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Logging: slog

simple log example
package main

import (
    "log/slog"
)

func main() {
    slog.Info("Info message")
}

# Output
# 2024/01/03 10:24:22 INFO Info message

Contextual attributes​

Attributes provide additional context about the logged event.

logger.Info(
  "incoming request",
  "method", "GET",
  "time_taken_ms", 158,
  "path", "/hello/world?q=search",
  "status", 200,
  "user_agent", "Googlebot/2.1 (+http://www.google.com/bot.html)",
)

# Output
# {
#   "time":"2023-02-24T11:52:49.554074496+01:00",
#   "level":"INFO",
#   "msg":"incoming request",
#   "method":"GET",
#   "time_taken_ms":158,
#   "path":"/hello/world?q=search",
#   "status":200,
#   "user_agent":"Googlebot/2.1 (+http://www.google.com/bot.html)"
# }

but better way to use typed values

logger.Info(
  "incoming request",
  slog.String("method", "GET"),
  slog.Int("time_taken_ms", 158),
  slog.String("path", "/hello/world?q=search"),
  slog.Int("status", 200),
  slog.String(
    "user_agent",
    "Googlebot/2.1 (+http://www.google.com/bot.html)",
  ),
)

Log level​

set fixes log level
func main() {
    opts := &slog.HandlerOptions{
        Level: slog.LevelDebug,
    }
    handler := slog.NewJSONHandler(os.Stdout, opts)

    logger := slog.New(handler)
    logger.Debug("Debug message")
    logger.Info("Info message")
    logger.Warn("Warning message")
    logger.Error("Error message")
}
set log level from var
func main() {
    ogLevel := &slog.LevelVar{} // INFO
    opts := &slog.HandlerOptions{
        Level: logLevel,
    }

    handler := slog.NewJSONHandler(os.Stdout, opts)
    . . .
}

JSON logs​

JSON logs
func main() {
    logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
    logger.Debug("Debug message")
    logger.Info("Info message")
    logger.Warn("Warning message")
    logger.Error("Error message")
}

# Output
# {"time":"2023-03-15T12:59:22.227408691+01:00","level":"INFO","msg":"Info message"}
# {"time":"2023-03-15T12:59:22.227468972+01:00","level":"WARN","msg":"Warning message"}
# {"time":"2023-03-15T12:59:22.227472149+01:00","level":"ERROR","msg":"Error message"}

Default logger​

Customize the default Logger
func main() {
    logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))

    slog.SetDefault(logger)

    // elsewhere in the application
    log.Println("Hello from old logger")
}

Docs & Links​

Structured Logging with slog
Структурированное логирование в Go с помощью Slog
Logging in Go with Slog: The Ultimate Guide