vai: document resync period

[moio]

Currently all Vai informers are started with a zero interval period, meaning they never force resync all objects.

It was suggested this should be made configurable and aligned with the behavior of other informers in lasso (rancher/lasso#65 (comment), part of rancher/rancher#47825)

Actually, there was a misunderstanding on the exact consequences of resync. This PR is to document exactly why.

Note also discussion in #463 (comment)

[tomleb]

I initially thought the resync was doing an actual LIST+WATCH resync with the apiserver, but it is simply taking whatever is in its cache and re-running handlers with it. (Confirmed by adding print statements in the various Store.Add, Store.Update, etc call and not seeing them with a custom configured interval of 2 seconds)

Given that there aren't any handlers registered with the vai informer, do we actually need this now? Or is it just a "future-proofing" kind of change?

[moio]

I initially thought the resync was doing an actual LIST+WATCH resync with the apiserver, but it is simply taking whatever is in its cache and re-running handlers with it. (Confirmed by adding print statements in the various Store.Add, Store.Update, etc call and not seeing them with a custom configured interval of 2 seconds)

Whoa.

I have to admit I was under the wrong assumption you mentioned to this day. Probably misguided by a document that circulated years ago as part of the Rancher developer onboarding: https://github.com/aiyengar2/k8s-docs/blob/main/docs/controllers/02_informers.md#introduction-to-the-informer-pattern

I checked client-go's code and indeed it seems like the periodic resync is only used to notify event handlers and setting the internal Reflector resync time. And all Reflector does is to call Resync on its associated Store - but is a no-op on "simple" stores like ours (which is modeled after ThreadSafeStore).

So that only leaves event handlers, no periodic re-listing.

Then this change is completely useless.

I will use this PR as an occasion to document the behavior well.

[ericpromislow]

ok