Tokio Async in Rust: tokio::join! vs tokio::spawn()

Rust’s Tokio async runtime provides two common ways to execute multiple async tasks concurrently: tokio::join! and tokio::spawn(). Choosing the right one is crucial for efficiency and proper task management. In this article, we’ll break down their differences, advantages, and when to use each with examples.

Understanding `tokio::join!`

tokio::join! is used when you need structured concurrency—where all async tasks must complete together before moving on.

How It Works

Executes multiple futures concurrently in the same parent task.
If one task blocks, all other tasks wait.
Returns all results at once.

Example Usage

use tokio::join;

async fn task1() -> String {
    "Task 1 completed".to_string()
}

async fn task2() -> String {
    "Task 2 completed".to_string()
}

#[tokio::main]
async fn main() {
    let (res1, res2) = join!(task1(), task2());
    println!("{}\n{}", res1, res2);
}

✅ Use case: When all tasks must complete before proceeding.

Understanding `tokio::spawn()`

tokio::spawn() runs each async function in a separate Tokio task, meaning they execute independently.

How It Works

Tasks run truly in parallel on Tokio’s scheduler.
If one task hangs or panics, others continue running.
Returns a JoinHandle<T>, which you must .await.

Example Usage

use reqwest::Client;
use tokio::task::JoinHandle;

const URLS: [&str; 3] = [
    "https://jsonplaceholder.typicode.com/todos/1",
    "https://jsonplaceholder.typicode.com/todos/2",
    "https://jsonplaceholder.typicode.com/todos/3",
];

#[tokio::main]
async fn main() {
    let client = Client::new();

    let handles: Vec<JoinHandle<()>> = URLS.iter()
        .map(|&url| {
            let client = client.clone();
            tokio::spawn(async move {
                if let Err(e) = fetch_url(&client, url).await {
                    eprintln!("Error fetching {}: {:?}", url, e);
                }
            })
        })
        .collect();

    for handle in handles {
        let _ = handle.await; // Ensure all tasks finish
    }
}

async fn fetch_url(client: &Client, url: &str) -> Result<(), reqwest::Error> {
    let res = client.get(url).send().await?;
    let content = res.text().await?;
    println!("{:?}", content);
    Ok(())
}

✅ Use case: When tasks should run independently and in the background.

Comparison: `tokio::join!` vs `tokio::spawn()`

Feature	`tokio::join!`	`tokio::spawn()`
Task Execution	Runs in the same parent task	Runs in separate Tokio tasks
Blocking Behavior	If one task hangs, all are delayed	If one task hangs, others continue
Error Handling	If one task panics, all are aborted	If one task panics, others keep running
Return Values	Returns all task results	Returns a `JoinHandle<T>` that you must `.await`
Overhead	Slightly lower (same task)	Slightly higher (separate tasks)
Use Case	When all tasks must complete together	When tasks should run independently

Which One Should You Use?

✅ Use tokio::join! when:
- You need all tasks to finish together before moving forward.
- Your tasks depend on each other.
- Example: Fetch multiple API responses, then process them together.
✅ Use tokio::spawn() when:
- Each task should run independently.
- A task may take longer, but others shouldn’t wait.
- Example: Handling multiple user requests in a web server.

Final Thoughts

Both tokio::join! and tokio::spawn() have their strengths. Your choice depends on whether tasks should complete together (join!) or run independently (spawn()). Understanding their differences will help you write more efficient and scalable Rust async applications.