Apple Watch brings distributed system headaches to your app
"My Toddler Loves Distributed Systems, So I Built Him a Radar"
The best day of a man’s life is the day he gets his shiny new Apple Watch MK Ultra.
The worst day of a man’s life is the day he tries to develop for it.
This is my story.
Watch Woes
Apple does not pull any punches when it comes to watch devex. You can’t just connect a cable, it only works via Wi-Fi. Specifically, it only works if you happen to have 2.4 GHz Wi-Fi. Even so, Xcode probably won’t notice your watch for several hours. A̷n̵d̴ w̶h̷e̸n̴ i̵t̷ d̶o̵e̷s̸, i̴t̵ w̶i̷l̴l̵ t̶a̸k̵e̶ a̵n̷o̸t̴h̵e̶r̷ h̵o̸u̶r̷ t̴o̵ c̶o̷p̸y̴ c̵a̶c̷h̸e̴ s̶y̷m̵b̸o̴l̶s̷. W̴a̶n̷t̸ t̵o̷ g̶e̸t̵ a̶ t̷e̵a̸m̶m̷a̴t̶e̸’̵s̷ U̶U̸I̵D̷ f̸o̶r̷ a̵d̸-̷h̶o̵c̷ b̸u̴i̷l̶d̸s̵? G̷o̸o̵d̶ l̷u̸c̵k̶: y̷o̸u̵ h̷a̶v̸e̵ t̶o̷ g̸o̵ t̷h̶r̸o̵u̷g̶h̸ t̵h̷i̶s̸ w̵h̷o̶l̸e̵ r̷i̶g̸m̷a̶r̸o̵l̷e̶ a̸g̵a̷i̶n̸. I̵͎f̷̰ y̶̝o̸͇ṳ̷ h̴͕a̷̻v̸͔e̶̪ a̵͉ S̷̟E̶̞? A̸̖l̷̠l̶̬ t̵̗h̸͙e̷͕s̶͓e̵̩ p̷̱r̸̥o̴̘b̷̝l̶͔e̵̙m̸̞s̷̺ a̶̱r̸̫e̷͈ 1̴͍0̸̲x̶̥ w̷̘o̶͎r̸̦s̵͚e̷̩. O̵̪̓h̷̬͒,̸͉̄ a̶̻̽n̷̮͛d̴̞͝ T̷̹̾e̵̙͌ș̸̑t̶̝̓F̷̙̍l̴̗͆i̵̜͝g̷͚̒h̶̳͛t̵͖̔ w̷͍̍a̵̫̾s̸̖̒ p̷̬̿a̶͚͊r̸͙͗t̵̩͠i̷̜̿a̶͚͊l̷͙͗l̵̩͠y̸̬͋ b̶͔͝ȑ̷̲o̴̰͐k̶̺̐e̷̫͑n̴̹̽ f̷̙͆o̵̳͠r̴̟͗ s̵̡̬̐ę̶̺͝v̵̩͕͊e̷̪̝̽r̴̗͎̍ȁ̸̦̤l̷̢͙͋ m̵̬̹͂o̷̮͉̔n̴͇̦̒t̴̛̲͑h̵͇̼͝s̷̨̬͗ o̴̰̼̍f̸̧̖̪̍ ẗ̵̛͚͎h̶̡̤̳̽i̶̢̯͔̇s̵̗͙͐ ÿ̸̡͚̘́ę̷̮̘̾a̶͇͚͖͌r̶̢̗̺͛.
Nevertheless, one speeds ahead. Probing one’s side projects for an excuse to make the most of one’s new toy. The good news: my one-and-a-half-year-old toddler has began getting really into planes when he sees them in the sky. We’re ready for round 2!
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Aviator v3.0
For anyone unfamiliar with my most popular article ever, I built a skeuomorphic radar toy using SwiftUI, shaders, and open-source APIs to help locate nearby planes in the sky.
My Toddler Loves Planes, So I Built Her A Radar
This is a tale of inspiration, of a vision taking shape, and of rapid feedback from that most discerning of customers: a 2-year-old girl. Keen to skip ahead to the final product? Go ahead and download Aviator — Radar on your Phone from the App Store now!
I re-implemented my radar as a Watch app!
This is a really cool miniature version of the full app, itself a neat SwiftUI wrapper over the OpenSky API.
But it doesn’t feel like an extension of my app. It feels like a little miniature version that runs totally separately. Because it is. This won’t please my dozen users. They have high expectations of me, for some reason. So let’s go back to basics.
At the very least, we should make sure the settings are shared between our two targets. This includes:
🗺️ The map/radar toggle
🇬🇧 The show/hide flags toggle
🔍 The zoom control
🎨 The colour selector
So we need our watch and our phone to talk. Like this:
But, like socially awkward British men (redundant phrasing), getting the two devices to communicate isn’t trivial.
iPhone and Apple Watch are separate systems: two small computers with their own file systems, their own secure enclaves, their own cell radios, and their own separate apps. These twin targets form a distributed system.
Each device is a writer node, both storing and mutating a replica of user settings.
Even the naïve approach introduces thorny state management problems. What happens to the data if my toddler and I modify the same settings on the phone and watch simultaneously? What happens to this data if they can’t connect? What happens when they do reconnect? What if the watch runs out of power, this reconnection takes a week, and an ancient settings change later propagates? Should our settings change happen instantly on each client or do we wait until the data is shared?
The mind boggles.
How am I, a lowly frontend developer, supposed to handle this?!
I threw my MacBook across the room in a fury.
And gasped.
The answer was right in front of me the whole time.
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