tracing-microjson

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

Quick Links: Why? | Who is this for? | Usage | Configuration | Comparisons | MSRV

Why?

Enabling the json feature on tracing-subscriber pulls in 7 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 = "0.3"

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)
            .with_thread_ids(true)      // include thread ID (default: false)
            .with_thread_names(true)    // include thread name (default: false)
            .flatten_event(true)        // flatten fields to top level (default: false)
            .without_time(),            // disable timestamps (default: SystemTimestamp)
    )
    .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	✅ Yes
Thread ID / name	✅ Yes	✅ Yes
Custom field formatters	✅ Yes	❌ No
Serialization deps	serde + serde_json + tracing-serde	✅ None

Dependencies

Both configurations start from tracing + tracing-subscriber with the fmt + registry features (14 crates in common).

Approach	Additional crates	Total
tracing-microjson	+1 (this crate)	15
tracing-subscriber json	+7 (serde ecosystem)	21

Binary size

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

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

_{aarch64-apple-darwin, Rust 1.93, strip = true, lto = true.}

Performance

Head-to-head timing benchmarks (lower is better):

Scenario	tracing-microjson	tracing-subscriber	Speedup
Simple event	315 ns	748 ns	2.4x
Event with fields	425 ns	1,045 ns	2.5x
Nested spans	877 ns	2,508 ns	2.9x

Heap allocations per event (after first-event warmup):

Scenario	tracing-microjson	tracing-subscriber
Simple event	0	0
Event with fields	0	0
Nested spans	6	17

_{Apple M1 Max, Rust 1.93, criterion 0.8. Run cargo bench --features _bench_internals to reproduce.}

MSRV

The minimum supported Rust version is 1.88 (edition 2024).

License

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