CROSSPLANE - Cloud Control Plane and more
Last updated
Last updated
Is a distributed systems, with:
Control Plane
Data Planes
It protects data plane using distributed systems best practices:
Rate limiting
Retries
Optimistic concurrency
All Crossplane controllers emit Prometheus metrics and k8s events.
Extends Control plane by adding new API endpoints that represent a related set of cloud primitives.
These managed APIs are called ManagedResources(MR)
A provider installs controllers that reconcile the desired state of its MRs with the data plane.
Examples:
provider-aws, provider-azure, provider-gcp, provider-kubernetes, provider-terraform, provider-sql
Most control plane consumers(e.g. Platform Engineers) won’t interact with MRs directly. Instead, they interact with higher-level API abstractions. When a consumer(e.g. Application Developer) calls these higher-level APIs, Crossplane reconciles their desired state into one or more MRs. Crossplane calls this composition: the higher-level APIs are composed of one or more MRs.
In k8s analogy, think Pod as MR and Deployment as Composition
Are higher level API abstractions that consumers call to specify the desired state of their data plane.
A claim is composed of one or more MRs, which is the source of terms like composite resource and composition.
Provider authors set MR types and schemes
ControlPlane curators set claim types and schemas and could be of any shape curators need.
XRD, lets you define a new kind of claim (special kind of API that adds new APIs to Crossplane)
Claim:
A single claim could have group of MRs representing several cloud primitives into a single API.
For example, a SimplePostgres claim cloud be composed of an RDS-instance MR, a firewall-rule MR, and an IAM role-binding MR.
XRD also have kind(type of your new claim) and spec(schema of this new claim)
When you create an XRD, Crossplane adds a new API endpoint for your kind of claim.
It also starts a controller that reconciles claims, called a claim controller.
Claim controller doesn't create MRs directly.
Between a claim and its MRs lies an intermediate API resource called a composite resource, or XR.
When claim is created, Crossplane automatically creates a matching XR.
The XR controller then creates MRs.
A composite resource definition defines the type and schema of a claim and a composite resource.
Most XRDs define both an XR and a claim.
All claims have a corresponding XR type, but not all XRs have a corresponding claim type.
You can create an XRD that defines only an XR, not a caim.
Allows you to define private API abstraction: once that control plane consumers can't use directly.
XR can be composed of other XRs.
Example, the security team defines a standard FirewallRule XR. The database team should be allowed to compose a FirewallRule XR into a higher-level SimplePostgres claim, but the average platform consumer should not be able to create a FirewallRule claim directly
A composite resource may be composed of other composite resources.
An XRD teaches Crossplane about a new consumer-facing API; a Composition teaches Crossplane what to do when that API is called.
In its simplest form, a Composition is a list of the MRs Crossplane should create, update, or delete when a control plane consumer creates, updates, or deletes a claim.
A composition configures how Crossplane composes man‐ aged resources from a given claim
Crossplane supports two styles of Composition: patch and transform Composition (P&T) and Composition Functions.
When using P&T Composition, you specify a base list of man‐ aged resource templates. Each of these base resources can be patched with values copied from the claim. A value can option‐ ally be transformed before it’s applied to the base, for example to convert megabytes to gigabytes or a string to an integer.
When using Composition Functions, you specify a pipeline of Open Container Initiative (OCI) containers. The claim’s XR is passed to these containers, which are responsible for returning managed resources for Crossplane to create, update, or delete.
There are two kinds of Crossplane packages:
A Provider package installs a Provider. It creates API endpoints for all of a Provider’s MRs, and it installs controllers to reconcile those MRs with the data plane. Most curators won’t create Provider packages but will install them.
A Configuration package configures your control plane with new capabilities. Typically a Configuration package will configure your control plane to serve one or more new kinds of claim API. It does this by installing a collection of Providers, XRDs, and Compositions. Most curators do curate their own Configuration packages.
A Crossplane package is an OCI image with special Crossplane specific metadata. This means Crossplane packages can be distributed by any kind of OCI registry, including Amazon’s Elastic Container Registry (ECR), Google Container Registry (GCR), and Upbound’s Marketplace, which is focused specifically on Crossplane packages.
