# Example
This directory includes example logger binaries which show how to use
component-base/logs and what effect the different command line options have.
Like most Kubernetes components, `cmd` uses Cobra and pflags. `stdlib` uses
just plain Go libraries. `test` contains a unit test with per-test output.
`slog2k8s` shows how an application using `log/slog` from Go 1.21 can include
packages from Kubernetes. `k8s2slog` is the other direction.
Below we can see examples of how some features work.
## Default
Run:
```console
go run ./staging/src/k8s.io/component-base/logs/example/cmd/logger.go
```
Expected output:
```
I0329 11:36:38.734334 99095 logger.go:44] "Oops, I shouldn't be logging yet!"
This is normal output via stdout.
This is other output via stderr.
I0329 11:36:38.734575 99095 logger.go:76] Log using Infof, key: value
I0329 11:36:38.734592 99095 logger.go:77] "Log using InfoS" key="value"
E0329 11:36:38.734604 99095 logger.go:79] Log using Errorf, err: fail
E0329 11:36:38.734619 99095 logger.go:80] "Log using ErrorS" err="fail"
I0329 11:36:38.734653 99095 logger.go:87] "Now the default logger is set, but using the one from the context is still better."
I0329 11:36:38.734693 99095 logger.go:94] "runtime" duration="1m0s"
I0329 11:36:38.734710 99095 logger.go:95] "another runtime" duration="1m0s"
```
## JSON
Run:
```console
go run ./staging/src/k8s.io/component-base/logs/example/cmd/logger.go --logging-format json
```
Expected output:
```
I0329 11:38:01.782592 99945 logger.go:44] "Oops, I shouldn't be logging yet!"
This is normal output via stdout.
This is other output via stderr.
{"ts":1648546681782.9036,"caller":"cmd/logger.go:76","msg":"Log using Infof, key: value\n","v":0}
{"ts":1648546681782.9392,"caller":"cmd/logger.go:77","msg":"Log using InfoS","v":0,"key":"value"}
{"ts":1648546681782.9763,"caller":"cmd/logger.go:79","msg":"Log using Errorf, err: fail\n"}
{"ts":1648546681782.9915,"caller":"cmd/logger.go:80","msg":"Log using ErrorS","err":"fail"}
{"ts":1648546681783.0364,"caller":"cmd/logger.go:87","msg":"Now the default logger is set, but using the one from the context is still better.","v":0}
{"ts":1648546681783.1091,"caller":"cmd/logger.go:94","msg":"runtime","v":0,"duration":"1m0s"}
{"ts":1648546681783.1257,"caller":"cmd/logger.go:95","msg":"another runtime","v":0,"duration":"1h0m0s","duration":"1m0s"}
```
## Verbosity
```console
go run ./staging/src/k8s.io/component-base/logs/example/cmd/logger.go -v1
```
The expected output now includes `Log less important message`:
```
I0329 11:38:23.145695 100190 logger.go:44] "Oops, I shouldn't be logging yet!"
This is normal output via stdout.
This is other output via stderr.
I0329 11:38:23.145944 100190 logger.go:76] Log using Infof, key: value
I0329 11:38:23.145961 100190 logger.go:77] "Log using InfoS" key="value"
E0329 11:38:23.145973 100190 logger.go:79] Log using Errorf, err: fail
E0329 11:38:23.145989 100190 logger.go:80] "Log using ErrorS" err="fail"
I0329 11:38:23.146017 100190 logger.go:83] Log less important message
I0329 11:38:23.146034 100190 logger.go:87] "Now the default logger is set, but using the one from the context is still better."
I0329 11:38:23.146074 100190 logger.go:94] "runtime" duration="1m0s"
I0329 11:38:23.146091 100190 logger.go:95] "another runtime" duration="1m0s"
```
## Contextual logging
Contextual logging enables the caller of the function to add a string prefix
and additional key/value pairs to a logger and then pass the updated logger
into functions via a `context` parameter.
At the moment, this functionality is controlled in Kubernetes with the
`ContextualLogging` feature gate and disabled by
default. `klog.LoggerWithValues`, `klog.LoggerWithName`, `klog.NewContext` just
return the original instance when contextual logging is
disabled. `klog.FromContext` doesn't check the context for a logger and instead
returns the global logger.
```console
go run ./staging/src/k8s.io/component-base/logs/example/cmd/logger.go --feature-gates ContextualLogging=true
```
The expected output now includes `example` (added by caller) and `myname`
(added by callee) as prefix and the caller's `foo="bar"` key/value pair:
```
I0329 11:47:36.830458 101057 logger.go:44] "Oops, I shouldn't be logging yet!"
This is normal output via stdout.
This is other output via stderr.
I0329 11:47:36.830715 101057 logger.go:76] Log using Infof, key: value
I0329 11:47:36.830731 101057 logger.go:77] "Log using InfoS" key="value"
E0329 11:47:36.830745 101057 logger.go:79] Log using Errorf, err: fail
E0329 11:47:36.830760 101057 logger.go:80] "Log using ErrorS" err="fail"
I0329 11:47:36.830795 101057 logger.go:87] "Now the default logger is set, but using the one from the context is still better."
I0329 11:47:36.830841 101057 logger.go:94] "example/myname: runtime" foo="bar" duration="1m0s"
I0329 11:47:36.830859 101057 logger.go:95] "example: another runtime" foo="bar" duration="1m0s"
```
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