logging

Logging

A sample app that demonstrates how to use Juno's built-in logging functions in Rust serverless functions.

Instead of ic_cdk::print (which shows as "Unknown" level in the Juno Console), this example uses junobuild_satellite::{info, debug, warn, error} to emit structured log entries with proper levels.

Key Files

• src/satellite/src/lib.rs — Hook that demonstrates all four log levels
• src/satellite/src/logging.rs — Reusable logging utility module

Getting started

git clone https://github.com/junobuild/examples
cd functions/rust/logging

How to Run

1. Install dependencies:

npm ci

2. Start Juno local emulator:

Requires the Juno CLI: npm i -g @junobuild/cli

juno emulator start

3. Create a Satellite for local dev:

• Visit http://localhost:5866 and follow the instructions.
• Update juno.config.ts with your Satellite ID.

4. Create required collection:

• notes in Datastore: http://localhost:5866/datastore

5. Start the frontend dev server (in a separate terminal):

npm run dev

6. Build the serverless functions (in a separate terminal):

juno functions build

The emulator will automatically upgrade your Satellite and live reload the changes.

7. Test logging:

• Sign in and save a note using the frontend.
• Check the Juno Console → Functions → Logs tab to see log entries with proper levels.
• Save a note with an empty body to trigger a warning-level log.

How It Works

Log Levels

Function	Level	Use Case
junobuild_satellite::debug()	Debug	Diagnostic messages for development
junobuild_satellite::info()	Info	Successful operations, state changes
junobuild_satellite::warn()	Warning	Skipped operations, unusual conditions
junobuild_satellite::error()	Error	Failed operations, unexpected errors

RNG Dependency

Custom loggers require a seeded RNG to generate unique document keys. Enable the on_init_random_seed feature in your Cargo.toml:

junobuild-satellite = { version = "0.2.0", features = ["on_set_doc", "on_init_random_seed"] }

Then implement the callback to know when loggers are ready:

#[unsafe(no_mangle)]
fn juno_on_init_random_seed() {
    // Custom loggers are now available
    logging::log_info("Satellite", "RNG seeded — loggers ready");
}

Why Not ic_cdk::print?

	ic_cdk::print	junobuild_satellite::info() etc.
Juno Console Level	"Unknown"	Info / Debug / Warning / Error
Availability	Always	After RNG seed
Best for	Quick debug, pre-seed fallback	Production logging

The logging.rs module in this example wraps both: it tries the Juno logger first and falls back to ic_cdk::print if it fails (e.g. before the RNG is seeded).