Reasoning about modern software systems can be hard. Debugging modern software systems can be hard. Whether you’re working on a multithreaded service or re-entrant UI code, tracking responses from network services or running many asynchronous IO tasks, understanding the overall state of your application especially when there’s a hard-to-reproduce bug can be very hard. Common styles of older multithreaded code are the worst. Shared state and locks, manual coordination between threads, fighting with race conditions, deadlocks and live-locks. It’s very hard to diagnose problems and common to have applications that have been running for weeks suddenly lock up and never…