Kotlin Coroutines have become one of the most important features of modern Kotlin development—especially across Android applications, backend services using Ktor or Spring, and even desktop or server-side tools. They offer developers a clean, elegant way to write asynchronous, concurrent code that is easy to read and reason about.

If you’re new to Kotlin or coming from Java, JavaScript, or another language, coroutines can feel a bit magical. They allow long-running work to be suspended without blocking threads, freeing your system to do more with less. In this article, we’ll walk step-by-step through what coroutines are, why they exist, how they work internally, and how to get started with them.

This introduction is designed for learners. We’ll use beginner-friendly explanations and code samples with clear commentary.

What Are Coroutines? Why They Were Introduced

Before Kotlin introduced coroutines, asynchronous programming in Android and Java used tools like:

• Threads
• Callbacks
• Executors
• AsyncTask (which is now deprecated)
• RxJava (still popular but complex for beginners)

These approaches work, but they introduce several problems:

1. Callback Hell

Callbacks inside callbacks inside more callbacks make code unreadable:

fetchUser { user ->
    fetchPosts(user.id) { posts ->
        fetchComments(posts) { comments ->
            // ...
        }
    }
}

2. Thread Creation Is Expensive

Creating lots of threads wastes memory and CPU resources.

3. Difficult Error Handling

Managing exceptions over multiple async operations is complicated.

4. Hard to Cancel Work

Stopping long-running tasks gracefully is difficult.

Coroutines were introduced to fix these issues

A coroutine is a lightweight, cooperative unit of work that can be suspended and resumed without blocking threads.

Key ideas:

• Coroutines allow asynchronous code to look synchronous.
• They use suspending functions instead of callbacks.
• They run on top of threads but are much cheaper to create and manage.
• Kotlin’s structured concurrency helps you manage lifecycles automatically.

Threads vs Coroutines

To understand the power of coroutines, we must compare them to threads.

Threads

• Provided by the operating system.
• Heavyweight: each thread requires megabytes of memory.
• Creating hundreds or thousands is expensive.
• Blocking a thread wastes system resources.
• Switching between threads (context switching) is costly.

Coroutines

• Managed by the Kotlin runtime.
• Lightweight: only need a few KB of memory.
• You can launch thousands of coroutines.
• Suspending a coroutine does not block the underlying thread.
• They cooperate instead of preemptively interrupting each other.

Let’s illustrate this:

// Launching 100,000 threads - likely to crash or freeze
repeat(100_000) {
    Thread {
        Thread.sleep(1000)
    }.start()
}

// Launching 100,000 coroutines - works fine
runBlocking {
    repeat(100_000) {
        launch {
            delay(1000)
        }
    }
}

Why do coroutines survive but threads don’t?

When you call delay(1000), the coroutine suspends itself, freeing the thread to do other work. No OS-level blocking happens.

How Coroutines Work Under the Hood

Coroutines seem magical: you can “pause” code with delay() and resume later. But under the hood, it’s all based on a few mechanisms.

Let’s break it down.

1. Suspending Functions

A suspending function can pause a coroutine without blocking the thread.

suspend fun fetchUser(): User { ... }

The compiler transforms this into a state machine internally.

2. Continuations

A continuation contains the state of a coroutine:

• Where to resume
• Local variables
• Exception handlers

When a coroutine suspends, Kotlin stores its continuation, then resumes it later.

3. Dispatchers

Coroutines choose which thread or thread pool to run on through dispatchers:

• Dispatchers.Main – UI thread (Android)
• Dispatchers.IO – disk or network work
• Dispatchers.Default – CPU-intensive tasks
• Dispatchers.Unconfined

4. Event Loop

Coroutines often resume through a small event loop that decides:

• Which coroutine to run
• When to schedule work on a dispatcher

This makes coroutines highly efficient.

5. Suspension vs Blocking

Blocking means:

Thread stops here and waits.

Suspension means:

Coroutine pauses here, thread is free to do something else.

This difference is what gives coroutines their power.

Example: How delay() works

delay(1000)

• Registers a timer
• Suspends coroutine
• Releases thread
• When timer finishes, coroutine resumes

No thread is blocked!

Structured Concurrency Overview

Early async programming suffered from “fire-and-forget” tasks.

Kotlin introduced structured concurrency to fix this.

Key Principles

1. Every coroutine must belong to a scope
   Examples:
   • GlobalScope (avoid if possible)
   • runBlocking
   • CoroutineScope
   • lifecycleScope (Android)
   • viewModelScope (Android)
2. Parent coroutines manage children
   • When a parent is cancelled, all children are cancelled.
   • No leaking background tasks.
3. Exception propagation works predictably
   • Child exceptions propagate to parents unless handled.

Example: Structured Concurrency in action

runBlocking {
    val job = launch {
        launch { delay(1000); println("Child 1 done") }
        launch { delay(500); println("Child 2 done") }
    }

    delay(200)
    job.cancel() // Cancels both child coroutines
}

This ensures you cannot accidentally create “zombie tasks” that keep running in the background.

Why it matters

• Android Activities and Fragments get destroyed.
• Backend requests should cancel work when clients disconnect.
• Predictable cleanup prevents memory leaks.

Why Coroutines Matter in Android and Backend Systems

Coroutines are now the recommended way to handle async work in Android and Kotlin backend frameworks.

1. On Android

Coroutines simplify:

• Networking (Retrofit supports suspend functions)
• Database operations (Room supports suspend)
• UI updates (Main dispatcher)
• Lifecycle-aware cancellation (viewModelScope)

Example:

viewModelScope.launch {
    val user = repository.getUser()  // suspend function
    _state.value = user
}

This code looks synchronous, but it’s async and non-blocking.

2. In Backend (Ktor, Spring WebFlux)

Coroutines help handle thousands of network requests using very few threads. This is critical for scalable systems.

Example Ktor route:

get("/users/{id}") {
    val id = call.parameters["id"]!!.toInt()
    val user = userService.getUserById(id)   // suspend
    call.respond(user)
}

Each HTTP request becomes a coroutine instead of a thread.

3. Coroutines > RxJava for most cases

• Easier to read
• Native Kotlin support
• Structured concurrency
• Better tooling (e.g., in Android Studio)
• Cleaner error handling

First Coroutine Example with runBlocking

Let’s build your first coroutine program using runBlocking.

runBlocking is mainly for:

• Running coroutines in main()
• Learning / examples
• Testing

It blocks the current thread until all its child coroutines complete.

Complete Example

import kotlinx.coroutines.*

fun main() = runBlocking {
    println("Start - Thread: ${Thread.currentThread().name}")

    val job = launch {
        println("Coroutine started - Thread: ${Thread.currentThread().name}")
        delay(1000)
        println("Coroutine finished")
    }

    println("Waiting for coroutine...")
    job.join()

    println("End")
}

Explanation

1. runBlocking starts a coroutine scope
   • It blocks the main thread until all coroutines inside it finish.
2. launch { ... } starts a new coroutine
   • This coroutine runs concurrently inside the runBlocking scope.
3. delay(1000) suspends the coroutine
   • The main thread is NOT blocked.
   • After 1 second, the coroutine resumes.
4. job.join() waits for the child coroutine
   • Ensures program doesn’t exit early.

Output example

Start - Thread: main
Coroutine started - Thread: main
Waiting for coroutine...
Coroutine finished
End

Conclusion

Kotlin coroutines represent one of the biggest advancements in the Kotlin ecosystem and modern concurrent programming. They allow developers to write asynchronous code that is easy to understand and maintain. Unlike traditional threads, coroutines are lightweight, scalable, and deeply integrated with Kotlin’s syntax and toolchain.