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slog is neat

23 October, 2025
  • tech
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    Why did golang get a new logging framework after ~10 years?

    Existing “log” package

    • It’s like printf but there’s a timestamp!
    • I can call Fatal and Panic; I guess that’s nice in main.
    log.Println("Hello, 世界")
log.Printf("I will see you in %s\n", time.Hour)
log.Fatal("Oh no it's all gone wrong")

    ▶ Go Playground

    2009/11/10 23:00:00 Hello, 世界
2009/11/10 23:00:00 I will see you in 1h0m0s
2009/11/10 23:00:00 Oh no it's all gone wrong
    • There is SetOutput; let’s come back to this later.
    • I can get a default logger, but I can’t set a default logger.

    What should libraries do?

    This approach is pretty great if you’re in main.go, but what if you’re writing a package for reuse?

    • Should libraries log at all?
      • Well, I can pass up errors (wrap them if you’re extra cool!).
      • What about that thing that seems wrong but I’m suppressing? I don’t want to break compatibilities, but I’m pretty sure this is wrong.
      • https://github.com/Azure/eno/pull/532
    • How do I get the logger?
      • Use a package-level variable or function?
      • Pass it everywhere and save it as a member?
      • Shove it onto context?
      • I have seen all of these approaches.

    • How do I log respectfully without over-logging?

    • What if I want to customize my log format?

    What about X

    • zap and logrus are pretty neat. They introduced level, structure, sinks, and a lot of the stuff I am going to talk about below.
    • But if I’m writing a library, how do I know what interface to get/log to?
    • I have seen several instances where logging adapters are regularly rewritten, usually losing something each time.
    • Sometimes it’s nice to just have the standard library.

    Enter Slog

    Gopkg and Blog

    Levels

    Debug/Info/Warn/Error

    • Throw all your junk into debug! Main.go can set level and toss it.
    • Indecisive? Warn away!
    • Make up your own log levels and go crazy.
    	slog.Debug("Hello, 世界")
	slog.Info("I will see you in ", "time", time.Hour)
	slog.Warn("It's getting hot in here")
	slog.Error("Nuts", "error", fmt.Errorf("next time gadget"))
	slog.Log(context.TODO(), slog.Level(10), "more important than an error"

    ▶ Go Playground

      2009/11/10 23:00:00 INFO I will see you in  time=1h0m0s
  2009/11/10 23:00:00 WARN It's getting hot in here
  2009/11/10 23:00:00 ERROR Nuts error="next time gadget"
  2009/11/10 23:00:00 ERROR+2 more important than an error

    Structure

    • String parsing stinks because we have Kusto and lots of other fancy databases!
    • Request IDs, other tracing items, latencies as durations, and general metadata all live happily here.
    • Attrs have lots of nice built-in types, and there are groups (I have not used groups).
    • You can set attributes globally on the default or a particular log instance with With/WithAttrs.
    slog.With("author", "pmiller")
	slog.Info("I will see you in ", "time", time.Hour, slog.Duration("strongtypedtime", time.Minute),
		slog.Float64("pi", math.Pi),
		slog.Group("plane", "make", "boeing", "model", "737"))
	attrs := []slog.Attr{
		slog.String("author", "jsonpaul"),
		slog.String("program", "playground"),
	}
	handler := slog.NewJSONHandler(os.Stdout, nil).WithAttrs(attrs)

	slog.SetDefault(slog.New(handler))
	slog.Info("I will see you in ", "time", time.Hour, slog.Duration("strongtypedtime", time.Minute),
		slog.Float64("pi", math.Pi),
		slog.Group("plane", "make", "boeing", "model", "737"))

    ▶ Go Playground

     2009/11/10 23:00:00 INFO I will see you in  time=1h0m0s strongtypedtime=1m0s pi=3.141592653589793 plane.make=boeing plane.model=737
{"time":"2009-11-10T23:00:00Z","level":"INFO","msg":"I will see you in ","author":"jsonpaul","program":"playground","time":3600000000000,"strongtypedtime":60000000000,"pi":3.141592653589793,"plane":{"make":"boeing","model":"737"}}

    Putting things on context?

    • Okay, what if I want to add attributes that everything below me in the call stack can use? ChatGPT
    • I can stick them on the context, but then the code I am calling has to pull them out.
    • There is an InfoContext and equivalent for each helper.
    • I didn’t find a great solution for this unless you control the handler too.
    • Then you can use https://github.com/PumpkinSeed/slog-context. Example blog
    • Written by someone named PumpkinSeed, so it must be good. I kind of wanted a standard library way to do it, though.

    Handlers!

    • The fools gave us an interface: Handle(context.Context, Record) error
    • Maybe you don’t want JSON; maybe you want BSON or Thrift or whatever compact nonsense.
    • Maybe you want to forward records to Kusto, Blob storage, Kafka, or Service Bus.
    • You can do whatever you want!
    • Sometimes you might just want to do an io.Writer, but then you lose per-record control.
    • My Azure append blob example.
      • [Started as a handler](https://github.com/paulgmiller/careme/blob/8621cce4cd5f240d881a0bf4e22cb26c281c8e8e/internal/logsink/appendblob.go
      • Hit an interesting deadlock
      • Went to a writer much simpler but regret it because now I might split log records

    Awesome slog

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