Deep Dive into policy logging

Rafael Fernández López

Policies are regular programs. As such they often have the need to log information. In general, we are used to make our programs log into standard output (stdout) and standard error (stderr) outputs.

However, policies run in a confined WebAssembly environment. For this mechanism to work as usual Kubewarden would need to set up the runtime environment in a way that the policy can write to stdout and stderr file descriptors, and upon completion, Kubewarden can check them – or stream log messages as they pop up.

Given Kubewarden uses waPC for allowing intercommunication between the guest (the policy) and the host (Kubewarden – the policy-server or kwctl if we are running policies manually), we have extended our language SDK’s so that they can log messages by using waPC internally.

Kubewarden has defined a contract between policies (guests) and the host (Kubewarden) for performing policy settings validation, policy validation, policy mutation and now, logging.

The waPC interface used for logging is therefore a contract, because once you have built a policy, it should be possible to run it in future Kubewarden versions. In this sense, Kubewarden keeps this contract behind the SDK of your preferred language, so you don’t have to deal with the details of how logging is implemented in Kubewarden. You just have to use your logging library of choice for the language you are working with.

Let’s look into how to take advantage of logging with Kubewarden in specific languages!

For Policy Authors


We are going to use the Go policy template as a starting point.

Our Go SDK provides integration with the onelog library. When our policy is built for the WebAssembly target, it will send the logs to the host through waPC. Otherwise, it will just print them on stderr – but this is only relevant if you happen to run your policy outside a Kubewarden runtime environment.

One of the first things our policy does on its main.go file is to initialize the logger:

var (
	logWriter = kubewarden.KubewardenLogWriter{}
	logger    = onelog.New(
		onelog.ALL, // shortcut for onelog.DEBUG|onelog.INFO|onelog.WARN|onelog.ERROR|onelog.FATAL

We are then able to use onelog API in order to produce log messages. We could, for example, perform structured logging with debugging level:

logger.DebugWithFields("validating object", func(e onelog.Entry) {
    e.String("name", gjson.GetBytes(payload, "request.object.metadata.name").String())
	e.String("namespace", gjson.GetBytes(payload, "request.object.metadata.namespace").String())

Or, with info level:

logger.InfoWithFields("validating object", func(e onelog.Entry) {
    e.String("name", gjson.GetBytes(payload, "request.object.metadata.name").String())
	e.String("namespace", gjson.GetBytes(payload, "request.object.metadata.namespace").String())

What happens under the covers is that our Go SDK sends every log event to the kubewarden host through waPC.


Let’s use the Rust policy template as our guide.

Our Rust SDK implements an integration with the slog crate. This crate exposes the concept of drains, so we have to define a global drain that we will use throughout our policy code:

use kubewarden::logging;
use slog::{o, Logger};
lazy_static! {
    static ref LOG_DRAIN: Logger = Logger::root(
        o!("some-key" => "some-value") // This key value will be shared by all logging events that use
                                       // this logger

Then, we can use the macros provided by slog to log on different levels:

use slog::{crit, debug, error, info, trace, warn};

Let’s log an info level message:

    "rejecting resource";
    "resource_name" => &resource_name

As happens with the Go SDK implementation, our Rust implementation of the slog drain sends this logging events to the host by using waPC.

You can read more about slog here.


We will be looking at the Swift policy template for this example.

As happens with Go and Rust’s SDK’s, the Swift SDK is instrumented to use Swift’s LogHandler from the swift-log project, so our policy only has to initialize it. In our Sources/Policy/main.swift file:

import kubewardenSdk
import Logging


Then, in our policy business logic, under Sources/BusinessLogic/validate.swift, we are able to log with different levels:

import Logging

public func validate(payload: String) -> String {
    // ...

    logger.info("validating object",
        metadata: [
            "some-key": "some-value",

    // ...

Following the same strategy as the Go and Rust SDK’s, the Swift SDK is able to push log events to the host through waPC.

For Cluster Administrators

Being able to log from within a policy is half of the story. Then, we have to be able to read and potentially collect these logs.

As we have seen, Kubewarden policies support structured logging that is then forwarded to the component running the policy. Usually, this is kwctl if you are executing the policy in a manual fashion, or policy-server if the policy is being ran in a Kubernetes environment.

Both kwctl and policy-server use the tracing crate to produce log events, either the events that are produced by the application itself, or by policies that are running in WebAssembly runtime environments.


The kwctl CLI tool takes a very straightforward approach to logging from policies: it will print them to the standard error file descriptor.


The policy-server supports different log formats: json, text and otlp.

otlp? I hear you ask. It stands for OpenTelemetry Protocol. We will look into that in a bit.

If the policy-server is run with the --log-fmt argument set to json or text, the output will be printed to the standard error file descriptor in JSON or plain text formats. These messages can be read using kubectl logs <policy-server-pod>.

If --log-fmt is set to otlp, the policy-server will use OpenTelemetry to report logs and traces.


Kubewarden is instrumented with OpenTelemetry, so it’s possible for the policy-server to send trace events to an OpenTelemetry collector by using the OpenTelemetry Protocol (otlp).

Our official Kubewarden Helm Chart has certain values that allow you to deploy Kubewarden with OpenTelemetry support, reporting logs and traces to, for example, a Jaeger instance:

  enabled: True
      endpoint: "all-in-one-collector.jaeger.svc.cluster.local:14250"

This functionality closes the gap on logging/tracing, given the freedom that the OpenTelemetry collector provides to us in terms of flexibility of what to do with this logs and traces.

You can read more about Kubewarden’s integration with OpenTelemetry in our documentation.

But this is a big enough topic on its own worth a future blog post. Stay logged!