Suppose you were creating a dozen lessons on distributed systems for people who are competent programmers but have never wrestled with partial failure, timing, and similar issues. You want each example to walk through construction of a simplified version of a real-world system (similar to https://third-bit.com/sdxpy/ and https://third-bit.com/sdxjs/). What single example would you add to the list in the next toot to bring the list up to a dozen, and why? 1/