[WIP] ✨ Add support for implicit paging in un/structured clients

[nimrodshn]

What this does?

Following #341 this patch adds implicit paging to the un/structured clients. Fixes #532.

Thoughts about the API:

This patch opts for adding a new Method called ListPages: This method lists large data in "chunks" using the Continue curser token with a callback to be used by the user.

Open Questions:

• Should ListPages be part of the Reader interface?

@ncdc @DirectXMan12 @vincepri @cben PTAL.

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[coderanger]

This feels weird to me. What is the use case for this? Generally when a pagination cursor exists, not using it will lead to lots of weird race condition bugs when the object set changes between requests.

[nimrodshn]

@coderanger I tend to agree. I guess the idea is to hide the use of Continue from the user. Albeit the user can simply use Continue and view pages of data as it is now. Please refer to #532 for more information.

[coderanger]

I don't think this is what #532 was referring to. In what situation would you ever want to ask for "page 4 of the results" when you have no idea what that will even contain? The only time I could see someone doing that is in error when they want to get all items via pagination, but using this code they would get all those bugs I mentioned, which is why a pagination cursor is returned.

[nimrodshn]

@coderanger I had implemented it this way b/c I thought that is what reported in #532 - how do you view it?

[coderanger]

That List() would automatically check if a continue token was returned, and (give or take an option flag) automatically fetch the next page and combine it with the the results you already have, and repeat until no more data is returned. When accessing a huge list of objects that change frequently (e.g. all Pods) this can spread out the load on the API server, trading increased effective latency to remove the massive IO spike that could disrupt API server stability.

[coderanger]

Or there's the option @vincepri mentioned of doing a similar looped get with an iterator function callback so you never have to hold the full list of objects in memory at once. Both could be useful in different situations.

[nimrodshn]

@coderanger Interesting.. I will try and tackle those ideas. Indeed this implementation is fairly naive.

[nimrodshn]

@coderanger Please re-tal; specifically at the implementation at the typed_client.go - is this the direction you had in mind? what about the callback do you think it is necessary? Should we add the ListPages method to the Reader interface?

cc: @cben

[coderanger]

This probably needs a better name since it's not the default for List() too.

[nimrodshn]

@vincepri @cben Any suggestions?

[coderanger]

We don't want both a callback function and a huge list return, we should pick one pattern or the other (or move them to separate functions/options). The reason to use a visitor callback function is to avoid keeping the whole list in memory. I think it would be more Go-like to just do the callback style probably, ExtractList() requires a lot of runtime reflect magic since there is no unified API for accessing list items. But having both available could be convenient.

[coderanger]

@vincepri Do you have opinions on which API to support (or both)?

[nimrodshn]

Gotcha. 👍

@coderanger Actoually thinking about it - if the user would like to accumulate the result she can simply use the same ExtractList inside the callback.

[nimrodshn]

That is, she can pass something like this:

out := &corev1.Pods{}
allItems := make([]runtime.Object, 0)
cl.ListPages(ctx, out,  func(obj client.ObjectList) error {
    items, err := apimeta.ExtractList(interimResult)
    if err != nil {
        return err
    }
    allItems = append(allItems, items...)
    return nil
})

[coderanger]

Indeed, or more likely cast it to the correct type and skip needing ExtractList :)

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@nimrodshn: PR needs rebase.

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