[StackOverflow/rust] How do I make asynchronous parallel requests in a separate thread?

### ROOT CAUSE The issue arises from a misunderstanding of how Rust handles asynchronous programming. The user may be conflating async/await with multithreading, assuming that async functions automatically run in separate threads. In Rust, async functions execute on a single thread (managed by a runtime like Tokio), and parallelism requires explicit multithreading or async concurrency. --- ### CODE FIX To make asynchronous parallel requests in Rust, use the `tokio` runtime for async operations and `std::thread` for true parallelism (if needed). Here's a step-by-step solution: 1. **Add Dependencies**: Include `tokio` and `reqwest` (for HTTP requests) in your `Cargo.toml`: ```toml [dependencies] tokio = { version = "1.0", features = ["full"] } reqwest = { version = "0.11", features = ["json"] } tokio-util = { version = "0.7" } ``` 2. **Use `tokio::task::spawn` for Async Parallelism**: ```rust use tokio::task; async fn fetch_data(url: &str) -> Result> { let response = reqwest::get(url).await?; Ok(response.text().await?) } #[tokio::main] async fn main() -> Result<(), Box> { let urls = vec![ "https://api.example.com/data1", "https://api.example.com/data2", ]; let mut handles = vec![]; for url in &urls { let handle = task::spawn(async { fetch_data(url).await }); handles.push(handle); } // Wait for all tasks to complete for handle in handles { handle.await?; } Ok(()) } ``` 3. **For True Parallelism (CPU-bound tasks)**: If you need to run blocking code in parallel threads, use `rayon`: ```rust use rayon::prelude::*; fn blocking_fetch(url: &str) -> String { // Blocking code here (e.g., thread pool managed by Rayon) reqwest::get(url).unwrap().text().unwrap() } fn main() { let urls = vec!["url1", "url2"]; urls.par_iter().for_each(|url| { let data = blocking_fetch(url); // Process data }); } ``` **Explanation**: - The first example uses `tokio::task::spawn` to run async tasks concurrently on a thread pool. - The second example uses `rayon` for true parallelism with blocking operations. Choose the approach based on whether your tasks are I/O-bound (use `tokio`) or CPU-bound (use `rayon`).