comprehensive-rust/src/async/control-flow/join.md

# Join

A join operation waits until all of a set of futures are ready, and
returns a collection of their results. This is similar to `Promise.all` in
JavaScript or `asyncio.gather` in Python.

```rust,editable,compile_fail
use anyhow::Result;
use futures::future;
use reqwest;
use std::collections::HashMap;

async fn size_of_page(url: &str) -> Result<usize> {
    let resp = reqwest::get(url).await?;
    Ok(resp.text().await?.len())
}

#[tokio::main]
async fn main() {
    let urls: [&str; 4] = [
        "https://google.com",
        "https://httpbin.org/ip",
        "https://play.rust-lang.org/",
        "BAD_URL",
    ];
    let futures_iter = urls.into_iter().map(size_of_page);
    let results = future::join_all(futures_iter).await;
    let page_sizes_dict: HashMap<&str, Result<usize>> =
        urls.into_iter().zip(results.into_iter()).collect();
    println!("{:?}", page_sizes_dict);
}
```

<details>

Copy this example into your prepared `src/main.rs` and run it from there.

* For multiple futures of disjoint types, you can use `std::future::join!` but
  you must know how many futures you will have at compile time. This is
  currently in the `futures` crate, soon to be stabilised in `std::future`.

* The risk of `join` is that one of the futures may never resolve, this would
  cause your program to stall. 

* You can also combine `join_all` with `join!` for instance to join all requests
  to an http service as well as a database query. Try adding a
  `tokio::time::sleep` to the future, using `futures::join!`. This is not a
  timeout (that requires `select!`, explained in the next chapter), but demonstrates `join!`.

</details>