How to launch a (Rust)Actix-web server as a lib from another thread

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05-10-2024

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Launching an Actix-web Server as a Library from Another Thread in Rust

Problem: You've built a powerful Rust application using the Actix-web framework. Now, you want to integrate this web server into a larger project, perhaps running it as a separate component or as part of a multi-threaded application. How do you launch and manage your Actix-web server as a library from a different thread?

Rephrased: Imagine you have a complex machine that requires a separate engine to function. You need to start this engine and ensure it runs smoothly while the machine performs other tasks. How can you safely and efficiently control this engine (your Actix-web server) from within your larger machine (your main application)?

Scenario:

Let's assume you have a simple Actix-web server defined in a module named web_server:

use actix_web::{App, HttpServer, web};

pub async fn index() -> String {
    String::from("Hello, World!")
}

pub async fn start_server() {
    HttpServer::new(|| App::new()
        .route("/", web::get().to(index))
    )
    .bind("127.0.0.1:8080")?
    .run()
    .await;
}

Now, you want to launch this server from another thread in your main application:

use std::thread;

mod web_server;

fn main() {
    let handle = thread::spawn(move || {
        web_server::start_server().await;
    });

    // ... Other main application logic ...

    handle.join().unwrap();
}

This code seems simple, but it has a major flaw: the main thread will block indefinitely while waiting for the server thread to complete. This defeats the purpose of using separate threads and prevents your application from progressing.

Analysis and Solution:

The key to successfully launching an Actix-web server as a library is to utilize asynchronous programming techniques and handle the server lifecycle correctly. We can achieve this by:

1. Using a dedicated tokio::runtime: Instead of relying on the default runtime, create a separate runtime for the web server thread. This ensures that the server's event loop operates independently.

2. Creating a custom Future: Wrap the HttpServer::run() call inside a Future that can be awaited separately. This allows us to monitor and control the server's lifecycle outside the main application.

Here's an improved version of the start_server function incorporating these changes:

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use actix_web::{App, HttpServer, web};
use tokio::runtime;

pub async fn index() -> String {
    String::from("Hello, World!")
}

pub fn start_server() -> impl Future<Output = ()> {
    let server = HttpServer::new(|| App::new()
        .route("/", web::get().to(index))
    )
    .bind("127.0.0.1:8080")
    .unwrap();

    async move {
        // Create a dedicated runtime for the web server
        let rt = runtime::Builder::new_multi_thread()
            .enable_all()
            .build()
            .unwrap();

        // Run the server within the dedicated runtime
        rt.block_on(async {
            server.run().await.unwrap();
        });
    }
}

Main application:

use std::thread;
use tokio::runtime;

mod web_server;

fn main() {
    let handle = thread::spawn(move || {
        runtime::Builder::new_multi_thread()
            .enable_all()
            .build()
            .unwrap()
            .block_on(web_server::start_server());
    });

    // ... Other main application logic ...

    handle.join().unwrap();
}

Explanation:

• The start_server function now returns an impl Future<Output = ()> instead of an async fn. This allows us to create and manage the future independently in the main application.
• We create a new tokio::runtime within the start_server function. This runtime provides the necessary resources for the Actix-web server to function correctly.
• We use rt.block_on to execute the server.run() future within the dedicated runtime, preventing blocking the main thread.

Additional Value:

This approach ensures that your Actix-web server operates on its own thread without hindering other operations within your application. It also provides flexibility for controlling the server's lifecycle, including graceful shutdown or restarting if needed.

References:

• Actix-web Documentation
• Tokio Documentation

By understanding the intricacies of asynchronous programming and utilizing proper threading and runtime management, you can successfully launch an Actix-web server as a library within a larger Rust application.