Non-blocking Activity task implementations

Issue

The TaskActivity interface is currently has a synchronous run method that returns that output of that activity:

public interface TaskActivity {
    Object run(TaskActivityContext ctx);
}

There's currently no way to define a non-blocking, asynchronous activity implementation. This can result in issues related to the scalability of applications if many activity tasks need to be executed concurrently.

Proposal

There should be an alternative mechanism for implementing non-blocking, asynchronous activity tasks. For example:

public interface AsyncTaskActivity {
    CompletionStage<Object> run(TaskActivityContext ctx);
}

The activity will finish when the CompletionStage is completed.

Other considerations

It should be noted that Azure Functions for Java doesn't yet support non-blocking functions: https://github.com/Azure/azure-functions-java-worker/issues/244. Support for non-blocking activity tasks likely won't benefit Azure Functions users until the Java worker for Azure Functions adds async function support. Unfortunately, there's no indication about if or when this support will be added.

TaskActivity インターフェースから始め、同期的な run の結果がどのように消費されるかを追跡します。実装範囲を定義する前に、提案されている AsyncTaskActivity コントラクトと、リンクされている Azure Functions の制限を確認します。完了とは、アクティビティのノンブロッキングな完了方法と、それを既存のアクティビティ実行フローに統合する方法について合意されたアプローチが定まっていることです。