The Beginner's Guide to Kotlin Coroutine Internals

Sonic Wang from DoorDash breaks down Kotlin coroutines from bytecode to scheduler. If you've ever wondered what actually happens when you call 'suspend', this deep dive reveals the magic.

DoorDash migrated from Python to Kotlin for backend services, requiring engineers to master coroutines quickly. This guide examines coroutines through three lenses: language compilation, standard library classes, and runtime scheduling. Wang uses bytecode inspection and decompilation to show exactly how coroutines work under the hood.

Key Takeaways

• Coroutines have smaller memory footprint and lower context switching overhead than Java threads
• Kotlin compiler uses switch statements and labeled code blocks to implement suspend/resume
• Default dispatcher uses work stealing scheduler with N/P expected workload per processor
• Each suspend function call becomes a case in a control flow switch statement
• Local variables are captured at compile time as continuation class member objects

The article reveals why Dispatchers.Unconfined behaves differently only until the first suspension point, and what that means for your code.