There are lots of ways to track an app’s performance—Apple and Google tools, plus plenty of third-party options. I’ll share how we ended up building our own performance monitoring suite at Booking.com, and why we went down that path. I’ll talk through the metrics we started with, what didn’t really work, and what turned out to be more useful. I’ll also cover what our current setup can do, how it’s been holding up after 4 years in production, and finish with a story from our two-year journey to track down and fix a really annoying fatal hang in our iOS app.