architecture.md

Korifi Architecture

Overview

Core Components

Korifi is built up of the following core components:

• Korifi CRDs: A set of Kubernetes custom resources under the korifi.cloudfoundry.org Group that implement the core set of V3 Cloud Foundry resources. Users can interact with these custom resources directly through the Kubernetes API (and clients such as kubectl) to extend Korifi and implement their own declarative workflows. As we are still in beta, the CRDs are still under development. For the latest list, check out our types in the source code.
• korifi-api Deployment: A Golang implementation of a core set of V3 Cloud Foundry APIs that is backed by the Korifi CRDs. Existing Cloud Foundry API clients (such as the CF CLI) can target the Korifi API and continue to use their existing CF developer workflows.
• korifi-controllers Deployment: A set of Kubernetes controllers that implement core CF subsystems by orchestrating and reconciling the Korifi CRDs into lower level Kubernetes resources. This Deployment also runs the default implementations of the builder, runner, and routing implementations.
• Korifi Webhooks: We rely on a set of mutating/validating admission webhooks (that run as part of the korifi-controllers Deployment) to enforce consistent rules and policy for both API and CRD users.

Extension Points

Korifi includes several custom resources that serve as extension points to provide additional flexibility to operators and developers. Currently, we provide the BuildWorkload, AppWorkload, and TaskWorkload custom resources as interfaces that abstract away the app staging and running subsystems from the rest of the project. Platform teams (and the Korifi community in general) are welcome to implement their own controllers for these resources to support other build systems and runtimes.

• BuildWorkload Resource: A custom resource that serves as an interface to the underlying build system used for staging applications. This resource contains all the information needed to stage an app and controller implementations communicate back via its status. The kpack-image-builder controller is our reference implementation for application staging that utilizes kpack and Cloud Native Buildpacks.

• AppWorkload Resource: A custom resource that serves as an interface to the underlying runtime. This resource contains all the information needed to run an app, and controller implementations communicate back to the rest of Korifi via its status. The statefulset-runner controller is our reference implementation that runs apps via Kubernetes StatefulSets. StatefulSets allow us to support features of CF such as the CF_INSTANCE_INDEX (an ordered numeric index for each container) environment variable and APIs, but there have been talks to loosen some of this support and use Deployments instead.

• TaskWorkload Resource: A custom resource that serves as an interface to the underlying runtime. This resource contains all the information needed to run a task, and controller implementations communicate back to the rest of Korifi via its status. The job-task-runner controller is our reference implementation that runs tasks via Kubernetes Jobs.

Although these are the three areas where we have explicitly defined extension points (see this proposal for more background on that) and pluggable controllers, CRDs by their nature lead to very loose coupling between components. The CFRoute resource, for example, can be considered both a core Korifi CRD (it is required for implementing the V3 Route endpoints) and an extension point resource since it includes all the necessary information needed to describe a Cloud Foundry route. Other resources (Service Instances and Bindings?) may also evolve overtime to support this kind of flexibility.

All of these default implementations are built into the korifi-controllers Deployment and can be toggled on and off individually via Helm properties.

Dependencies

As mentioned earlier, we aim to be loosely coupled with our dependencies and interact with them through defined, pluggable interfaces. That said, currently we depend on the following for both core Korifi / the provided controllers that implement some of our pluggable subsystems.

• cert-manager: We use cert-manager to generate and rotate the internal certs used for our webhooks.
• metrics-server: We use metrics-server to expose app container metrics to developers.
• kpack: We use kpack and Cloud Native Buildpacks to stage apps via the kpack-image-builder controller. This dependency sits behind our BuildWorkload abstraction and the kpack-image-builder controller could be replaced with alternative staging implementations.
• Contour: We use Contour as our default ingress controller. Contour is a CNCF project that serves as a control plane for Envoy Proxy that provides a robust, lightweight ingress routing solution. Our CFRoute resources are reconciled into Gateway API HTTPRoute and K8s Service resources to implement app ingress routing. However, Contour could be substituted by any Gateway API implementation.
• Service Bindings for Kubernetes: Korifi supports the Service Bindings for Kubernetes ServiceBinding resource. A ServiceBinding reconciler (such as service-binding-controller) is required for those resources to be reconciled correctly and volume mounted on to app workloads.

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Deeper Dives

Korifi Core

Korifi provides a CF API translation layer that implements a subset of the v3 Cloud Foundry APIs. The v3 CF APIs are generally RESTful and enable granular interaction with the CF resources. A simple command such as "cf push" will call dozens of discrete API endpoints in order to orchestrate the push.

1. POST /v3/spaces/<space_guid>/actions/apply_manifest
2. GET /v3/jobs/<job_guid>
3. GET /v3/apps?name=<app_name>&space_guids=<space_guid>
4. GET /v3/packages
5. POST /v3/packages/<package_guid>/upload
6. GET /v3/packages/<package_guid>
7. POST /v3/builds
8. and so on...

To implement these various endpoints, we ported over the relevant CF resources in the form of Kubernetes custom resources. So instead of having a row in the apps table in Cloud Controller's database, we have a CFApp custom resource. Similar resources exist for Processes, Routes, etc. Two core Korifi components handle the translation and lifecycle of these resources: the Korifi API and Korifi Controllers.

Korifi API

The Korifi API performs three primary functions:

1. It translates requests from a CF API client (such as the CLI) into Kubernetes API requests that create, read, update, and otherwise interpret the various CF custom resources.
2. It translates responses from the Kubernetes logging endpoints and metrics-server into CF compatible responses.
3. It uses libraries from the kpack project to convert application source code from the CF CLI (zip files) into OCI images

Korifi Controllers

The Korifi Controllers component is a single process that runs a set of Kubernetes controllers that were built using the kubebuilder framework as scaffolding. These controllers watch their respective custom resources and either transform them into downstream resources (e.g. a CFRoute becomes a Gateway API HTTPRoute and Kubernetes Service) and/or do bookkeeping such as propagate status and actual state upwards so that the Korifi API can return it back to CF clients (e.g. is a Pod running or did a kpack Build succeed). For the most part these controllers are translational and the actual developer outcomes are handled by the components we integrate with.

Korifi CRDs

Although we expect most users to interact with Korifi using existing CF API clients, the true "API" for Korifi is actually its custom resources. Since these resources extend the Kubernetes API and authentication is handled by it, users can use any K8s API client (such as kubectl, client-go, k9s, etc.) to view and manipulate the Korifi resources directly.

Some examples:

• Orgs and spaces can be declaratively managed via the CFOrg and CFSpace resources and roles can be applied by creating K8s RoleBindings.
• An app developer could orchestrate their own "cf push" by directly manipulating the CFApp, CFPackage, and CFBuild resources.
• An operator could use k9s or a generic Kubernetes GUI to visualize their Korifi installation and all apps running on the cluster.

Authentication and Authorization

Korifi relies on the Kubernetes API and RBAC (Roles, ClusterRoles, RoleBindings, etc.) for authentication and authorization (aka auth(n/z)). Users authenticate using their Kubernetes cluster credentials and either interact with the CRDs directly or send their credentials to the Korifi API layer to interact with the resources on their behalf. Cloud Foundry roles (such as SpaceDeveloper) have corresponding ClusterRoles on the cluster and commands like cf set-space-role result in RoleBindings being created in the appropriate namespaces.

Check out the User Authentication Overview docs for more details on our auth(n/z) strategy.

Organization and Space Hierarchy / Multi-tenancy

Cloud Foundry has a tiered tenancy system consisting of the cluster or "foundation" level, organization level, and space level. The CF foundation will contain one or more organizations which will themselves contain one or more spaces. CF roles typically allow for read/write access in these various areas. For example, a "CF Admin" user can make shared domains for the entire CF installation as well as interact with apps within an individual space, while a "Space Developer" user will typically be able to view things within their org as well as push apps within their space.

We model these using Kubernetes namespaces. There is a root "cf" namespace that can contain multiple CFOrg custom resources. These trigger the creation of K8s namespaces for each org which themselves will contain CFSpace resources that point to additional namespaces for each space. This is convenient because it maps closely to the CF model in terms of app isolation and user permissions on Kubernetes. Initially we used the Hierarchical Namespaces Controller project to manage this hierarchy, but moved away to a custom implementation for various reasons.

Routing

We integrate with the Kubernetes Gateway API to implement routing to both the Korifi API and app workloads. The CFRoute custom resource supports the CF route management APIs and is converted into GatewayAPI HTTPRoute and Kubernetes Service resources. We use a validating webhook to apply Cloud Controller's validation rules to the routes (e.g. no duplicate routes, route has a matching CFDomain, etc).

By leveraging the Gateway API, we abstract Korifi away from concrete networking implementations. Users can deploy whatever networkers they want as long as they are a Gateway API implementation.

Service Management

We currently support the CF APIs for managing and binding user-provided service instances (UPSIs) by providing two custom resources: the CFServiceInstance and CFServiceBinding. These CRs primarily exist to back the CF APIs and store additional state that isn't relevant in downstream service resources. They implement the ProvisionedService "duck type" from the Service Bindings for Kubernetes spec which allow them to interoperate directly with other projects surrounding the Kubernetes Service Bindings ecosystem (kpack, ServiceBinding reconcilers, etc.).

UPSIs enable devs to provide their own service credentials (can be sourced from anywhere) and have them projected onto workloads in the same way that CF would project credentials from managed services. For Korifi we aim to provide compatibility for existing Cloud Foundry applications by aggregating these UPSI credentials and providing them to workloads through the VCAP_SERVICES environment variable. Additionally, we integrate with the reference Service Bindings reconciler via the "Service Bindings for Kubernetes" ServiceBinding resource to volume mount these credentials onto workload Pods. This enables apps that are pushed with Korifi to interoperate with binding-aware Cloud Native Buildpacks as well as updated app frameworks like Spring Cloud Bindings.

Logging and Metrics

Korifi supports best effort access to current logs and resource metrics through the "cf app", "cf logs", and "cf push" (staging logs) commands. We do this by implementing the CF APIs for accessing these resources and querying the Kubernetes metrics-server component for Pod container metrics (memory and CPU) and the Kubernetes API's Pod log endpoint to fetch logs from the staging/running containers for Apps. The Korifi API translates these metrics and logs into CF API responses that existing CF clients understand.

We do not plan on porting over the existing CF for VMs logging and metrics stack due to its complexity and the fact that there are alternatives available in the Kubernetes community. For more reliable access to app logs/metrics and more durable storage we recommend using Kubernetes-native tools like Prometheus for collecting app metrics and fluentbit sidecars for log egress.

Object Storage for App Artifacts

Korifi does not use an object store / blobstore (e.g. Amazon S3, WebDav, etc.) to store app source code packages and runnable app droplets like CF for VMs. Instead, we rely on a container registry (e.g. DockerHub, Harbor, etc.) since all Kubernetes clusters require one to source their image. App source code (via the CFPackage resource) is transformed into a single layer OCI-spec container image and stored on the container registry instead of as a zip file on a blobstore. Likewise, we no longer use the custom "droplet" (zip file container runnable app source) + "stack" concept from CF for VMs. The build system produces container images (also stored in the container registry) that can be run anywhere.

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Misc

Architecture Decision Records and Proposals

We primarily propose changes and record decisions in two places:

1. As Architecture Decision Records (ADRs)
2. As Proposal documents

Check out those out for more information on past decisions and potential upcoming changes.

We typically use ADRs to record context about decisions that were already made or discussed as a team and proposals to discuss larger changes or check out tracks of work. More details on each type of document and their templates can be found here.

CF <--> Kubernetes Resource Mappings

Cloud Foundry Concept/Resource	Kubernetes Representation
CF User	Kubernetes RBAC User / Service Account
UAA Oauth2 Token	Kubernetes auth token or client cert/key
Roles/Permissions	Kubernetes RBAC resources
Organization	CFOrg + Namespace
Space	CFSpace + Namespace
App	CFApp
App Env Vars	Kubernetes Secret
Package (app source code)	CFPackage + OCI Container Image
Build	CFBuild, BuildWorkload, Kpack Image
Droplet	CFBuild, BuildWorkload, OCI Container Image
Process	CFProcess, AppWorkload (eventually StatefulSet and Pod)
Task	CFTask, TaskWorkload (eventually Kubernetes Job and Pod)
Route	CFRoute, Gateway API HTTPRoute, K8s Service
Domain	CFDomain
Service Instance	CFServiceInstance (ProvisionedService)
Service Binding	CFServiceBinding + ServiceBinding for Kubernetes
Service credentials	Kubernetes Secret
Diego Desired LRP	AppWorkload + StatefulSet
Diego Actual LRP	Kubernetes Pod