Coroutines introduction

Coroutines are lightweight concurrent tasks designed for structured async I/O—Kotlin's alternative to callback chains in JavaScript and thread pools in Java. On Gradle projects you use kotlinx-coroutines; the playground simulates concurrency with threads.

Concepts

• suspend functions can pause without blocking a thread
• CoroutineScope and structured concurrency cancel child work when parents fail
• async/await and launch schedule work on dispatchers

runBlocking (local / Gradle only)

In Android Studio or a Gradle module with coroutines on the classpath, runBlocking { } bridges blocking main to suspend functions for demos and tests. Do not use runBlocking on production request threads—prefer suspend all the way up.

// Local Gradle — not in playground
runBlocking {
    val result = fetchUser()
    println(result)
}

This lesson's runnable code uses kotlin.concurrent.thread and Thread.sleep to preview overlapping work without the coroutines library.

Important interview questions and answers

1. Q: Coroutine vs thread?
   A: Coroutines are cooperatively scheduled on thread pools—cheaper to spawn thousands for I/O-bound work; threads are OS resources with higher overhead.
2. Q: What is runBlocking for?
   A: Blocking entry point that runs a coroutine scope to completion—fine for main and tests, not for reactive server threads.

Self-check

1. Why avoid runBlocking on a servlet thread?
2. What keyword marks a function that can suspend?

Tip: Real projects add kotlinx-coroutines and use runBlocking only in main or tests—not on HTTP worker threads.