What is the best way to handle large file downloads with Reqwest?

Handling large file downloads efficiently is important to ensure your application does not run out of memory and to provide a smooth user experience. When using Reqwest, a popular HTTP client for Rust, you should stream the file content instead of loading the entire file into memory at once.

Here's an example of how to handle large file downloads using Reqwest with asynchronous Rust:

use std::fs::File;
use std::io::Write;
use reqwest::Client;
use tokio::io::AsyncWriteExt;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    // URL of the large file you want to download
    let url = "http://example.com/largefile.zip";

    // Initialize an HTTP client
    let client = Client::new();

    // Start the GET request
    let mut response = client.get(url).send().await?;

    // Ensure the request was successful
    if response.status().is_success() {
        // Open a file to write the stream to
        let mut file = File::create("largefile.zip").await?;

        // Stream the response body and write it to the file chunk by chunk
        while let Some(chunk) = response.chunk().await? {
            file.write_all(&chunk).await?;
        }

        println!("File downloaded successfully.");
    } else {
        eprintln!("Download error: {}", response.status());
    }

    Ok(())
}

Here's a step-by-step breakdown of what the example code does:

1. We set up an asynchronous Rust environment using Tokio, which is a runtime for asynchronous Rust applications.
2. We specify the URL of the large file we want to download.
3. We create a new instance of reqwest::Client.
4. We make a GET request to the URL and await the response.
5. We check if the response status indicates success.
6. If successful, we create a new file to write the downloaded content. Note that we're using the asynchronous version of File::create.
7. We loop through the response body in chunks using response.chunk().await?. In each iteration, we write the chunk to the file using file.write_all(&chunk).await?. This ensures that we only hold a small part of the file in memory at any given time.
8. If the download is successful, we print a success message; otherwise, we print an error message.

It's important to note that the chunk() method on the Response object asynchronously retrieves a chunk of the body of the response. This allows us to handle the response body as a stream, which is ideal for large file downloads.

Make sure to add the required dependencies in your Cargo.toml:

[dependencies]
reqwest = { version = "0.11", features = ["json", "stream"] }
tokio = { version = "1", features = ["full"] }

When dealing with large files, you might also want to consider other factors like error handling, logging progress, handling network errors, and resuming interrupted downloads. You can incorporate these features depending on your application requirements.