This page answers some common questions encountered while evaluating, setting up, and using Metacontroller.

If you have any questions that aren't answered here, please ask on the mailing list or Slack channel.

Evaluating Metacontroller

How does Metacontroller compare with other tools?

See the features page for a list of the things that are most unique about Metacontroller's approach.

In general, Metacontroller aims to make common patterns as simple as possible, without necessarily supporting the full flexibility you would have if you wrote a controller from scratch. The philosophy is analogous to that of CustomResourceDefinition (CRD), where the main API server does all the heavy lifting for you, but you don't have as much control as you would if you wrote your own API server and connected it through aggregation.

Just like CRD, Metacontroller started with a small set of capabilities and is expanding over time to support more customization and more use cases as we gain confidence in the abstractions. Depending on your use case, you may prefer one of the alternative tools that took the opposite approach of first allowing everything and then building "rails" over time to encourage best practices and simplify development.

What is Metacontroller good for?

Metacontroller is intended to be a generic tool for creating many kinds of Kubernetes controllers, but one of its earliest motivating use cases was to simplify development of custom workload automation, so it's particularly well-suited for this.

For example, if you've ever thought, "I wish StatefulSet would do this one thing differently," Metacontroller gives you the tools to define your own custom behavior without reinventing the wheel.

Metacontroller is also well-suited to people who prefer languages other than Go, but still want to benefit from the efficient API machinery that was developed in Go for the core Kubernetes controllers.

Lastly, Metacontroller is good for rapid development of automation on top of APIs that already exist as Kubernetes resources, such as:

  • ad hoc scripting ("make an X for every Y")
  • configuration abstraction ("when I say A, that means {X,Y,Z}")
  • higher-level automation of custom APIs added by Operators
  • gluing an external CRUD API into the Kubernetes control plane with a simple translation layer

What is Metacontroller not good for?

Metacontroller is not a good fit when you need to examine a large number of objects to answer a single hook request. For example, if you need to be sent a list of all Pods or all Nodes in order to decide on your desired state, we'd have to call your hook with the full list of all Pods or Nodes any time any one of them changed. However, it might be a good fit if your desired behavior can be naturally broken down into per-Pod or per-Node tasks, since then we'd only need to call your hook with each object that changed.

Metacontroller is also not a good fit for writing controllers that perform long sequences of imperative steps -- for example, a single hook that executes many steps of a workflow by creating various children at the right times. That's because Metacontroller hooks work best when they use a functional style (no side effects, and output depends only on input), which is an awkward style for defining imperative sequences.

Do I have to use CRD?

It's common to use CRD, but Metacontroller doesn't know or care whether a resource is built-in or custom, nor whether it's served by CRD or by an aggregated API server.

Metacontroller uses API discovery and the dynamic client to treat all resources the same, so you can write automation for any type of resource. Using the dynamic client also means Metacontroller doesn't need to be updated when new APIs or fields are added in subsequent Kubernetes releases.

What does the name Metacontroller mean?

The name Metacontroller comes from the English words meta and controller. Metacontroller is a controller controller -- a controller that controls other controllers.

How do you pronounce Metacontroller?

Please see the pronunciation guide.

Setting Up Metacontroller

Do I need to be a cluster admin to install Metacontroller?

Installing Metacontroller requires permission to both install CRDs (representing the Metacontroller APIs themselves) and grant permissions for Metacontroller to access other resources on behalf of the controllers it hosts.

Why is Metacontroller shared cluster-wide?

Metacontroller currently only supports cluster-wide installation because it's modeled after the built-in kube-controller-manager component to achieve the same benefits of sharing watches and caches.

Also, resources in general (either built-in or custom) can only be installed cluster-wide, and a Kubernetes API object is conventionally intended to mean the same thing regardless of what namespace it's in.

Why does Metacontroller need these permissions?

During alpha, Metacontroller simply requests wildcard permission to all resources so the controllers it hosts can access anything they want. For this reason, you should only give trusted users access to the Metacontroller APIs that create hosted controllers.

By contrast, core controllers are restricted to only the minimal set of permissions needed to do their jobs.

Does Metacontroller have to be in its own namespace?

The default installation manifests put Metacontroller in its own namespace to make it easy to see what's there and clean up if necessary, but it can run anywhere. The metacontroller namespace is also used in examples for similar convenience reasons, but you can run webhooks in any namespace or even host them outside the cluster.

Developing with Metacontroller

Which languages can I write hooks in?

You can write lambda hooks (the business logic for your controller) in any language, as long as you can host it as a webhook that accepts and returns JSON. Regardless of which language you use for your business logic, Metacontroller uses the efficient machinery written in Go for the core controllers to interact with the API server on your behalf.

How do I access the Kubernetes API from my hook?

You don't! Or at least, you don't have to, and it's best not to. Instead, you just declare what objects you care about and Metacontroller will send them to you as part of the hook request. Then, your hook should simply return a list of desired objects. Metacontroller will take care of reconciling your desired state.

Can I call external APIs from my hook?

Yes. Your hook code can do whatever it wants as part of computing a response to a Metacontroller hook request, including calling external APIs.

The main thing to be careful of is to avoid synchronously waiting for long-running tasks to finish, since that will hold up one of a fixed number of concurrent slots in the queue of triggers for that hook. Instead, if your hook needs to wait for some condition that's checked through an external API, you should return a status that indicates this pending state, and set a resync period so you get a chance to check the condition again later.

How can I make sure external resources get cleaned up?

If you allocate external resources as part of your hook, you should also implement a finalize hook to make sure you get a chance to clean up those external resources when the Kubernetes API object for which you created them goes away.

Does Metacontroller support "apply" semantics?

Yes, Metacontroller enforces apply semantics, which means your controller will play nicely with other automation as long as you only fill in the fields that you care about in the objects you return.

How do I host my hook?

You can host your lambda hooks with an HTTP server library in your chosen language, with a standalone HTTP server, or with a Functions-as-a-Service platform. See the examples page for approaches in various languages.

How can I provide a programmatic client for my API?

Since Metacontroller uses the dynamic client on your behalf, you can write your controller's business logic without any client library at all. That also means you can write a "dynamically typed" controller without creating static schema (either Kubernetes' Go IDL or OpenAPI) or generating a client.

However, if you want to provide a static client for users of your API, nothing about Metacontroller prevents you from writing Go IDL or OpenAPI and generating a client the same way you would without Metacontroller.

What are the best practices for designing controllers?

Please see the dedicated best practices guide.

How do I troubleshoot problems?

Please see the dedicated troubleshooting guide.