tracing

tracing provides structured, contextual logging for async Rust. Spans follow requests across tasks; fields are typed and filterable at runtime.

Recipe

[dependencies]
tracing = "0.1"
tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }

use tracing::{info, instrument};
 
#[instrument]
async fn fetch_user(user_id: u64) -> String {
    info!(%user_id, "fetching user");
    format!("user-{user_id}")
}
 
#[tokio::main]
async fn main() {
    tracing_subscriber::fmt()
        .with_env_filter("info")
        .init();
    fetch_user(42).await;
}

When to reach for this:

• Production services needing request IDs and latency fields
• Async code where println! loses ordering
• OpenTelemetry export via tracing-opentelemetry

Working Example

use tracing::{info_span, Instrument};
 
async fn work() {
    let span = info_span!("job", job_id = 7);
    async {
        tracing::info!("started");
        tokio::time::sleep(std::time::Duration::from_millis(1)).await;
        tracing::info!("done");
    }
    .instrument(span)
    .await;
}

What this demonstrates:

• Spans nest and attach metadata
• .instrument(span) propagates context across .await
• Subscribers format output (text or JSON)

Deep Dive

Layers

use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
 
tracing_subscriber::registry()
    .with(tracing_subscriber::fmt::layer())
    .with(tracing_subscriber::EnvFilter::from_default_env())
    .init();

Set RUST_LOG=info,my_crate=debug in production for targeted verbosity.

Cross-Link

See tracing in Tokio and Observability for metrics and OTLP.

Gotchas

• Initializing subscriber twice - panics in tests. Fix: OnceLock or tracing_subscriber::try_init().
• Huge span fields - logging full payloads is expensive. Fix: log ids and sizes.
• Missing .instrument on spawned tasks - broken parent links. Fix: instrument before tokio::spawn.
• println! in libraries - bypasses filters. Fix: tracing macros only.
• Debug in hot loops - guard with tracing::enabled! or level checks.

Alternatives

FAQs

Related

• tracing in Tokio
• Observability
• tokio
• Axum Service Skill

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.