gatekeeper VS opa-envoy-plugin

When the Gatekeeper validatingwebhook came up, I was really worried that'd be the issue! Regardless I'd recommend anyone who cares about their cluster not collapsing to change the gatekeeper webhook to only intercept resources you care about: https://github.com/open-policy-agent/gatekeeper/pull/1806

Kyverno adds an extra layer of security where only the allowed type of manifest is deployed onto kubernetes, otherwise, it will reject or we can set validationFailureAction to audit which only logs the policy violation message for reporting. Kubewarden and Gatekeeper are alternative tools available to enforce policies on Kubernetes CRD.

Now, we have the hardest part resolved and let's turn our attention to the OPA Gatekeeper. Gatekeeper uses the OPA Constraint Framework to describe and enforce policy. Right now there are mainly 3 parts we should pay attention:

They enable you to establish the policies and regulations that govern cluster deployments and applications. Using predefined policies, policy engines can dynamically modify or create configurations. Policy engines such as Gatekeeper and Kyverno can be leveraged to meet legal and compliance requirements while maintaining operational flexibility and development speed.

Setup policy around what resource requirements can be requested by an app per environment. OPA and gatekeeper or kyverno can help. Setup access control for who can create or modify apps.

OPA Gatekeeper is an open source, general purpose policy engine. OPA decouples policy decisions from other responsibilities of an application, like those commonly referred to as business logic. OPA works equally well making decisions for Kubernetes, Microservices, functional application authorization and more, thanks to its single unified policy language.

Open Policy Agent is a well-known general-purpose policy engine and has emerged as a policy enforcer across the stacks be it API gateways, service meshes, Kubernetes, microservice, CICD, or IAC. OPA decouples decision making from policy enforcement such that whenever your software needs to make a decision regarding the incoming requests, it queries OPA. OPA-Envoy extends OPA with a gRPC server that implements the Envoy External Authorization API, thus making itself compatible to be as an external authz server to Emissary.

I'm observing the quick_start.yaml provided by OPA and trying to comprehend what's happening in the inject.rego resource (proper name?) under the ConfigMap. Can anyone break it down for me a bit? I think I have an inkling of what's happening but not sure.