axum-code-review

Axum Code Review

Review Workflow

1. Check Cargo.toml — Note axum version (0.6 vs 0.7+ have different patterns), tower, tower-http features
2. Check routing — Route organization, method routing, nested routers
3. Check extractors — Order matters (body extractors must be last), correct types
4. Check state — Shared state via State<T>, not global mutable state
5. Check error handling — IntoResponse implementations, error types

Output Format

Report findings as:

[FILE:LINE] ISSUE_TITLE
Severity: Critical | Major | Minor | Informational
Description of the issue and why it matters.

Quick Reference

Issue Type	Reference
Route definitions, nesting, method routing	references/routing.md
State, Path, Query, Json, body extractors	references/extractors.md
Tower middleware, layers, error handling	references/middleware.md

Review Checklist

Routing

• Routes organized by domain (nested routers for /api/users, /api/orders)
• Fallback handlers defined for 404s
• Method routing explicit (.get(), .post(), not .route() with manual method matching)
• No route conflicts (overlapping paths with different extractors)

Extractors

• Body-consuming extractors (Json, Form, Bytes) are the LAST parameter
• State<T> requires T: Clone — typically Arc<AppState> or direct Clone derive
• Path<T> parameter types match the route definition
• Query<T> fields are Option for optional query params with #[serde(default)]
• Custom extractors implement FromRequestParts (not body) or FromRequest (body)

State Management

• Application state shared via State<T>, not global mutable statics
• Database pool in state (not created per-request)
• State contains only shared resources (pool, config, channels), not request-specific data
• Clone derived or manually implemented on state type

Error Handling

• Handler errors implement IntoResponse for proper HTTP error codes
• Internal errors don't leak to clients (no raw error messages in 500 responses)
• Error responses use consistent format (JSON error body with code/message)
• Result<impl IntoResponse, AppError> pattern used for handlers

Middleware

• Tower layers applied in correct order (outer runs first on request, last on response)
• tower-http used for common concerns (CORS, compression, tracing, timeout)
• Request-scoped data passed via extensions, not global state
• Middleware errors don't panic — they return error responses

Severity Calibration

Critical

• Body extractor not last in handler parameters (silently consumes body, later extractors fail)
• SQL injection via path/query parameters passed directly to queries
• Internal error details leaked to clients (stack traces, database errors)
• Missing authentication middleware on protected routes

Major

• Global mutable state instead of State<T> (race conditions)
• Missing error type conversion (raw sqlx::Error returned to client)
• Missing request timeout (handlers can hang indefinitely)
• Route conflicts causing unexpected 405s

Minor

• Manual route method matching instead of .get(), .post()
• Missing fallback handler (default 404 is plain text, not JSON)
• Middleware applied per-route when it should be global (or vice versa)
• Missing tower-http::trace for request logging

Informational

• Suggestions to use tower-http layers for common concerns
• Router organization improvements
• Suggestions to add OpenAPI documentation via utoipa or aide

Valid Patterns (Do NOT Flag)

• #[axum::debug_handler] on handlers — Debugging aid that improves compile error messages
• Extension<T> for middleware-injected data — Valid pattern for request-scoped values
• Returning impl IntoResponse from handlers — More flexible than concrete types
• Router::new() per module, merged in main — Standard organization pattern
• ServiceBuilder for layer composition — Tower pattern, not over-engineering
• axum::serve with TcpListener — Standard axum 0.7+ server setup

Before Submitting Findings

Load and follow beagle-rust:review-verification-protocol before reporting any issue.