Our checkout service went down for 47 minutes because the transactional email vendor had an outage. Placing an order does not need an email to be sent at that moment, but the order handler called the email API synchronously, the calls hung until timeout, the worker pool filled up and every other request queued behind them. A dependency that should have caused "confirmation emails are late" instead caused "nobody can buy anything". That incident made us rewrite how we think ab...