proposal of yurthub cache refactoring

[Congrool]

What type of PR is this?

/kind documentation
/kind design
/sig storage

What this PR does / why we need it:

When implementing the feature of pool-coordinator, I encountered serval problems to be solved first. So, it is this pr. Details are in the proposal.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

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In response to this:

What type of PR is this?

/kind documentation
/kind design
/sig storage

What this PR does / why we need it:

When implementing the feature of pool-coordinator, I encountered serval problems to be solved first. So, it is this pr. Details are in the proposal.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[rambohe-ch]

@Congrool Maybe we should also consider the interface for reading objects from yurthub cache?

[Congrool]

@rambohe-ch Currently we have Get/List/ListKeys only for reading objects in diskStorage. In the case of Pool-Coordiantor, the read request will be send to the APIServer in pool-coordinator, so it does not need to use the read interface to read object from cache directly.

The problem i can find is that the function of List is so weak for using.

  // List will retrieve all contents whose keys have the prefix of rootKey.
  // If rootKey is empty, ErrKeyIsEmpty will be returned.
    // If the rootKey does not exist in the store, ErrStorageNotFound will be returned.
  // If the rootKey exists in the store but no keys has the prefix of rootKey,
    // an empty content slice will be returned.
  List(rootKey Key) ([][]byte, error)

We can see that List only has rootKey as its parameter, which means it cannot list objects with selector. Thus, we can only support one kind of selector for each resource per component as described in #265 . It will be a problem if one component need to list the same resource with different selectors. But currently, we haven't encountered such a component. We can do this enhancement when we have to be faced with it.

[rambohe-ch]

@rambohe-ch Currently we have Get/List/ListKeys only for reading objects in diskStorage. In the case of Pool-Coordiantor, the read request will be send to the APIServer in pool-coordinator, so it does not need to use the read interface to read object from cache directly.

@Congrool ok, the conclusion is that pool-coordinator will not implement Get/List/ListKeys funcs for Storage interface because yurthub read from kube-apiserver of pool-coordinator directly.

[Congrool]

need revision
/hold

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In response to this:

/lgtm

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[Congrool]

revised according to the newest implementation

[codecov]

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[rambohe-ch]

@Congrool how about the definition of KeyBuildInfo and Key struct?

[Congrool]

The definition of Key is like:

type Key interface {
	Key() string
}

type KeyBuildInfo struct {
	Component string
	Namespace string
	Name      string
	Resources string
	Group     string
	Version   string
}

I'll update it in this proposal.

[Congrool]

proposal is updated with definition of Key and new interface names

[openyurt-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Congrool, rambohe-ch

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[rambohe-ch]

/lgtm