rust 75 lines · 8 steps

Substate extraction with FromRef in Axum

How Axum's FromRef lets handlers pull just the piece of app state they need instead of the whole struct.

Explained by highlit
Axum Intermediate state-management routing middleware dependency-injection type-safety
1use std::sync::Arc;
2 
3use axum::{
4 extract::{FromRef, State},
5 http::StatusCode,
6 routing::{get, post},
7 Json, Router,
8};
9use serde::{Deserialize, Serialize};
10 
11#[derive(Clone)]
12struct AppState {
13 db: Arc<PgPool>,
14 auth: AuthState,
15}
16 
17#[derive(Clone)]
18struct AuthState {
19 jwt_secret: Arc<str>,
20 sessions: Arc<SessionStore>,
21}
22 
23impl FromRef<AppState> for AuthState {
24 fn from_ref(state: &AppState) -> Self {
25 state.auth.clone()
26 }
27}
28 
29impl FromRef<AppState> for Arc<PgPool> {
30 fn from_ref(state: &AppState) -> Self {
31 state.db.clone()
32 }
33}
34 
35#[derive(Deserialize)]
36struct LoginRequest {
37 email: String,
38 password: String,
39}
40 
41#[derive(Serialize)]
42struct TokenResponse {
43 token: String,
44}
45 
46fn public_routes() -> Router<AppState> {
47 Router::new()
48 .route("/health", get(|| async { StatusCode::OK }))
49 .route("/auth/login", post(login))
50}
51 
52fn authenticated_routes() -> Router<AppState> {
53 Router::new()
54 .route("/me", get(current_user))
55 .route("/posts", post(create_post))
56 .layer(axum::middleware::from_fn(require_bearer_token))
57}
58 
59pub fn app(state: AppState) -> Router {
60 Router::new()
61 .nest("/api", public_routes().merge(authenticated_routes()))
62 .with_state(state)
63}
64 
65async fn login(
66 State(auth): State<AuthState>,
67 State(db): State<Arc<PgPool>>,
68 Json(body): Json<LoginRequest>,
69) -> Result<Json<TokenResponse>, StatusCode> {
70 let user = User::verify_credentials(&db, &body.email, &body.password)
71 .await
72 .map_err(|_| StatusCode::UNAUTHORIZED)?;
73 let token = auth.issue_token(user.id);
74 Ok(Json(TokenResponse { token }))
75}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1FromRef decouples handlers from the full state struct so each one asks only for the substate it uses.
  2. 2Wrapping shared resources in Arc makes cloning state per-request cheap regardless of what it holds.
  3. 3Splitting routes into groups lets you attach middleware like auth to exactly the endpoints that need it.

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