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Aug 11, 2025
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📖 Add more docs to SyncPeriod #3269

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11 changes: 11 additions & 0 deletions pkg/cache/cache.go
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Expand Up @@ -172,6 +172,17 @@ type Options struct {
// is "done" with an object, and would otherwise not requeue it, i.e., we
// recommend the `Reconcile` function return `reconcile.Result{RequeueAfter: t}`,
// instead of `reconcile.Result{}`.
//
// SyncPeriod will trigger update events with the old object being equal to the new
// object, except when the cache was out of sync.
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@sbueringer sbueringer Aug 11, 2025 •
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except when the cache was out of sync

I'm not sure that this is true. Do we have evidence that informer sometimes trigger update events with old object != new object during resync?

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I wrote that, because I thought that this is what SyncPeriod should do: Check if the local cache is in-sync with the api-server. But the question of Stefan makes sense: Do we have evidence?

@alvaroaleman Do you know more about that?

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@sbueringer sbueringer Aug 12, 2025 •
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(Simplified version) Every syncPeriod the informer takes all objects that are stored locally and sends out update events for them.

I took a look at the implementation, but for me it's not clear if oldObj/newObj are always identical or not

xref: https://github.com/kubernetes/kubernetes/blob/5cca03792748714547c39330ac9cb8cb9c7c60ae/staging/src/k8s.io/client-go/tools/cache/controller.go#L567

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I thought that this is a sync between the api-server and the local cache :-)

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Nope :) But that's a very frequent misunderstanding

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Re-reading the rest of the godoc above. While it contains a lot of correct information I can see how it's misleading

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@sbueringer then it is time for a new PR: 📖 Docs for SyncPeriod: No sync from api-server. by guettli · Pull Request #3277 · kubernetes-sigs/controller-runtime

// If you filter update events like this:
// Controller.Watch(
// &source.Kind{Type: v1.MyCustomKind},
// &handler.EnqueueRequestForObject{},
// predicate.Or(predicate.GenerationChangedPredicate{},
// predicate.LabelChangedPredicate{}))
// then the SyncPeriod will not trigger a Reconcile call, because the update event
// will be ignored by the predicate.
SyncPeriod *time.Duration

// ReaderFailOnMissingInformer configures the cache to return a ErrResourceNotCached error when a user
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