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.

// Copyright 2024 Google LLC
// SPDX-License-Identifier: Apache-2.0

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:?}");
}

This slide should take about 4 minutes.

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!.

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