How to compose Observables to avoid the given nested and dependent callbacks?

Question

In this blog, he gives this (copy/pasted the following code) example for the callback hell. However, there is no mention of how the issue can be eliminated by using Reactive

Answer 1

I'm the original author of the referenced blog post about callbacks and Java Futures. Here is an example of using flatMap, zip and merge to do service composition asynchronously.

It fetches a User object, then concurrently fetches Social and PersonalizedCatalog data, then for each Video from the PersonalizedCatalog concurrently fetches a Bookmark, Rating and Metadata, zips those together, and merges all of the responses into a progressive stream output as Server-Sent Events.

return getUser(userId).flatMap(user -> {
    Observable> catalog = getPersonalizedCatalog(user)
            .flatMap(catalogList -> catalogList.videos().> flatMap(
                    video -> {
                        Observable bookmark = getBookmark(video);
                        Observable rating = getRatings(video);
                        Observable metadata = getVideoMetadata(video);
                        return Observable.zip(bookmark, rating, metadata, (b, r, m) -> combineVideoData(video, b, r, m));
                    }));

    Observable> social = getSocial(user).map(s -> {
        return s.getDataAsMap();
    });

    return Observable.merge(catalog, social);
}).flatMap(data -> {
    String json = SimpleJson.mapToJson(data);
    return response.writeStringAndFlush("data: " + json + "\n");
});

This example can be seen in context of a functioning application at https://github.com/Netflix/ReactiveLab/blob/952362b89a4d4115ae0eecf0e73f273ecb27ba98/reactive-lab-gateway/src/main/java/io/reactivex/lab/gateway/routes/RouteForDeviceHome.java#L33

Since I can't possibly provide all of the information here you can also find an explanation in presentation form (with link to video) at https://speakerdeck.com/benjchristensen/reactive-streams-with-rx-at-javaone-2014?slide=32.