This is a high-level overview of what Metacontroller provides for Kubernetes controller authors.

Dynamic Scripting

With Metacontroller’s hook-based design, you can write controllers in any language while still taking advantage of the efficient machinery we developed in Go for core controllers.

This makes Metacontroller especially useful for rapid development of automation in dynamic scripting languages like Python or JavaScript, although you’re also free to use statically-typed languages like Go or Java.

To support fast ramp-up and iteration on your ideas, Metacontroller makes it possible to write controllers with:

  • No schema/IDL
  • No generated code
  • No library dependencies
  • No container image build/push

Controller Best Practices

Controllers you write with Metacontroller automatically behave like first-class citizens out of the box, before you write any code.

All interaction with the Kubernetes API happens inside the Metacontroller server in response to your instructions. This allows Metacontroller to implement best practices learned from writing core controllers without polluting your business logic.

Even the simplest Hello, World example with Metacontroller already takes care of:

  • Label selectors (for defining flexible collections of objects)
  • Orphan/adopt semantics (controller reference)
  • Garbage collection (owner references for automatic cleanup)
  • Watches (for low latency)
  • Caching (shared informers/reflectors/listers)
  • Work queues (deduplicated parallelism)
  • Optimistic concurrency (resource version)
  • Retries with exponential backoff
  • Periodic relist/resync

Declarative Watches

Rather than writing boilerplate code for each type of resource you want to watch, you simply list those resources declaratively:

- apiVersion: v1
  resource: pods
- apiVersion: v1
  resource: persistentvolumeclaims

Behind the scenes, Metacontroller sets up watch streams that are shared across all controllers that use Metacontroller.

That means, for example, that you can create as many lambda controllers as you want that watch Pods, and the API server will only need to send one Pod watch stream (to Metacontroller itself).

Metacontroller then acts like a demultiplexer, determining which controllers will care about a given event in the stream and triggering their hooks only as needed.

Declarative Reconciliation

A large part of the expressiveness of the Kubernetes API is due to its focus on declarative management of cluster state, which lets you directly specify an end state without specifying how to get there. Metacontroller expands on this philosophy, allowing you to define controllers in terms of what they want without specifying how to get there.

Instead of thinking about imperative operations like create/read/update/delete, you just generate a list of all the things you want to exist. Based on the current cluster state, Metacontroller will then determine what actions are required to move the cluster towards your desired state and maintain it once its there.

Just like the built-in controllers, the reconciliation that Metacontroller performs for you is level-triggered so it’s resilient to downtime (missed events), yet optimized for low latency and low API load through shared watches and caches.

However, the clear separation of deciding what you want (the hook you write) from running a low-latency, level-triggered reconciliation loop (what Metacontroller does for you) means you don’t have to think about this.

Declarative Declarative Rolling Update

Another big contributor to the power of Kubernetes APIs like Deployment and StatefulSet is the ability to declaratively specify gradual state transitions. When you update your app’s container image or configuration, for example, these controllers will slowly roll out Pods with the new template and automatically pause if things don’t look right.

Under the hood, implementing gradual state transitions with level-triggered reconcilation loops involves careful bookkeeping with auxilliary records, which is why StatefulSet originally launched without rolling updates. Metacontroller lets you easily build your own APIs that offer declarative rolling updates without making you think about all this additional bookkeeping.

In fact, Metacontroller provides a declarative interface for configuring how you want to implement declarative rolling updates in your controller (declarative declarative rolling update), so you don’t have to write any code to take advantage of this feature.

For example, adding support for rolling updates to a Metacontroller-based rewrite of StatefulSet looks essentially like this:

   - apiVersion: v1
     resource: pods
+    updateStrategy:
+      method: RollingRecreate
+      statusChecks:
+        conditions:
+        - type: Ready
+          status: "True"

For comparison, the corresponding pull request to add rolling updates to StatefulSet itself involved over 9,000 lines of changes to business logic, boilerplate, and generated code.