Contextual Logging in Kubernetes 1.24

Wednesday, May 25, 2022

Authors: Patrick Ohly (Intel)

The Structured Logging Working Group has added new capabilities to the logging infrastructure in Kubernetes 1.24. This blog post explains how developers can take advantage of those to make log output more useful and how they can get involved with improving Kubernetes.

Structured logging

The goal of structured logging is to replace C-style formatting and the resulting opaque log strings with log entries that have a well-defined syntax for storing message and parameters separately, for example as a JSON struct.

When using the traditional klog text output format for structured log calls, strings were originally printed with \n escape sequences, except when embedded inside a struct. For structs, log entries could still span multiple lines, with no clean way to split the log stream into individual entries:

I1112 14:06:35.783529  328441 structured_logging.go:51] "using InfoS" longData={Name:long Data:Multiple
lines
with quite a bit
of text. internal:0}
I1112 14:06:35.783549  328441 structured_logging.go:52] "using InfoS with\nthe message across multiple lines" int=1 stringData="long: Multiple\nlines\nwith quite a bit\nof text." str="another value"

Now, the < and > markers along with indentation are used to ensure that splitting at a klog header at the start of a line is reliable and the resulting output is human-readable:

I1126 10:31:50.378204  121736 structured_logging.go:59] "using InfoS" longData=<
	{Name:long Data:Multiple
	lines
	with quite a bit
	of text. internal:0}
 >
I1126 10:31:50.378228  121736 structured_logging.go:60] "using InfoS with\nthe message across multiple lines" int=1 stringData=<
	long: Multiple
	lines
	with quite a bit
	of text.
 > str="another value"

Note that the log message itself is printed with quoting. It is meant to be a fixed string that identifies a log entry, so newlines should be avoided there.

Before Kubernetes 1.24, some log calls in kube-scheduler still used klog.Info for multi-line strings to avoid the unreadable output. Now all log calls have been updated to support structured logging.

Contextual logging

Contextual logging is based on the go-logr API. The key idea is that libraries are passed a logger instance by their caller and use that for logging instead of accessing a global logger. The binary decides about the logging implementation, not the libraries. The go-logr API is designed around structured logging and supports attaching additional information to a logger.

This enables additional use cases:

The caller can attach additional information to a logger:
- WithName adds a prefix
- WithValues adds key/value pairs
When passing this extended logger into a function and a function uses it instead of the global logger, the additional information is then included in all log entries, without having to modify the code that generates the log entries. This is useful in highly parallel applications where it can become hard to identify all log entries for a certain operation because the output from different operations gets interleaved.
When running unit tests, log output can be associated with the current test. Then when a test fails, only the log output of the failed test gets shown by go test. That output can also be more verbose by default because it will not get shown for successful tests. Tests can be run in parallel without interleaving their output.

One of the design decisions for contextual logging was to allow attaching a logger as value to a context.Context. Since the logger encapsulates all aspects of the intended logging for the call, it is part of the context and not just using it. A practical advantage is that many APIs already have a ctx parameter or adding one has additional advantages, like being able to get rid of context.TODO() calls inside the functions.

Another decision was to not break compatibility with klog v2:

Libraries that use the traditional klog logging calls in a binary that has set up contextual logging will work and log through the logging backend chosen by the binary. However, such log output will not include the additional information and will not work well in unit tests, so libraries should be modified to support contextual logging. The migration guide for structured logging has been extended to also cover contextual logging.
When a library supports contextual logging and retrieves a logger from its context, it will still work in a binary that does not initialize contextual logging because it will get a logger that logs through klog.

In Kubernetes 1.24, contextual logging is a new alpha feature with ContextualLogging as feature gate. When disabled (the default), the new klog API calls for contextual logging (see below) become no-ops to avoid performance or functional regressions.

No Kubernetes component has been converted yet. An example program in the Kubernetes repository demonstrates how to enable contextual logging in a binary and how the output depends on the binary's parameters:

$ cd $GOPATH/src/k8s.io/kubernetes/staging/src/k8s.io/component-base/logs/example/cmd/
$ go run . --help
...
      --feature-gates mapStringBool  A set of key=value pairs that describe feature gates for alpha/experimental features. Options are:
                                     AllAlpha=true|false (ALPHA - default=false)
                                     AllBeta=true|false (BETA - default=false)
                                     ContextualLogging=true|false (ALPHA - default=false)
$ go run . --feature-gates ContextualLogging=true
...
I0404 18:00:02.916429  451895 logger.go:94] "example/myname: runtime" foo="bar" duration="1m0s"
I0404 18:00:02.916447  451895 logger.go:95] "example: another runtime" foo="bar" duration="1m0s"

The example prefix and foo="bar" were added by the caller of the function which logs the runtime message and duration="1m0s" value.

The sample code for klog includes an example for a unit test with per-test output.

klog enhancements

Contextual logging API

The following calls manage the lookup of a logger:

FromContext: from a context parameter, with fallback to the global logger
Background: the global fallback, with no intention to support contextual logging
TODO: the global fallback, but only as a temporary solution until the function gets extended to accept a logger through its parameters
SetLoggerWithOptions: changes the fallback logger; when called with ContextualLogger(true), the logger is ready to be called directly, in which case logging will be done without going through klog

To support the feature gate mechanism in Kubernetes, klog has wrapper calls for the corresponding go-logr calls and a global boolean controlling their behavior:

Usage of those functions in Kubernetes code is enforced with a linter check. The klog default for contextual logging is to enable the functionality because it is considered stable in klog. It is only in Kubernetes binaries where that default gets overridden and (in some binaries) controlled via the --feature-gate parameter.

ktesting logger

The new ktesting package implements logging through testing.T using klog's text output format. It has a single API call for instrumenting a test case and support for command line flags.

klogr

klog/klogr continues to be supported and it's default behavior is unchanged: it formats structured log entries using its own, custom format and prints the result via klog.

However, this usage is discouraged because that format is neither machine-readable (in contrast to real JSON output as produced by zapr, the go-logr implementation used by Kubernetes) nor human-friendly (in contrast to the klog text format).

Instead, a klogr instance should be created with WithFormat(FormatKlog) which chooses the klog text format. A simpler construction method with the same result is the new klog.NewKlogr. That is the logger that klog returns as fallback when nothing else is configured.

Reusable output test

A lot of go-logr implementations have very similar unit tests where they check the result of certain log calls. If a developer didn't know about certain caveats like for example a String function that panics when called, then it is likely that both the handling of such caveats and the unit test are missing.

klog.test is a reusable set of test cases that can be applied to a go-logr implementation.

Output flushing

klog used to start a goroutine unconditionally during init which flushed buffered data at a hard-coded interval. Now that goroutine is only started on demand (i.e. when writing to files with buffering) and can be controlled with StopFlushDaemon and StartFlushDaemon.

When a go-logr implementation buffers data, flushing that data can be integrated into klog.Flush by registering the logger with the FlushLogger option.

Various other changes

For a description of all other enhancements see in the release notes.

logcheck

Originally designed as a linter for structured log calls, the logcheck tool has been enhanced to support also contextual logging and traditional klog log calls. These enhanced checks already found bugs in Kubernetes, like calling klog.Info instead of klog.Infof with a format string and parameters.

It can be included as a plugin in a golangci-lint invocation, which is how Kubernetes uses it now, or get invoked stand-alone.

We are in the process of moving the tool into a new repository because it isn't really related to klog and its releases should be tracked and tagged properly.

Next steps

The Structured Logging WG is always looking for new contributors. The migration away from C-style logging is now going to target structured, contextual logging in one step to reduce the overall code churn and number of PRs. Changing log calls is good first contribution to Kubernetes and an opportunity to get to know code in various different areas.

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