slog

Logging package for Go.

• Concurrent safe
• Three levels; slog.Err, slog.Warn, and slog.Info
• Children loggers for sub-processes
• Built-in zero-memory slog.NilLogger to easily logging off without changing calling code
• Custom reporters to send logs anywhere
• Reporters function for reporting to many places

Simple

The slog.Logger interface is simple.

// make a logger that logs everything
logger := slog.New("prefix", slog.Everything)

// throughout your code, call the method with empty
// params to see whether the logger cares about that
// level or not.
if logger.Info() {
  logger.Info("something happened")
}

// errors too...
if err != nil && logger.Err() {
  logger.Err("error occurred", err)
}

// when you're finished with it - stop it
logger.Stop()
<-logger.StopChan() // wait for it to stop

Different levels

If you only care about errors, use the slog.Err level:

logger := slog.New("prefix", slog.Err)

You can change the level at runtime:

logger.SetLevel(slog.Info)

Children

Children loggers report their findings to the parent, and changes to the parent will also affect the children.

logger := slog.New("parent", slog.Everything)

childLogger := logger.New("child")
go otherProcess(childLogger)

// stopping the parent will make sure children
// are stopped too.
logger.Stop()
<-logger.StopChan() // wait for everything to stop

NilLogger

If you want to disable logging entirely, the most memory efficient way to do so is to pass a slog.NilLogger wherever a Logger is needed.

thing := &MyThing{
  Logger: slog.NilLogger,
}

thing.DoStuff() // will do stuff without logging

Custom reporting

If you want to control where the logs get reported to, you can call SetReporter or SetReporterFunc on a RootLogger.

logger := slog.New("prefix", slog.Info)
logger.SetReporterFunc(func(l *slog.Log) {
  // publish logs to a messaging queue
  queuePublisher.Publish("log-channel", l)
})

• You can only change the Reporter of a RootLogger (i.e. parent), children loggers will automatically report through the specified method too.

Multiple reporters

If you want to report logs to multiple locations, you can use the Reporters function.

logger := slog.New("prefix", slog.Info)
logger.SetReporter(slog.Reporters(slog.Stdout, msgQueueReporter, databaseReporter))

Notes

• Avoid catching logic inside if log.Info() blocks, changing log levels or instance of Logger should not affect flow.