#logging #tracing #serialization


An alternative, structured, compatibility layer for serializing trace data with serde

2 releases

Uses new Rust 2021

0.1.1 Jun 8, 2022
0.1.0 Jun 8, 2022

#124 in Debugging

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Used in 3 crates

MIT license

599 lines


An alternative, structured, adapter for serializing tracing types using serde.



tracing is a framework for instrumenting Rust programs to collect scoped, structured, and async-aware diagnostics.tracing-serde-structured enables serializing tracing types using serde.

Traditional logging is based on human-readable text messages. tracing gives us machine-readable structured diagnostic information. This lets us interact with diagnostic data programmatically. With tracing-serde-structured, you can implement a Subscriber to serialize your tracing types and make use of the existing ecosystem of serde serializers to talk with distributed tracing systems.

Serializing diagnostic information allows us to do more with our logged values. For instance, when working with logging data in JSON gives us pretty-print when we're debugging in development and you can emit JSON and tracing data to monitor your services in production.

The tracing crate provides the APIs necessary for instrumenting libraries and applications to emit trace data.

Differences with the tracing-serde crate

Unlike the upstream tracing-serde crate, tracing-serde-structured does this serialization in a structured manner, making the data compatible with binary formats such as postcard, while also allowing deserialization of the data.

tracing-serde-structured is still compatible with serialization and deserialization to/from JSON, though it does change the format of the JSON data, meaning it is not a 100% drop-in replacement.

The following is an example of the difference between tracing-serde and tracing-serde-structured data:

pub fn main() {
    // 1 - new span
    let span = tracing::span!(Level::TRACE, "outer_span");
    // 2 - enter span
    let _span = span.enter();
    // 7 - exit span

mod do_thing {
    pub fn doit() {
        // 3 - new span
        let span = tracing::span!(Level::TRACE, "my span");
        // 4- enter span
        span.in_scope(|| {
            // 5 - event
            event!(Level::INFO, "something has happened!");
            // 6 - exit span
# 1 - new span
- '{"name":"outer_span","target":"tracing_playground","level":"TRACE","module_path":"tracing_playground","file":"src/main.rs","line":34,"fields":[],"is_span":true,"is_event":false}'
+ '{"name":"outer_span","target":"tracing_playground","level":"TRACE","module_path":"tracing_playground","file":"src/main.rs","line":34,"fields":[],"is_span":true,"is_event":false}'
# 2 - enter span
- '{"metadata":{"name":"outer_span","target":"tracing_playground","level":"TRACE","module_path":"tracing_playground","file":"src/main.rs","line":34,"fields":[],"is_span":true,"is_event":false},"parent":null,"is_root":false}'
+ '{"metadata":{"name":"outer_span","target":"tracing_playground","level":"TRACE","module_path":"tracing_playground","file":"src/main.rs","line":34,"fields":[],"is_span":true,"is_event":false},"parent":null,"is_root":false}'
- '[1]'
+ '{"id":1}'
# 3 - new span
- '{"name":"my span","target":"tracing_playground::do_thing","level":"TRACE","module_path":"tracing_playground::do_thing","file":"src/main.rs","line":74,"fields":[],"is_span":true,"is_event":false}'
+ '{"name":"my span","target":"tracing_playground::do_thing","level":"TRACE","module_path":"tracing_playground::do_thing","file":"src/main.rs","line":74,"fields":[],"is_span":true,"is_event":false}'
# 4 - enter span
- '{"metadata":{"name":"my span","target":"tracing_playground::do_thing","level":"TRACE","module_path":"tracing_playground::do_thing","file":"src/main.rs","line":74,"fields":[],"is_span":true,"is_event":false},"parent":null,"is_root":false}'
+ '{"metadata":{"name":"my span","target":"tracing_playground::do_thing","level":"TRACE","module_path":"tracing_playground::do_thing","file":"src/main.rs","line":74,"fields":[],"is_span":true,"is_event":false},"parent":null,"is_root":false}'
- '[2]'
+ '{"id":2}'
# 5 - event
- '{"name":"event src/main.rs:76","target":"tracing_playground::do_thing","level":"INFO","module_path":"tracing_playground::do_thing","file":"src/main.rs","line":76,"fields":["message"],"is_span":false,"is_event":true}'
+ '{"name":"event src/main.rs:76","target":"tracing_playground::do_thing","level":"INFO","module_path":"tracing_playground::do_thing","file":"src/main.rs","line":76,"fields":["message"],"is_span":false,"is_event":true}'
- '{"metadata":{"name":"event src/main.rs:76","target":"tracing_playground::do_thing","level":"INFO","module_path":"tracing_playground::do_thing","file":"src/main.rs","line":76,"fields":["message"],"is_span":false,"is_event":true},"message":"something has happened!"}'
+ '{"fields":{"message":{"Debug":"something has happened!"}},"metadata":{"name":"event src/main.rs:76","target":"tracing_playground::do_thing","level":"INFO","module_path":"tracing_playground::do_thing","file":"src/main.rs","line":76,"fields":["message"],"is_span":false,"is_event":true},"parent":null}'
# 6 - exit span
- '[2]'
+ '{"id":2}'
# 7 - exit span
- '[1]'
+ '{"id":1}'


First, add this to your Cargo.toml:

tracing = "0.1"
tracing-serde-structured = "0.1"

Next, add this to your crate:

use tracing_serde::AsSerde;

Please read the tracing documentation for more information on how to create trace data.

This crate provides the as_serde function, via the AsSerde trait, which enables serializing the Attributes, Event, Id, Metadata, and Record tracing values.

Implement a Subscriber to format the serialization of tracing types how you'd like.

pub struct JsonSubscriber {
    next_id: AtomicUsize, // you need to assign span IDs, so you need a counter

impl Subscriber for JsonSubscriber {

    fn new_span(&self, attrs: &Attributes) -> Id {
        let id = self.next_id.fetch_add(1, Ordering::Relaxed);
        let id = Id::from_u64(id as u64);
        let json = json!({
        "new_span": {
            "attributes": attrs.as_serde(),
            "id": id.as_serde(),
        println!("{}", json);
    // ...

After you implement your Subscriber, you can use your tracing subscriber (JsonSubscriber in the above example) to record serialized trace data.

Crate Feature Flags

The following crate feature flags are available:

  • std: Depend on the Rust standard library (enabled by default).

    no_std users may disable this feature with default-features = false:

    tracing-serde-structured = { version = "0.1", default-features = false }

Unstable Features

These feature flags enable unstable features. The public API may break in 0.1.x releases. To enable these features, the --cfg tracing_unstable must be passed to rustc when compiling.

The following unstable feature flags are currently available:

  • valuable: Enables [Visit::record_value] implementations, for serializing values recorded using the valuable crate.

Enabling Unstable Features

The easiest way to set the tracing_unstable cfg is to use the RUSTFLAGS env variable when running cargo commands:

RUSTFLAGS="--cfg tracing_unstable" cargo build

Alternatively, the following can be added to the .cargo/config file in a project to automatically enable the cfg flag for that project:

rustflags = ["--cfg", "tracing_unstable"]


This crate is a fork of the tracing-serde library, as provided by the Tokio project.


This project is licensed under the MIT license.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, shall be licensed as MIT, without any additional terms or conditions.


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