How `kubectl exec` works

How It Works — kubectl exec

If you are interested in finding out how kubectl exec works, then I hope you will find this post useful. We will look into how the command works by examing the relevant code in kubectl, K8s API Server, Kubelet and the Container Runtime Interface (CRI) Docker API.

About This Command

Thekubectl exec command is an invaluable tool for those of us who regularly work with containerized workloads on Kubernetes. It allows us to inspect and debug our applications, by executing commands inside our containers.

Let’s use kubectl v1.15.0 to run an example:

The first exec command runs a date command inside my Nginx container. The second exec command uses the -i and -t flags to get a shell to my Nginx container.

The CLI Code

Let’s repeat the command with increased log verbosity:

Notice that there are two HTTP requests:

1. a GET request to fetch the pod
2. a POST request to the exec subresource of the pod.

https://kubernetes.io/docs/reference/using-api/api-concepts/

The server responds with a 101 Upgrade response header indicating to the client that it has switched to the SPDY protocol.

The API Server Code

Let’s examine the API Server’s code to see how it registers the rest.ExecRest handler to handle /exec subresource requests. This handler is used to determine the node endpoint to exec to.

One of the things that the API Server does when starting is to instruct its embedded GenericAPIServer to install the ‘legacy’ API: