tracing-microjson

A tracing layer that outputs JSON-formatted logs without pulling in serde, serde_json, or tracing-serde.

Why?

Enabling the json feature on tracing-subscriber pulls in 9 additional crates (serde, serde_json, tracing-serde, and their transitive dependencies). tracing-microjson produces the same output format using a hand-written JSON formatter with zero serialization framework dependencies.

Who is this for?

Projects where compile time and binary size matter
Environments with strict dependency auditing requirements
Anyone who wants structured JSON logging with a minimal dependency footprint

Usage

[dependencies]
tracing-microjson = "0.1"

use tracing_microjson::JsonLayer;
use tracing_subscriber::prelude::*;

tracing_subscriber::registry()
    .with(JsonLayer::new(std::io::stderr))
    .init();

Configuration

use tracing_microjson::JsonLayer;
use tracing_subscriber::prelude::*;

tracing_subscriber::registry()
    .with(
        JsonLayer::new(std::io::stderr)
            .with_target(true)          // include event target (default: true)
            .with_file(true)            // include source filename (default: false)
            .with_line_number(true)     // include source line number (default: false)
            .flatten_event(true),       // flatten fields to top level (default: false)
    )
    .init();

Comparisons

All comparisons are against tracing-subscriber with its json feature enabled.

Features

Feature	tracing-subscriber `json`	tracing-microjson
JSON event output	✅ Yes	✅ Yes
Span fields & nesting	✅ Yes	✅ Yes
Target, file, line	✅ Yes	✅ Yes
`flatten_event`	✅ Yes	✅ Yes
Custom timestamps	✅ Yes	🔜 Not yet
Thread ID / name	✅ Yes	🔜 Not yet
Custom field formatters	✅ Yes	🔜 Not yet
Serialization deps	serde + serde_json + tracing-serde	✅ None

Dependencies

Both configurations start from tracing-subscriber with the fmt + registry features (7 crates).

Approach	Additional crates	Total
`tracing-microjson`	+1 (this crate)	8
`tracing-subscriber` `json` feature	+9 (serde ecosystem)	16

Binary size

Minimal "hello world" JSON logging binary (release, LTO, stripped):

Approach	Size
`tracing-microjson`	377 KiB (23% smaller)
`tracing-subscriber` with `json` feature	490 KiB

aarch64-apple-darwin, Rust 1.85, strip = true, lto = true.

Performance

Head-to-head benchmarks on the same workload (lower is better):

Scenario	tracing-microjson	tracing-subscriber	Speedup
Simple event	685 ns	737 ns	1.08x
Event with fields	824 ns	1,039 ns	1.26x
Nested spans	1,313 ns	2,475 ns	1.89x

Apple M1 Max, Rust 1.85, criterion 0.5. Run cargo bench --features _bench_internals to reproduce.

MSRV

The minimum supported Rust version is 1.85 (edition 2024).

License

Licensed under the GNU General Public License v3.0 or later.

tracing-microjson 0.1.0