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.
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
There's a default logger that's used when you call package-level functions (as opposed to methods on an instance of slog.Logger). The default logger is probably what you configured in your main package.
In my opinion this is perfectly idiomatic Go. Sometimes the package itself hosts one global instance. If you think that's "magic" then you must think all of Go is magic. It helps to think of a package in Go as equivalent to a single Java class. Splitting up a Go package's code into multiple files is purely cosmetic.
I’m not sure I understand what you mean by “magic for config”. You create and configure a logger using slog.New(…). You can use the default logger instead, slog.Default(), which is just a global and has a default config. You can also set the default logger using slog.SetDefault(…).
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.
It's fine for application logging but I have two gripes with slog:
1) If you're writing a library that can be used by many different applications and want to emit logs, you'll still need to write a generic log interface with adapters for slog, zap, charmlog, etc. That the golang team refuses to bless a single interface for everyone to settle on both makes sense given their ideological standpoint on shipping interfaces and also causes endless mild annoyance and code duplication.
2) I believe it's still impossible to see the correct callsite in test logs when using slog as the logger. For more information, see https://github.com/neilotoole/slogt?tab=readme-ov-file#defic.... It's possible I'm out of date here — please correct me if this is wrong, it's actually a much larger annoyance for me and one of the reasons I still use uber/zap or charmbracelet/log.
Overall, especially given that it performs worse than uber/zap and everyone has basically standardized on that and it provides essentially the same interface, I recommend using uber/zap instead.
EDIT: just to expand further, take a look at the recommended method of wrapping helper methods that call logs. Compare to the `t.Helper()` approach. And some previous discussion. Frustrating!
> The key decision is thus between two patterns: using a global logger or using dependency injection. The former is extremely convenient but adds a hidden dependency that’s hard to test, while the latter is more verbose but makes dependencies explicit, resulting in highly testable and flexible code.
Curious how different people handle this. I personally pretty much always pass a logger into function, classes, structs (what have you) so it has the context I need it to. It's a tad more verbose I guess, but it's such a minor lift I've always found it worth it.
I really like structured logs and am pleased the Go team saw the benefits of bringing it into the standard library.
However, I feel like errors should be able to hold slog attributes. It makes for some very useful and easy error logging, especially when the logging takes place far up the execution chain from where the error happened.
This is easily possible with a custom error type and some log functions. I have published on GitHub my small and crude implementation that I use in a few hobby projects, MIT licensed, if anyone is interested. https://github.com/sveinnthorarins/sterlo
I felt the same way, and made my own package for adding slog attributes to an error for easier logging. It's usually all I need for my own custom errors.
I can understand why it's not in the stdlib though, it seems easy enough to run into key overwriting issues if a dependency returned custom errors with attributes.
I appreciate the built in support slog has for slog.GroupValue and the slog.LogValuer interface that enables everyone to build a solution best for their needs.
To guarantee correctness, you must use the strongly-typed slog.Attr helpers. They make it impossible to create an unbalanced pair by catching errors at compile time
I take their point, but given the typical value of most logging lines, this does not seem worth the tax you pay in readability. This is a gripe I have with oTel too --- it really cruds your code up --- but with oTel you're getting long-term value that logging (which is still my go-to o11y) doesn't.
Great article! super helpful for someone like me who recently started building services in Go.
I especially appreciated the section on slog.Attr and the !BADKEY issue, it’s one of those little things that can go unnoticed until logs break in prod.
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[ 4.7 ms ] story [ 41.6 ms ] threadThe 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.
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.
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
In my opinion this is perfectly idiomatic Go. Sometimes the package itself hosts one global instance. If you think that's "magic" then you must think all of Go is magic. It helps to think of a package in Go as equivalent to a single Java class. Splitting up a Go package's code into multiple files is purely cosmetic.
It’s extremely unmagical in my opinion.
1) If you're writing a library that can be used by many different applications and want to emit logs, you'll still need to write a generic log interface with adapters for slog, zap, charmlog, etc. That the golang team refuses to bless a single interface for everyone to settle on both makes sense given their ideological standpoint on shipping interfaces and also causes endless mild annoyance and code duplication.
2) I believe it's still impossible to see the correct callsite in test logs when using slog as the logger. For more information, see https://github.com/neilotoole/slogt?tab=readme-ov-file#defic.... It's possible I'm out of date here — please correct me if this is wrong, it's actually a much larger annoyance for me and one of the reasons I still use uber/zap or charmbracelet/log.
Overall, especially given that it performs worse than uber/zap and everyone has basically standardized on that and it provides essentially the same interface, I recommend using uber/zap instead.
EDIT: just to expand further, take a look at the recommended method of wrapping helper methods that call logs. Compare to the `t.Helper()` approach. And some previous discussion. Frustrating!
- https://pkg.go.dev/log/slog#example-package-Wrapping
- https://github.com/golang/go/issues/59145#issuecomment-14770...
Curious how different people handle this. I personally pretty much always pass a logger into function, classes, structs (what have you) so it has the context I need it to. It's a tad more verbose I guess, but it's such a minor lift I've always found it worth it.
I really like structured logs and am pleased the Go team saw the benefits of bringing it into the standard library.
However, I feel like errors should be able to hold slog attributes. It makes for some very useful and easy error logging, especially when the logging takes place far up the execution chain from where the error happened.
This is easily possible with a custom error type and some log functions. I have published on GitHub my small and crude implementation that I use in a few hobby projects, MIT licensed, if anyone is interested. https://github.com/sveinnthorarins/sterlo
https://github.com/Danlock/pkg/blob/main/errors/attr_test.go
I can understand why it's not in the stdlib though, it seems easy enough to run into key overwriting issues if a dependency returned custom errors with attributes.
I appreciate the built in support slog has for slog.GroupValue and the slog.LogValuer interface that enables everyone to build a solution best for their needs.
I take their point, but given the typical value of most logging lines, this does not seem worth the tax you pay in readability. This is a gripe I have with oTel too --- it really cruds your code up --- but with oTel you're getting long-term value that logging (which is still my go-to o11y) doesn't.
I especially appreciated the section on slog.Attr and the !BADKEY issue, it’s one of those little things that can go unnoticed until logs break in prod.
Thanks for putting this together!