http://stackoverflow.com/questions/30399125/particular-asynchrony-of-requests-with-retrofit-an-d-rxjava

gistfile1.java

public Observable<Response> upload(final Image image) {
    // create new ArrayList in case if image.getChunks() returns immutable collection
    final List<Chunk> chunks = new ArrayList<Chunk>(image.getChunks());

    if (chunks.isEmpty()) {
        return Observable.empty();
    }

    if (chunks.size() == 1) {
        return api.upload(chunks.get(0));
    }

    final Observable<Response> first = api.upload(chunks.get(0));
    final Observable<Response> last = api.upload(chunks.get(chunks.size() - 1));

    chunks.remove(chunks.size() - 1);
    chunks.remove(0);

    return first.concat(chunks.flatMap(chunk -> return api.upload(chunk))).concat(last);
}