How to handle complex side-effects in Redux?

When you want complex async dependencies, just use Bacon, Rx, channels, sagas, or another asynchronous abstraction. You can use them with or without Redux. Example with Redux:

observeSomething()
  .flatMap(someTransformation)
  .filter(someFilter)
  .map(createActionSomehow)
  .subscribe(store.dispatch);

You can compose your asynchronous actions any way you like—the only important part is that eventually they turn into store.dispatch(action) calls.

Redux Thunk is enough for simple apps, but as your async needs get more sophisticated, you need to use a real asynchronous composition abstraction, and Redux doesn’t care which one you use.

Update: Some time has passed, and a few new solutions have emerged. I suggest you to check out Redux Saga which has become a fairly popular solution for async control flow in Redux.