With config in place, we can bring up an actual server. We split this across two files: main.rs, which owns startup and shutdown, and app.rs, which owns building the Router itself. Keeping build_app separate from main gives us a clean composition boundary. Later, integration tests can construct the application directly without starting a real TCP listener.

crates/pokemon-api/src/app.rs
use axum::Router;


pub fn build_app() -> Router {
    Router::new()
}

build_app doesn’t do anything asynchronous yet, so it stays a plain synchronous function rather than async fn. It also can’t fail today, so it returns Router directly instead of anyhow::Result<Router>. Async initialization, and anything that can fail, belongs in main; building the router itself is just composition.

crates/pokemon-api/src/main.rs
use tokio::net::TcpListener;


use pokemon_api::app::build_app;
use pokemon_api::config::AppConf;


#[tokio::main]
async fn main() -> anyhow::Result<()> {
    tracing_subscriber::fmt::init();


    let config = AppConf::init()?;
    let addr = config.server.addr();


    let listener = TcpListener::bind(addr).await?;


    tracing::info!(%addr, "Listening");


    let app = build_app();


    axum::serve(listener, app).await?;


    Ok(())
}

We reach for Tokio’s TcpListener here, not std::net::TcpListener. Both can bind a socket, but Tokio’s version does it asynchronously and hands back a listener axum::serve can poll without blocking the runtime, which is the natural pairing with everything else in this function already being async.

main reads config, binds a listener to the address it produces, builds the app, and hands both off to axum::serve. Nothing here is Pokemon-specific yet. The routes that make this a Pokemon API get merged into build_app starting with the next part.

Terminal window
$ make run
INFO Listening addr=0.0.0.0:8080

In another terminal:

Terminal window
$ curl -i http://localhost:8080
HTTP/1.1 404 Not Found

A 404 is the right outcome here. There are no routes yet, so nothing should match, but the response coming back at all proves the whole path works: config produced a SocketAddr, that address became a TcpListener, and Axum served an empty Router over it. Part 4 gives that router its first real route.