router: support fetching backend list from external interface

[djshow832]

What problem does this PR solve?

Issue Number: close None

Problem Summary:

In the serverless tier, the gateway offers an interface to query the backend list.

What is changed and how it works:

• Add an interface BackendFetcher
• There are 3 implementations of BackendFetcher: StaticFetcher, PDFetcher, and ExternalFetcher.

Some TODOs in next PRs:

• Not always init an ETCD client (TBD)
• Update router
• Stop the observer when there's no connections

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[xhebox]

Why can not let they pass an implementation of interface BackendFetcher?

[djshow832]

It's OK to pass a BackendFetcher from the Router, but it exposes map[string]*BackendInfo.
But it's not easy to pass it from the gateway. The fetcher requires a token as the parameter, and the router can wrap it.

[xhebox]

But it's not easy to pass it from the gateway. The fetcher requires a token as the parameter, and the router can wrap it.

I don't understand. It is completely possible to do:

type gatewayFetcher struct {
  token string
}

func (f *gatewayFetcher) GetBackendList(context.Context) map[string]*BackendInfo {
	...
}

The only imperfect is that we exposed BackendInfo.

[djshow832]

Updated, the BackendObserver receives a customFetcher. But I still think the fetcher should be passed from the router because of the token parameter.

[djshow832]

type gatewayFetcher struct {
  token string
}

This is different from the interface we negotiated with them. This just makes things complicated. Besides offering a GetBackends, they have to also create a fetcher.

[xhebox]

This is different from the interface we negotiated with them. This just makes things complicated. Besides offering a GetBackends, they have to also create a fetcher.

If the only method of the interface is GetBackendList(context.Context) map[string]*BackendInfo, with all BackendInfo an empty struct, what is the difference?

Three lines of code to declare gatewayFetcher type? They will need to store that token somewhere anyway, for example, a variable captured by the lambda function.

If we want to support, grpc pushing/notification of backends changes, BackendFetcher interface will change somehow, i.e. not just a fetcher. It is likely that ExternalFetcher will not work anymore.