Logging the body of a request in a tracing layer

[ehamberg]

I have a handler that expects a JSON body and responds with a 422 if the JSON can't be deserialized:

async fn handle(
    State(pool): State<DbPool>,
    Json(push_message): Json<pubsub::PushMessage<MyType>>,
) -> Result<(), MyError>

… and I want to trace the requests to this API.
I can add a tracing layer, but how can I access the body of the request?

I can log requests, but the body will only log Body: Body(Streaming) (even though the body has probably been fully read at that point, since it has been rejected…):

    let app = Router::new()
        .route("/", axum::routing::post(handle))
        .layer(
            TraceLayer::new(
                tower_http::classify::StatusInRangeAsFailures::new(400..=599)
                    .into_make_classifier(),
            )
            .on_request(|request: &hyper::Request<axum::body::Body>, _: &'_ _| {
                tracing::info!("Body: {:?}", request.body());
            }),
        )

Is it possible to log the full body? Ideally, I would only do this when the JSON is rejected (and a 422 is returned), but doing it for all requests would also work for this API.

[davidpdrsn]

The Trace middleware doesn't support that. It's a bit complicated since the body is an async stream, so in order to log it you need to buffer the whole thing, which doesn't work for infinite streams and Trace needs to support those.

You can roll your own tracing middleware pretty easily though and can consume the body like this https://github.com/tokio-rs/axum/blob/main/examples/consume-body-in-extractor-or-middleware/src/main.rs

[ehamberg]

Thanks! I'll have a look at writing my own middleware to do this in the (few) cases where this is needed. The example you linked to doesn't work any more in hyper 1.x, but it should be easy enough to adjust. :)