feat: multi-provider implementation

[liran2000]

Readme describes the provider.

References

• Spec
• blog post by @emmawillis

[aepfli]

Thank you, this looks really cool. But especially the Strategy initialization and the handling of 'exception-less Errors' might be problematic

[aepfli]

Not all providers throw exceptions sometimes, they return the information in the provider evaluation instead of throwing as exceptions for flows is expensive see open-feature/java-sdk#1095 - applies to all strategies

[liran2000]

updated accordingly.

[aepfli]

This might be a problem, as the map is generated in the multi-provider; we need these values to be generated before we pass the strategy to the provider. Maybe we should also put those as parameters to the evaluate method else it will be hard to use any other strategy than the default

[liran2000]

I updated it a bit. The providers can be reached via BaseStrategy, as done at MultiProviderTest. What do you think ?

[aepfli]

i am not sure, now we do have to separate maps where the state needs to be in sync. this might cause problems. we should centralize this on the provider, what if we change the signature of the evaluate method like

public interface Strategy {
    <T> ProviderEvaluation<T> evaluate(Map<...> providers, String key, T defaultValue, EvaluationContext ctx,
           Function<FeatureProvider, ProviderEvaluation<T>> providerFunction);
}

[liran2000]

it is commonly the same providers instance at the multi-provider and the strategy, and it is built as unmodifiable when initialized. Why can this be a problem ? I am fine with your approach above too, just want to understand better first.

another thing, reg. ProviderEvaluation> providerFunction as parameter:
it is pretty straight-forward to implement with it, but I have some concerns that if a custom strategy implementation want to do change the function call and/or arguments, it will limit it. Other approach can be to pass the type. e.g. Boolean.class instead of the function, and convert it to the specific evaluation method. What do you think ?

[aepfli]

if i eg. switch to the FirstSuccessful implementation my code would look like

var providers = List.of(...);
var strategy = new FirstSuccessful(providers);
var provider = new MultiProvider(providers, strategy);

but that means that theoretically the providers list could be a different one, or am i missing here something?

Regarding your providerFunction, i do think that this is the clearest way also maintainence wise - trying to hassle with type information adds unnecessary overhead in my opinion

[liran2000]

ok, updated accordingly.

[beeme1mr]

Please remove this changelog. A new one should be created by Release Please.

[liran2000]

done

[beeme1mr]

The file path is providers/multiprovider. Please either update the path name or this configuration.

[liran2000]

done

[liran2000]

BTW, originally I had "multi-provider" like other providers, but it failed validation on Invalid Automatic-Module-Name:

❌ no - allowed

[beeme1mr]

Why would Maven Central allow invalid names to be uploaded?