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Do you measure your app's uptime?
If so, than how do you measure it? Any libraries, tools, services, etc worth investigating?
Pretty much has to be an external service, since you want it running independently of your app. And, so you don't have to also monitor and run your own uptime monitoring, most people just pay for something.
I like and recommend HoneyBadger.io will do uptime checks, check-ins, and error monitoring all-in-one. Nice to have those features together in the same app rather than paying for multiple things. They have a free solo-dev option that comes in handy.
You've also got tools like Pingdom that will do performance / uptime. Lots of other options out there too.