I'm surprised this isn't a standard base pattern in languages, to be honest. Apache's commons-logging library was a standard part of enterprise java placements for many years, and only started to go away when Log4J came along.

I'm a big fan of slog, and this is a great overview.

The fact it is so flexible and composable, while still maintaining a simple API is just great design. I wasn't aware of the performance overhead compared to something like zerolog, but this shouldn't be a concern for most applications.

My biggest gripe with slog is that there is no clear guidance on supported types of attributes.

One could argue that supported types are the ones provided by Attr "construct" functions (like slog.String, slog.Duration, etc), but it is not enough. For example, there is no function for int32 – does it mean it is not supported? Then there is slog.Any and some support in some handlers for error and fmt.Stringer interfaces. The end result is a bit of a mess.

  req := expvar.NewInt("requests")
  req.Add(1)
  
  attr := slog.Any("requests", req)
  
  slog.New(slog.NewTextHandler(os.Stderr, nil)).Info("text", attr)
  slog.New(slog.NewJSONHandler(os.Stderr, nil)).Info("json", attr)

  time=2025-09-12T13:15:42.125+02:00 level=INFO msg=text requests=1
  {"time":"2025-09-12T13:15:42.125555+02:00","level":"INFO","msg":"json","requests":{}}

    type StringerValue struct {
        fmt.Stringer
    }

    func (v StringerValue) LogValue() slog.Value {
        return slog.StringValue(v.String())
    }

    slog.Any("requests", StringerValue{req})

I have a gripe with slog - it uses magic for config

What I mean is, if you configure slog in (say) your main package, then, by magic, that config is used by any call to slog within your application.

There's no "Oh you are using this instance of slog that has been configured to have this behaviour" - it's "Oh slog got configured so that's the config you have been given"

I've never tried to see if I can split configs up, and I don't have a usecase, it just strikes me as magic is all

The thing that gets me about slog is that the output key for the slog JSON handler is msg, but that's not compatible with Googles own GCP Stackdriver logging. Since that key is a constant I now need to use an attribute replacer to change it from msg to message (or whatever it is stackdriver wants). Good work Google.