When reviewing a design for an asynchronous system, there are some simple questions you can ask.

• What happens if this message is delayed?
• What happens if these messages arrive out of order?
• What happens if this message is processed twice? Concurrently? After a significant delay?
• What happens if two related messages are processed concurrently?
• What happens if this message handler fails partway through? Repeatedly?

In my experience, you are likely to find some bugs!

After you find some bugs, take some time to feel good about it then share this blog post with the designers. Next time they can be the ones to look smart!

For techniques to handle these issues, see failure patterns.