Understanding Kubernetes controllers part IV – putting it all together

In the last few posts, we have looked in detail at the various parts of the machinery behind Kubernetes controllers – informers, queues, stores and so forth. In this final post, we will wrap up a bit and see how all this comes together in the Kubernetes sample controller.

The flow of control

The two main source code files that make up the sample controller are main.go and controller.go. The main function is actually rather simple. It creates a clientset, an informer factory for Foo objects and an informer factory for deployments and then uses those items to create a controller.

Once the controller exists, main starts the informers using the corresponding functions of the factory, which will bring up the goroutines of the two informers. Finally, the controllers main loop is started by calling its Run method.

The controller is more interesting. When it is created using NewController, it attaches event handlers to the informes for Deployments and Foo resources. As we have seen, both event handlers will eventually put Foo objects that require attention into a work queue.

The items in this work queue are processed by worker threads that are created in the controllers Run method. As the queue servers act as a synchronization point, we can run as many worker threads that we want and still make sure that each event is processed by only one worker thread. The main function of the worker thread is processNextWorkItem which retrieves elements from the queue, i.e. the keys of the Foo objects that need to be reconciled, and calls syncHandler for each of them. If that functions fails, the item is put back onto the work queue.

The syncHandler function contains the actual reconciliation logic. It first splits the key into namespace and name of the Foo resource. Then it tries to receive an existing deployment for this Foo resource and creates one if it does not yet exist. If a deployment is found, but deviates from the target state, it is replaced by a new deployment as returned by the function newDeployment. Finally, the status subresource of the Foo object is updated.

So this simple controller logic realizes some of the recommendations for building controllers.

  • It uses queues to decouple workers from event handlers and to allow for concurrent processing
  • Its logic is level based, not edge based, as a controller might be down for some time and miss updates
  • It uses shared informers and caches. The cached objects are never updated directly, but if updates are needed, deep copies are used and modified and updates are done via the API
  • It waits for all caches to sync before starting worker threads
  • It collapses all work to be done into a single queue

Package structure and generated code

If you browse the sourcecode of the sample controller, you might at the first glance be overwhelmed by the number of files in the repository. However, there is good news – most of this code is actually generated! In fact, the diagram below shows the most important files and packages. Generated files are in italic font, and files that serve as input for the code generator or are referenced by the generated code are in bold face.

SampleControllerPackages

We see that the number of files that we need to provide is comparatively small. Apart from, of course, the files discussed above (main.go and controller.go), these are

  • register.go which adds our new types to the scheme and contains some lookup functions that the generated code will use
  • types.go which defines the Go structures that correspond to the Foo CRD
  • register.go in the top-level directory of the API package which defines the name of the API group as a constant

All the other files are created by the Kubernetes API code generator. This generator accepts a few switches (“generators”) to generate various types of code artifacts – deep-copy routines, clientsets, listers and informers. We will see the generator in action in a later post. Instead of using the code generator directly, we could of course as well use the sample controller as a starting point for our own code and make the necessary modifications manually, the previous posts should contain all the information that we need for this.

This post completes our short mini-series on the machinery behind custom controllers. In the next series, we will actually apply all this to implement a controller that operators a small bitcoin test network in a Kubernetes cluster.

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