Logging

Up until this point, whenever you needed to see some output from your application you used the println!() macro. This works, but it's like relying exclusively on Console.WriteLine() in .NET. It's not really up to scratch when you are building production ready applications. So what is the alternative in Rust?

The log crate

The log crate a logging facade provides a single logging API that abstracts over the actual logging implementation. It provides API's for writing log messages, with seperate crates that implement the actual log output. If you're familiar with Serilog in the .NET ecosystem, the various different Sinks provided, and the interplay with the ILogger<T> interface this will feel familiar.

You configure your logger once when your application starts up and from there you write log messages using:

log::trace!("Trace message");
log::info!("Info message");
log::warn!("Warning message");
log::error!("Error message");

Configuring the log crate

You can see the available logging implementations in the docs for the log crate. For the purposes of this workshop, you'll be focused on structured logging in a JSON format. For that, you'll need to add the structured-logger crate.

info

You'll need to configure your logger in your main() function. It is generally recommended to make your logger configuration the first thing that happens when your application runs. That way, you have structured logging available from the start.

use structured_logger::{async_json::new_writer, Builder};

#[tokio::main]
async fn main() -> Result<(), ApplicationError> {
    let log_level = std::env::var("LOG_LEVEL").unwrap_or("INFO".to_string());
    // Initialize the logger.
    Builder::with_level(&log_level)
        .with_target_writer("*", new_writer(tokio::io::stdout()))
        .init();

    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await.map_err(|e| ApplicationError::ApplicationError(e.to_string()))?;
    
    log::info!("listening on {}", listener.local_addr().unwrap());

    // Prints:
    // {"level":"INFO","message":"listening on 0.0.0.0:3000","target":"module_10_rust_app","timestamp":1746536374630}
}

You can configure the desired log level passing the desired level (DEBUG, TRACE, INFO, WARN, ERROR) to the with_level() function on the structured logger Builder.