local_remote_repository.java

@NonNull
@Override public Observable<User> getUser(int id) {
return localDataSource.getUser(id)
        .onErrorResumeNext(
                remoteDataSource.getUser(id).doOnNext(new Action1<User>() {
                        @Override public void call(User user) {
                            localDataSource.saveUser(user);
                        }
                })
        )
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread());
}