How many uses of makeNotifierKit should use makeSubscriptionKit instead?

[erights]

Outside of the notifier package, and test and demo directories, it looks like we have 34 calls to makeNotifierKit in agoric-sdk. With those same filters, we currently have 1 call to makeSubscriptionKit. This may or may not be a good split --- it depends on the semantics of the notifications being pushed. Fortunately, 35 is small enough to inspect. From

• the general familiarity with notifiers
• the general unfamiliarity with subscriptions
• the very dangerous misfeature that a notifier will often happen to accidentally work when a subscription is called for

I suspect that some of these notifier uses really should be subscription uses. The example which made me aware of this is represented well by #3783 . The code it corrects happened to work with a notifier by accident. #3783 demonstrates how painless it is to convert code from using a notifier to using a subscription. Please take a look.

[zarutian]

Defenitely Notifiers on ERTP purses shoulde Subscriptions instead, specially on Zoe fee purses of running contract vats. The latter for fuel gauge alarm reasons alone.

[erights]

https://github.com/Agoric/agoric-sdk/issues/3756#issuecomment-911121024

[Tartuffo]

@Chris-Hibbert This needs an area label to get picked up the right Wed planning meeting. This looked like Core Economy, please change if that is not right.

[dtribble]

A general note is that notifiers should be used only to report states, not transitions. Thus dropping one update doesn't prevent getting the latest state update. To use notifiers to communicate events, the update for a getUpdateSince(x) must include all the events since the specified time x or the "state" should be the entire list of events.

[erights]

To use notifiers to communicate events,

or, if you are communicating events, use subscriptions rather than notifiers. If subscriptions are not better at events than notifiers, then subscriptions should either be fixed or retired.

[erights]

Hi @gibson042 , I just added you