palindrome-policy

README

Palindrome Policy

This Kubewarden policy can be used to avoid the deployment of Pod with palindrome labels.

Introduction

The policy looks at the labels of a Pod and rejects the request if it contains any label that is a palindrome.

The policy can be configured to whitelist a set of labels and to tolerate a threshold via the runtime settings of the policy. The configuration of the policy is expressed via this structure:

{
  "whitelisted_labels": [ "level" ],
  "threshold": 1
}

If the threshold is set then the request is rejected only if the number of palindromes exceed the value set. The whitelisted_labels doesn't count to exceed the threshold.

i.e.: with the previous settings a Pod with the ["level", "radar"] labels would be approved, since the level label is whitelisted and the threshold is 1.

Code organization

The code that takes care of parsing the settings can be found inside of the settings.go file.

The actual validation code is defined inside of the validate.go file.

The main.go contains only the code which registers the entry points of the policy.

Implementation details

DISCLAIMER: WebAssembly is a constantly evolving topic. This document describes the status of the Go ecosystem at April 2021.

Currently the official Go compiler cannot produce WebAssembly binaries that can be run outside of the browser. Because of that, Kubewarden Go policies can be built only with the TinyGo compiler.

TinyGo doesn't yet support all the Go features (see here to see the current project status). Currently its biggest limitation is the lack of a fully supported reflect package. Among other things, that leads to the inability to use the encoding/json package against structures and user defined types.

Kubewarden policies need to process JSON data like the policy settings and the actual request received by Kubernetes. However it's still possible to write a Kubewarden policy by using some 3rd party libraries.

This is a list of libraries that can be useful when writing a Kubewarden policy:

• Parsing JSON: queries against JSON documents can be written using the gjson library. The library features a powerful query language that allows quick navigation of JSON documents and data retrieval.
• Mutating JSON: changing the contents of a JSON document can be done using the sjson library.
• Generic set implementation: using Set data types can significantly reduce the amount of code inside of a policy, see the union, intersection, difference,... operations provided by a Set implementation. The mapset can be used when writing policies.

Last but not least, this policy takes advantage of helper functions provided by Kubewarden's Go SDK.

Testing

This policy comes with a set of unit tests implemented using the Go testing framework.

As usual, the tests are defined inside of the _test.go files. Given these tests are not part of the final WebAssembly binary, the official Go compiler can be used to run them. Hence they can take advantage of the encoding/json package to reduce some testing boiler plate.

The unit tests can be run via a simple command:

make test

It's also important the test the final result of the TinyGo compilation: the actual WebAssembly module.

This is done by a second set of end-to-end tests. These tests use the kwctl cli provided by the Kubewarden project to load and execute the policy.

The e2e tests are implemented using bats: the Bash Automated Testing System.

The end-to-end tests are defined inside of the e2e.bats file and can be run via this command:

make e2e-tests

Automation

This project contains the following GitHub Actions:

• e2e-tests: this action builds the WebAssembly policy, installs the bats utility and then runs the end-to-end test
• unit-tests: this action runs the Go unit tests
• release: this action builds the WebAssembly policy and pushes it to a user defined OCI registry (ghcr is a perfect candidate)