📖 Explain what may happen when using OnlyMetadata and Get on the concrete type instead of PartialMetadataObject

[maelvls]

In #1660, we found that it is too easy for users to duplicate cache projections, for example by creating a watch with OnlyMetadata, which creates a metadata cache, and then using the concrete type to get the watched object, which automatically creates a cache of the concrete projection.

Duplicating the cached projections is probably never what the user wanted to do, and leads to:

• Increased memory usage,
• Race conditions since caches aren't synchronized.

At first, I wanted to follow the suggestion in #1660 (comment):

We could add a check for this into pkg/cache and error out if someone requests something that we already have in a different projection

I then realized that I don't really know how to make this change.

Can I suggest starting with a quick warning that people would see when looking at OnlyMetadata? In this PR, I added the warning.

To sum up my goal in this PR, I wish to document the following:

1. The three following Get calls will create three different caches:

   // Concrete client:
   pod := &v1.Pod{}
   mgr.GetClient().Get(ctx, nsAndName, pod)
   
   // Unstructured client:
   pod := &unstructured.Unstructured{}
   mgr.GetClient().Get(ctx, nsAndName, pod)
   
   // Metadata-only client:
   pod := &metav1.PartialObjectMetadata{}
   mgr.GetClient().Get(ctx, nsAndName, pod)

2. Caches are not synchronized, meaning that you may end up with data races when using two different cache projections.

3. Using OnlyMetadata option should not be used with the concrete nor the unstructured clients.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Maël Valais (fcd8310)

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[joelanford]

@maelvls is this still a work in progress? As far as I can tell, this looks like a great clarification and something we could go ahead and merge.

[maelvls]

Thanks, I think it is ready to go!

[alvaroaleman]

Doesn't this need typemeta set?

[maelvls]

That's a very good point, controller-runtime would have a hard time figuring out which type we are referring to with just &metav1.PartialObjectMetadata{} 😅

I fixed this in 3318359.

[alvaroaleman]

thank you!

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This pull-request has been approved by: alvaroaleman, maelvls

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