Where this leaves you

You have the pipeline from the introduction’s diagram, assembled, with every model-backed stage optional and every failure named. Run it with every credential and it uses every stage; run it with one key and it uses the stages that key can run; run it with none and it prints one line and exits. Nothing it shows was unchecked, on any of those paths.

What you do not yet know is how well it answers. That is a different kind of question from any this book has asked so far, and the next chapter spends money to answer it.