community/contributors/devel/sig-instrumentation/migration-to-structured-logging.md

Structured Logging migration instructions

This document describes instructions for migration proposed by Structured Logging KEP. It describes new structured functions introduced in klog (Kubernetes logging library) and how log calls should be changed to utilize new features. This document was written for the initial migration of kubernetes/kubernetes repository proposed for Alpha stage, but should be applicable at later stages or for other projects using klog logging library.

Goal of Alpha migration

The first step is to introduce structure to the high percentage of logs generated in Kubernetes by changing only a small number of logs API calls. Based on criteria described in the selecting most important logs section, the selected 22 log calls are estimated to impact 99.9% of log volume. The up to date list of these log calls is provided in the Enhancement Issue.

Structured logging in Kubernetes

With this enhancement a set of new functions were added to klog. Structured logging functions the follow interface based on logr, which has a different design than other klog functions which are based on glog. It is recommended to familiarize yourself with logr.

Here are the prototypes of functions added to klog that will be utilized during migration:

package klog

// InfoS structured logs to the INFO log.
// The msg argument used to add constant description to the log line.
// The key/value pairs would be join by "=" ; a newline is always appended.
//
// Examples:
// >> klog.InfoS("Pod status updated", "pod", klog.KObj(pod), "status", "ready")
// output:
// >> I1025 00:15:15.525108       1 controller_utils.go:116] "Pod status updated" pod="kube-system/kubedns" status="ready"
func InfoS(msg string, keysAndValues ...interface{})

// ErrorS structured logs to the ERROR, WARNING, and INFO logs.
// the err argument used as "err" field of log line.
// The msg argument used to add constant description to the log line.
// The key/value pairs would be join by "=" ; a newline is always appended.
//
// Examples:
// >> klog.ErrorS(err, "Failed to update pod status")
// output:
// >> E1025 00:15:15.525108       1 controller_utils.go:114] "Failed to update pod status" err="timeout"
func ErrorS(err error, msg string, keysAndValues ...interface{})

// KObj is used to create ObjectRef when logging information about Kubernetes objects
// Examples:
// >> klog.InfoS("Pod status updated", "pod", klog.KObj(pod), "status", "ready")
// output:
// >> I1025 00:15:15.525108       1 controller_utils.go:116] "Pod status updated" pod="kube-system/kubedns" status="ready"
func KObj(obj KMetadata) ObjectRef

// KRef is used to create ObjectRef when logging information about Kubernetes objects without access to metav1.Object
// Examples:
// >> klog.InfoS("Pod status updated", "pod", klog.KRef(podNamespace, podName), "status", "ready")
// output:
// >> I1025 00:15:15.525108       1 controller_utils.go:116] "Pod status updated" pod="kube-system/kubedns" status="ready"
func KRef(namespace, name string) ObjectRef

// ObjectRef represents a reference to a kubernetes object used for logging purpose
// In text logs it is serialized into "{namespace}/{name}" or "{name}" if namespace is empty
type ObjectRef struct {
	Name      string `json:"name"`
	Namespace string `json:"namespace,omitempty"`
}

// KMetadata is a subset of the kubernetes k8s.io/apimachinery/pkg/apis/meta/v1.Object interface
// this interface may expand in the future, but will always be a subset of the
// kubernetes k8s.io/apimachinery/pkg/apis/meta/v1.Object interface
type KMetadata interface {
	GetName() string
	GetNamespace() string
}

Migration

1. Change log functions to structured equivalent
2. Remove string formatting from log message
3. Name arguments
4. Use klog.KObj and klog.KRef for Kubernetes object references
5. Verify log output

Change log functions to structured equivalent

Structured logging functions follow a different logging interface design than other functions in klog. They follow minimal design from logr thus there is no one-to-one mapping.

Simplified mapping between functions:

• klog.Infof, klog.Info, klog.Infoln, klog.InfoDepth -> klog.InfoS
• klog.V(N).Infof, klog.V(N).Info, klog.V(N).Infoln -> klog.V(N).InfoS
• klog.Warning, klog.Warningf, klog.Warningln, klog.WarningDepth -> klog.InfoS
• klog.Error, klog.Errorf, klog.Errorln, klog.ErrorDepth -> klog.ErrorS
• klog.Fatal, klog.Fatalf, klog.Fatalln, klog.FatalDepth -> klog.ErrorS

Removing Depth

Functions with depth (klog.InfoDepth, klog.WarningDepth, klog.ErrorDepth, klog.FatalDepth) are used to indicate that the source of the log (added as metadata in log) is different than the invocation of logging library. This is usually used when implementing logging util functions. As logr interface doesn't support depth, those functions should return logging arguments instead of calling klog directly.

For example

func Handle(w http.ReponseWriter, r *http.Request) {
    logHTTPRequest(r)
    handle(w, r)
}

func logHTTPRequest(r *http.Request) {
    klog.InfoDepth(1, "Received HTTP %s request", r.Method)
}

should be replaced with

func Handle(w http.ReponseWriter, r *http.Request) {
    klog.InfoS("Received HTTP request", httpRequestLog(r)...)
    handle(w, r)
}

func httpRequestLog(r *http.Request) []interface{} {
    return []interface{}{
        "verb", r.Method,
    }
}

Using ErrorS

With klog structured logging borrowing the interface from logr it also inherits it's differences in semantic of error function. Logs generated by ErrorS command may be enhanced with additional debug information (such as stack traces) or be additionally sent to special error recording tools. Errors should be used to indicate unexpected behaviours in code, like unexpected errors returned by subroutine function calls.

Calling ErrorS with nil as error is semi-acceptable if there is error condition that deserves a stack trace at this origin point. For expected errors (errors that can happen during routine operations) please consider using klog.InfoS and pass error in err key instead.

Replacing Fatal calls

Use of Fatal should be discouraged and it's not available in new functions. Instead of depending on the logger to exit the process, you should call os.Exit() yourself.

Fatal calls use a default exit code of 255. When migrating, please use an exit code of 1 and include an "ACTION REQUIRED:" release note.

Remove string formatting from log message

With structured logging, log messages are no longer formatted, leaving argument marshalling up to the logging client implementation. This allows messages to be a static description of event.

All string formatting (%d, %v, %w, %s) should be removed and log message string simplified. Describing arguments in log messages is no longer needed and should be removed leaving only a description of what happened.

Additionally we can improve messages to comply with good practices:

• Start from a capital letter.
• Do not end the message with a period.
• Use active voice. Use complete sentences when there is an acting subject ("A could not do B") or omit the subject if the subject would be the program itself ("Could not do B").
• Use past tense ("Could not delete B" instead of "Cannot delete B")
• When referring to an object, state what type of object it is. ("Deleted pod" instead of "Deleted")

For example

klog.Infof("delete pod %s with propagation policy %s", ...)

should be changed to

klog.InfoS("Deleted pod", ...)

Some logs are constructed solely from string formats. In those cases a message needs to be derived from the context of the log call.

For example http access logs

func LogHTTP(r *http.Request) {
   klog.Infof("%s %s: (%v) %v%v%v [%s %s]", ...)
}

should be changed to

func LogHTTP(r *http.Request) {
   klog.InfoS("Received HTTP request", ...)
}

Name arguments

Even though new structured logging functions have very similar function prototype func (string, ...interface{}) it has different meaning for variadic arguments. Instead of just passing arguments, now we are passing key value pairs of argument name and argument value. This means when migrating a log call we need to add an additional string before each argument, that will be used as it's name.

For example

func LogHTTP(r *http.Request) {
   klog.Infof("Received HTTP request, path: %s, method: %s", r.Path, r.Method)
}

should be changed to

func LogHTTP(r *http.Request) {
   klog.InfoS("Received HTTP request", "path", r.Path, "method", r.Method)
}

Names of arguments should use lowerCamelCase and be alphanumeric. Arguments names in one log call should be unique. Names should be picked based on semantic meaning of value itself, not the context in which is used (log message should imply the context). For example names like status should be used over (desiredStatus, oldStatus, badStatus) thus allowing to query and join different log lines of the status field.

Kubernetes objects should be referenced using only their kind, no matter their api group or version. Example argument names: deployment, pod, node, replicaSet. For objects of unknown type, is ok to log them under object key with addition of apiVersion and kind fields describing the k8s object type.

In situations when we want to the log value of the same meaning twice (e.g. transition between state) it is ok to name an additional argument based on context, but leaving one most current/correct value with canonical name.

Examples of keys (strongly suggested, will be extended when pattern emerge, no standard schema yet):

• err - error when using klog.InfoS. Used for expected errors that are not klog.ErrorS.
• object - reference to k8s objects of unknown type. Should be used with kind and apiVersion.
• kind - kind of k8s object of unknown type.
• apiVersion - API version of k8s object of unknown type.

Example:

func ChangeStatus(newStatus, currentStatus string) {
  err := changeStatus(newStatus)
  if err != nil {
    klog.ErrorS(err, "Failed changing status", "desiredStatus", newStatus, "status", currentStatus)
  }
  klog.InfoS("Changed status", "previousStatus", currentStatus, "status", newStatus)
}

Use klog.KObj and klog.KRef for Kubernetes objects

As part of structured logging migration we want to ensure that kubernetes objects references are consistent within the codebase. Two new utility functions were introduced to klog klog.KObj and klog.KRef. Any reference (name, uid, namespace) to Kubernetes Object (Pod, Node, Deployment, CRD) should be rewritten to utilize those functions. In situations when object UID is would be beneficial for log, it should be added as separate field with UID suffix.

For example

func updatePod(pod *covev1.Pod) {
   ...
   klog.Infof("Updated pod %s in namespace %s", pod.Name, pod.Namespace)
}

should be changed to

func updatePod(pod *covev1.Pod) {
   ...
   klog.InfoS("Updated pod", "pod", klog.KObj(pod))
}

And

func updatePod(pod *covev1.Pod) {
   ...
   klog.Infof("Updated pod with uid: %s", pod.Uid)
}

should be changed to

func updatePod(pod *covev1.Pod) {
   ...
   klog.InfoS("Updated pod", "pod", klog.KObj(pod), "podUID", pod.Uid)
}

klog.KObj requires passing a kubernetes object (struct implementing metav1.Object interface). In situations where the object is not available, we can use klog.KRef. Still it is suggested to rewrite the code to use object pointer instead of strings where possible.

func updatePod(podName, podNamespace string) {
   ...
   klog.InfoS("Updated pod", "pod", klog.KRef(podNamespace, podName))
}

For non-namespaced object we can pass empty string to namespace argument

func updateNode(nodeName string) {
   ...
   klog.InfoS("Updated node", "node", klog.KRef("", nodeName))
}

Verify log output

With the introduction of structured functions log arguments will be formatted automatically instead of depending on the caller. This means that we can remove the burden of picking the format by caller and ensure greater log consistency, but during migration it's important to ensure that we avoid degradation of log quality. We should ensure that during migration we preserve properties like:

• meaning of event described by log
• verbosity of stored information

PRs migrating logs should include examples of outputted logs before and after the change, thus helping reviewers understand the impact of change.

Example code to compare httplog.go#168

package main

import (
	"fmt"
	"k8s.io/klog/v2"
	"net/http"
	"time"
)

type respLogger struct {
	status         int
	statusStack    string
	addedInfo      string
	req *http.Request
}

func (rl *respLogger) Log(latency time.Duration) {
	klog.InfoDepth(1, fmt.Sprintf("verb=%q URI=%q latency=%v resp=%v UserAgent=%q srcIP=%q: %v%v",
		rl.req.Method, rl.req.RequestURI,
		latency, rl.status,
		rl.req.UserAgent(), rl.req.RemoteAddr,
		rl.statusStack, rl.addedInfo,
	))
}

func (rl *respLogger) LogArgs(latency time.Duration) []interface{} {
    return []interface{}{
        "verb", rl.req.Method,
        "URI", rl.req.RequestURI,
        "latency", latency,
        "resp", rl.status,
        "userAgent", rl.req.UserAgent(),
        "srcIP", rl.req.RemoteAddr,
    }
}

func main() {
	klog.InitFlags(nil)

    // Setup
	rl := respLogger{
		status:             200,
		req:                &http.Request{
			Method:           "GET",
			Header:           map[string][]string{"User-Agent": {"Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0. 2272.118 Safari/537.36."}},
			RemoteAddr:       "127.0.0.1",
			RequestURI:       "/metrics",
		},
	}
	latency := time.Second

    // Before migration
    rl.Log(latency)

    // After migration
	klog.InfoS("Received HTTP request", rl.LogArgs(latency)...)
}

Log output before migration

I0528 19:15:22.737538   47512 logtest.go:52] verb="GET" URI="/metrics" latency=1s resp=200 UserAgent="Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0. 2272.118 Safari/537.36." srcIP="127.0.0.1":

After

I0528 19:15:22.737588   47512 logtest.go:55] "Received HTTP request" verb="GET" URI="/metrics" latency="1s" resp=200 userAgent="Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0. 2272.118 Safari/537.36." srcIP="127.0.0.1"