How to Output Logs as JSON in Rust

Use the tracing crate with the tracing-subscriber JSON layer to output logs as structured JSON. Add tracing and tracing-subscriber to your Cargo.toml, then initialize the subscriber in main before logging.

use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt, fmt::Layer};

fn main() {
    let json_layer = Layer::new()
        .json()
        .with_target(false)
        .with_thread_ids(false)
        .with_thread_names(false)
        .with_file(false)
        .with_line_number(false);

    tracing_subscriber::registry()
        .with(tracing_subscriber::filter::LevelFilter::INFO)
        .with(json_layer)
        .init();

    tracing::info!(message = "Application started", user_id = 123);
}

Add this to your Cargo.toml:

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

Run your application to see JSON-formatted logs like:

{"message":"Application started","user_id":123}

Outputting logs as JSON in Rust configures your program to print structured data instead of plain text. This helps tools like log aggregators parse and search your logs automatically. Think of it as labeling every log entry with clear tags so machines can read them easily.