example

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:

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:

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

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.

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"