#tracing-subscriber #subscriber #tracing #logging-tracing #json #logging #json-log


Customizable layer and subscriber for tracing that emits logs in JSON

3 releases

0.1.2 Jun 16, 2024
0.1.1 Jun 14, 2024
0.1.0 Jun 13, 2024

#188 in Debugging

Download history 44/week @ 2024-06-07 415/week @ 2024-06-14 28/week @ 2024-06-21

487 downloads per month

MIT license



json-subscriber is (mostly) a drop-in replacement for tracing_subscriber::fmt().json().

It provides helpers to be as compatible as possible with tracing_subscriber while also allowing for simple extensions to the format to include custom data in the log lines.

The end goal is for each user to be able to define the structure of their JSON log lines as they wish. Currently, the library only allows what tracing-subscriber plus OpenTelemetry trace and span IDs.


However you created your FmtSubscriber or fmt::Layer, the same thing should work in this crate.

For example in README.md in Tracing, you can see an yak-shaving example where if you just change tracing_subscriber to json_subscriber, everything will work the same, except the logs will be in JSON.

use tracing::info;
use json_subscriber;

fn main() {
    // install global collector configured based on RUST_LOG env var.

    let number_of_yaks = 3;
    // this creates a new event, outside of any spans.
    info!(number_of_yaks, "preparing to shave yaks");

    let number_shaved = yak_shave::shave_all(number_of_yaks);
        all_yaks_shaved = number_shaved == number_of_yaks,
        "yak shaving completed."

Most configuration under tracing_subscriber::fmt should work equivalently. For example one can create a layer like this:

    // .json()

Calling .json() is not needed and the method does nothing and is marked as deprecated. It is kept around for simpler migration from tracing-subscriber though.

Trying to call .pretty() or .compact() will however result in an error. json-tracing does not support any output other than JSON.



To include trace ID and span ID from opentelemetry in log lines, simply call with_opentelemetry_ids. This will have no effect if you don't also configure a tracing-opentelemetry layer.

let tracer = todo!();
let opentelemetry = tracing_opentelemetry::layer().with_tracer(tracer);
let json = json_subscriber::layer()


This will produce log lines like for example this (without the formatting):

  "fields": {
    "message": "shaving yaks"
  "level": "INFO",
  "openTelemetry": {
    "spanId": "35249d86bfbcf774",
    "traceId": "fb4b6ae1fa52d4aaf56fa9bda541095f"
  "target": "readme_opentelemetry::yak_shave",
  "timestamp": "2024-06-06T23:09:07.620167Z"

See the readme-opentelemetry example for full code.


You can also specify custom static fields to be added to each log line, or serialize extensions provided by other Layers:

struct Foo(String);

impl<S: Subscriber + for<'lookup> LookupSpan<'lookup>> Layer<S> for FooLayer {
    fn on_new_span(&self, attrs: &Attributes<'_>, id: &Id, ctx: Context<'_, S>) {
        let span = ctx.span(id).unwrap();
        let mut extensions = span.extensions_mut();
        let foo = Foo("hello".to_owned());

fn main() {
  let foo_layer = FooLayer;

  let mut layer = json_subscriber::JsonLayer::stdout();



This crate strives to be faster than tracing_subscriber. See benchmarks.

Supported Rust Versions

json-subscriber is built against the latest stable release. The minimum supported version is 1.65. The current version is not guaranteed to build on Rust versions earlier than the minimum supported version.

json-subscriber follows the same compiler support policies as the Tokio project. The current stable Rust compiler and the three most recent minor versions before it will always be supported. For example, if the current stable compiler version is 1.69, the minimum supported version will not be increased past 1.66, three minor versions prior. Increasing the minimum supported compiler version is not considered a semver breaking change as long as doing so complies with this policy.


This project is licensed under the MIT license.


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


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