allows calls (i.e. can await), but does not allow responses? That might new, or affect the default reject handler.

[crusso]

Not entirely sure, but it sounds like the same restrictions we enforce on oneways by forcing the ignore async {...} abomination.

[FloorLamp]

Hi Claudio, any chance we can get this added? Right now I have to install a separate rust heartbeat canister to call my motoko ones, this isn't very ideal 😅

[jzxchiang1]

This definitely sounds useful for any Motoko app that needs cron-like functionality.

You might've seen this, but here is an example of a Rust canister that uses the heartbeat: https://github.com/akhi3030/heartbeat-example/blob/master/src/lib.rs

[ninegua]

Here is my very naive & hacky attempt at implementing it ninegua@6a0a645

Probably made a lot of mistakes, and I don't have time to write any test case. It is better to wait for official implementation.

Example:

import Debug "mo:base/Debug";

actor {
    var count = 0;
    public shared func inc() : async () {
        count := count + 1;
        Debug.print("count = " # debug_show(count));
    };

    system func heartbeat() : async () {
        Debug.print("heartbeat");
        ignore inc();
    };
};

[crusso]

@ninegua thanks for that! Looks quite plausible (skimming only)

@rossberg shall we add support for the heartbeat method?

[crusso]

(I would probably make it just return (), not async(), like a oneway though.)

[rossberg]

Fine by me!

[ninegua]

(I would probably make it just return (), not async(), like a oneway though.)

That was my first try. But with only () I cannot make it pass the type checker if I have ingore inc() in the function body. Maybe you can help?

Also my suggestion is to rename upgrade { .. } record to system { ... } to better match the intention.

[jzxchiang1]

Any update on this? Thanks!

[Motokoder]

Please give my canister a heartbeat.