tomevault-io Axum Core Architecture
Overview
Axum is an async web framework for Rust, built and maintained by the Tokio team. ALWAYS start from the founding fact: Axum implements almost nothing itself. It is a thin composition over three lower layers, tokio, hyper, and tower. Every architectural question about Axum resolves to "which layer owns this".
Core principle: routing is data and handlers are ordinary async fns. There are no
routing macros. The single macro a minimal Axum app uses is #[tokio::main], and that
macro belongs to tokio, not to Axum.
This skill covers the composition model, the request lifecycle, the no-macros design,
axum::serve, the axum-core crate boundary, and when to choose Axum. For the router
API see axum-core-router. For handler eligibility see axum-syntax-handlers. For
diagnosing handler errors see axum-errors-handler-trait.
Quick Reference
The three layers Axum composes
| Layer | Crate | Owns |
|---|---|---|
| Runtime | tokio | executor, TcpListener, timers, tokio::sync, task scheduling |
| HTTP | hyper 1.x | HTTP/1 and HTTP/2, parsing socket bytes into http::Request<Body> |
| Service abstraction | tower | the Service trait, the universal async fn(Request) -> Result<Response, Error>, and the tower / tower-http middleware ecosystem |
Axum itself adds the Router, the concrete extractors, the Handler trait,
IntoResponse, and axum::serve. Because everything routable in Axum is a
tower::Service, Axum inherits timeouts, tracing, compression, CORS, and rate limiting
for free. NEVER reimplement middleware that tower or tower-http already provides.
Axum 0.7 and later require hyper ^1.1.0 and http ^1.0.0. This is true for both 0.7
and 0.8.
Minimal app (axum 0.7 and 0.8, identical)
// axum 0.7 and 0.8: this entry point is byte-for-byte identical.
use axum::{routing::get, Router};
#[tokio::main]
async fn main() {
let app = Router::new().route("/", get(|| async { "Hello, World!" }));
let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await.unwrap();
axum::serve(listener, app).await.unwrap();
}
NEVER add a 0.7-vs-0.8 version split to the entry point. axum::serve was introduced
in 0.7 as the replacement for the removed axum::Server, and its shape did not change
across the 0.7 to 0.8 boundary.
The axum::serve signature (verified, identical 0.7 and 0.8)
pub fn serve<L, M, S>(listener: L, make_service: M) -> Serve<L, M, S>
where
L: Listener,
M: for<'a> Service<IncomingStream<'a, L>, Error = Infallible, Response = S>,
S: Service<Request, Response = Response, Error = Infallible> + Clone + Send + 'static,
S::Future: Send;
ALWAYS remember the Serve future "will never actually complete or return an error".
main runs an infinite server until the process is killed or a graceful-shutdown
signal future resolves. A main that returns immediately means the runtime was never
blocked on serve. See references/methods.md for the full Serve API.
The request lifecycle
tokio::net::TcpListeneraccepts an incoming TCP connection.- hyper reads bytes off the socket and parses them into
http::Request<Body>. axum::servedrives theRouteras atower::Serviceby callingService::call.- The
Routermatches the request path and selects aMethodRouter. - The
MethodRouterselects the handler for the request's HTTP method, or its 405 fallback when no method matches. - Each handler argument runs its extractor:
FromRequestPartsfor parts-only extractors,FromRequestfor the single body extractor, which MUST be last. - The handler
async fnbody runs and returns a value. IntoResponse::into_responseturns that value intohttp::Response<Body>.- hyper serializes the response back to socket bytes.
Middleware (tower::Layer and tower::Service) wraps steps 3 through 8: an outer
layer sees the request before the router and the response after the handler.
Decision Trees
When to choose Axum
Need a Rust web framework?
|- Want a framework maintained by the Tokio team with tight hyper
| integration and the full tower / tower-http middleware ecosystem? -> Axum
|- Want routing as composable data, unit-testable with no macro expansion? -> Axum
|- Specifically want attribute-macro routing such as #[get("/path")]? -> Rocket or
| Actix Web
|- Need a non-tokio async runtime? -> not Axum
ALWAYS expect that Axum gives building blocks, not a full stack. There is no
built-in auth, ORM, or templating engine. ALWAYS expect handler futures to be Send,
because tokio's multi-threaded scheduler can migrate tasks between worker threads, so
!Send types such as Rc, RefCell, and std::sync::MutexGuard cannot be held
across .await.
Dependency choice: axum vs axum-core
What are you building?
|- An application binary, a runnable server? -> depend on `axum`
|- A reusable library whose only Axum surface is impls of
| FromRequest / FromRequestParts / IntoResponse? -> depend on `axum-core`
The official rule, verbatim: "Libraries authors that want to provide FromRequest,
FromRequestParts, or IntoResponse implementations should depend on the axum-core
crate, instead of axum if possible." Depending on axum-core insulates a library
from breaking changes in the router and extractor surface.
Diagnosing the cryptic Handler error
Compiler says: the trait bound `fn(..) {handler}: Handler<_, _>` is not satisfied
|- STEP 1: add #[axum::debug_handler] above the handler (needs the `macros` feature)
|- STEP 2: recompile and read the precise plain-language error it emits
|- STEP 3: fix the root cause it names. The 5 causes are:
|- an argument is not an extractor, for example a bare `bool`
|- a body extractor is not the last handler argument
|- the return type does not implement IntoResponse
|- the function is not declared `async`
|- the future is !Send, an Rc / RefCell / std::sync::MutexGuard held across .await
NEVER guess the cause from the raw error. The no-macros design concentrates every
handler-shape mistake into this one message, and #[debug_handler] is the
deterministic tool to decode it. Full diagnosis lives in axum-errors-handler-trait.
Patterns
Pattern: Axum is a composition, not a monolith
ALWAYS map a feature to the layer that owns it before reaching for an Axum-specific
answer. Sockets, timers, and spawn_blocking belong to tokio. HTTP/1 and HTTP/2 wire
behavior belongs to hyper. Middleware, timeouts, tracing, compression, and CORS belong
to tower and tower-http. Axum owns only routing, extraction, handler dispatch, and
response conversion. This map prevents reimplementing what a lower layer already
provides and explains why an Axum app pulls in tokio, hyper, and tower transitively.
Pattern: the no-macros design and its one trade-off
Routes are ordinary values: Router::new().route("/", get(handler)) is plain function
calls producing data. Handlers are ordinary async fns whose eligibility is enforced
purely by the Handler trait bounds, not by an attribute macro. The benefit is that
routing composes, returns from functions, merges, nests, and unit-tests cleanly, and
no macro expansion obscures compiler errors.
The trade-off: when a function fails the Handler trait bounds, the compiler emits the
cryptic Handler is not satisfied error that does not name the failing requirement.
The mitigation is the one macro Axum ships for this, #[debug_handler] from the
axum-macros crate (also re-exported as axum::debug_handler, requires the macros
feature). It generates a precise diagnostic and is a development-only aid.
Pattern: axum::serve is the glue
axum::serve binds a tokio::net::TcpListener to a make-service. A Router<()> is
accepted directly; a MethodRouter or Handler becomes a make-service via
.into_make_service(). ALWAYS supply all required state with .with_state(...) before
serving, because only a Router<()> is serveable. axum::serve requires the tokio
feature plus one of http1 or http2; these are on by default and matter only when
default-features = false is set.
For deployments add .with_graceful_shutdown(signal), where signal is a future that
resolves on ctrl_c() and on the Unix SIGTERM signal. See references/examples.md
and the axum-impl-deployment skill.
Pattern: the axum-core crate boundary
axum-core is a separate crate holding the foundational traits FromRequest,
FromRequestParts, IntoResponse, IntoResponseParts, and the Error type. The
axum crate depends on axum-core and adds the router, the concrete extractors, and
serve. An application binary depends on axum. A reusable library that only
implements core traits depends on axum-core.
Pattern: a Router IS a tower::Service
A fully configured Router<()> implements tower::Service. This is why a router can
be passed to axum::serve, nested inside another router, merged, and tested via
tower::ServiceExt::oneshot with no extra crate. ALWAYS treat the router as a service
value, not as a special framework object. See axum-impl-testing for the oneshot
test pattern.
Common Mistakes
| Mistake | Fix |
|---|---|
Adding a 0.7-vs-0.8 cfg split around axum::serve |
The entry point is identical on both versions, write it once |
Expecting axum::serve(...).await to return |
It never completes, main blocks on it until the process is killed |
Using axum::Server on 0.8 |
axum::Server was removed in 0.8, use axum::serve |
| Reimplementing a timeout or trace layer by hand | Use the tower-http layer, Axum inherits the tower ecosystem |
Guessing at Handler is not satisfied |
Add #[debug_handler] and read the precise error |
A custom-extractor library depending on full axum |
Depend on axum-core instead |
Full anti-pattern analysis with root causes is in references/anti-patterns.md.
Reference Links
references/methods.md: complete API signatures foraxum::serve, theServestruct,into_make_service,#[tokio::main],#[debug_handler], and theaxum-coreexports, with feature gates.references/examples.md: working, version-annotated code for the minimal app, graceful shutdown, a router used as atower::Service,#[debug_handler], anaxum-corelibrary extractor, and theaxum::Servertoaxum::servemigration.references/anti-patterns.md: real architecture mistakes with "why this fails" explanations and fixes.
Related skills:
axum-core-router: theRouterAPI, route matching, nesting, merging, fallbacks.axum-core-state:State<S>,with_state,FromRefsubstate composition.axum-syntax-handlers: theHandlertrait and what makes a valid handler.axum-errors-handler-trait: full diagnosis ofHandler is not satisfied.axum-core-version-migration: the complete 0.7 to 0.8 breaking-change matrix.axum-impl-deployment: graceful shutdown and production entry points.
Source: Impertio-Studio/Axum-Claude-Skill-Package — distributed by TomeVault.