ContainerImage.Pinniped/doc/architecture.md

# Architecture

The principal purpose of Pinniped is to allow users to access Kubernetes
clusters. Pinniped hopes to enable this access across a wide range of Kubernetes
environments with zero configuration.

This integration is implemented using a credential exchange API which takes as
input a credential from the external IDP (or internal federation trust
relationship) and returns a credential which is understood by the host
Kubernetes cluster. To learn more about this integration, see [Cluster
Integration Strategies](#cluster-integration-strategies).

<img src="img/pinniped_architecture.svg" alt="Pinniped Architecture Sketch" width="300px"/>

## External Identity Provider Integrations

Pinniped will consume identity from one or more external identity providers
(IDPs). Administrators will configure external IDPs via [Kubernetes custom
resources](https://kubernetes.io/docs/concepts/extend-kubernetes/api-extension/custom-resources/),
allowing Pinniped to be managed using GitOps and standard Kubernetes tools.

IDP integration support will be driven by empirical use case.

IDPs that support only just-in-time flows (such as OIDC) can be optionally
paired with a separate directory backend to enable directory-based flows such as
first-class support for policy editing UX.

### Supported External Identity Provider Types

The currently supported external IDP types are outlined here. More will be added
in the future.

1. Any webhook which implements the
   [Kubernetes TokenReview API](https://kubernetes.io/docs/reference/access-authn-authz/authentication/#webhook-token-authentication).

   In addition to allowing the integration of any existing IDP which implements this API, webhooks also
   serve as an extension point for Pinniped by allowing for integration of arbitrary custom authenticators.
   While a custom implementation may be in any language or framework, this project provides a
   sample implementation in Golang. See the `ServeHTTP` method of
   [cmd/local-user-authenticator/main.go](../cmd/local-user-authenticator/main.go).

## Cluster Integration Strategies

Pinniped will issue a cluster credential by leveraging cluster-specific
functionality.  In the near term, cluster integrations will happen via different
cluster-specific flows depending on the type of cluster. In the longer term,
Pinniped hopes to contribute and leverage upstream Kubernetes extension points that
cleanly enable this integration.

### Supported Cluster Integration Strategies

The currently supported cluster integration strategies are outlined here. More
will be added in the future.

1. Pinniped hosts a credential exchange API endpoint via a Kubernetes aggregated API server.
This API returns a new cluster-specific credential using the cluster's signing keypair to
issue short-lived cluster certificates. (In the future, when the Kubernetes CSR API
provides a way to issue short-lived certificates, then the Pinniped credential exchange API
will use that instead of using the cluster's signing keypair.)

## `kubectl` Integration

With any of the above IDPs and integration strategies, `kubectl` commands receive the
cluster-specific credential via a
[Kubernetes client-go credential plugin](https://kubernetes.io/docs/reference/access-authn-authz/authentication/#client-go-credential-plugins).
Users may use the Pinniped CLI as the credential plugin, or they may use any proprietary CLI
built with the [Pinniped Go client library](generated).

## Example Cluster Authentication Sequence Diagram

![example-cluster-authentication-sequence-diagram](img/pinniped.svg)