Observable Method that optionally uses a thread to fetch data if not available in memory. Either way the response type is an Observable callback that can be composed using Rx operators.

ObservableMethod.groovy

/**
 * Non-blocking method that immediately returns the value
 * if available or  uses a thread to fetch the value and 
 * callback via `onNext()` when done. 
 */
def Observable<T> getData(int id) {
    if(availableInMemory) {
        // if data available return immediately with data
        return Observable.create({ observer ->
            observer.onNext(valueFromMemory);
            observer.onCompleted();
        })
    } else {
        // else spawn thread or async IO to fetch data
        return Observable.create({ observer ->
            executor.submit({
                try {
                    // do work on separate thread
                    T value = getValueFromRemoteService(id);
                    // callback with value
                    observer.onNext(value);
                    observer.onCompleted();
                }catch(Exception e) {
                    observer.onError(e);
                }
            })
        });
    }
}