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Log Levels

Log levels aren't specific to structured logging, but log/slog provides a built-in way to handle them. They're a convention for labeling each log with a severity. The standard library defines four levels by default, and you can define custom levels if needed:

slog.LevelError: Error messages, indicating failures or issues that need attention.
slog.LevelWarn: Warning messages, indicating potential issues that are not critical.
slog.LevelInfo: Informational messages, typically used for general application events.
slog.LevelDebug: Debug messages, useful for development and debugging.

You'll usually only need these four.

I don't even use Warn messages very often if truth be told...

Filtering

Have you ever added print statements while debugging? Sometimes you need that same visibility in production because you can't reproduce the issue locally. You still don't want to flood your normal logs with extra noise, so you use Debug. Then if something else goes wrong, you can filter debug logs out or send them elsewhere. For example, you can use different handlers with different minimum levels:

// logs DEBUG and above
debugLogger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{
	Level: slog.LevelDebug,
}))

// logs ERROR and above
errorLogger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{
	Level: slog.LevelError,
}))

Combining Loggers

A more practical approach is a single logger that routes logs to different destinations by level. For example, everything goes to STDERR, but only INFO and higher go to a file. As of Go 1.26, this is easy with slog.NewMultiHandler:

debugHandler := slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{
	Level: slog.LevelDebug,
})

logFile, err := os.OpenFile("linko.access.log", os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0o644)
if err != nil {
	return err
}
defer logFile.Close()
infoHandler := slog.NewTextHandler(logFile, &slog.HandlerOptions{
	Level: slog.LevelInfo,
})

logger := slog.New(slog.NewMultiHandler(
	debugHandler,
	infoHandler,
))

Assignment

Split logs by severity, but keep one app-wide logger.

Use slog.Handlers to configure your STDERR logs to include DEBUG and above, and your file logs to include INFO and above.
Use slog.NewMultiHandler to combine both handlers into one logger used throughout the app.
Update the startup and shutdown messages to be logged at the DEBUG level.
Update any error-condition logs (e.g. "failed to create store") to use the Error level.

Start your server with LINKO_LOG_FILE=linko.access.log set:

LINKO_LOG_FILE=linko.access.log go run . 2>&1 | sh -c 'trap "" INT; tee linko.out.log'

Run and submit the CLI tests from the root of the Linko repo.