Virtual Kubelet providers are written using the Go programming language. While Go is a great general purpose programming language, it is however a fact that other programming languages exist. The problem that Virtual Kubelet solves is as applicable to applications written in those languages as it is for those written using Go. This provider aims to serve as a bridge between technology stacks and programming languages, as it were, by adapting the Virtual Kubelet provider interface using a web endpoint, i.e., this provider is a thin layer that forwards all calls that Kubernetes makes to the virtual kubelet to a pre-configured HTTP endpoint. This frees the provider's implementor to write their code in any programming language and technology stack as they see fit.
The providers/web/web-rust folder contains a sample provider implemented in
the Rust programming language. Here's a diagram that depicts the interaction
between Kubernetes, the virtual kubelet web provider and the Rust app.
+----------------+ +---------------------------+ +------------------------------+
| | | | HTTP | |
| Kubernetes | <-----> | Virtual Kubelet: Web | <------> | Provider written in Rust |
| | | | | |
+----------------+ +---------------------------+ +------------------------------+
The web provider uses an environment variable to determine the endpoint to use
for forwarding requests. The environment variable must be named
WEB_ENDPOINT_URL and must implement the following HTTP API:
| Path | Verb | Query | Request | Response | Description |
|---|---|---|---|---|---|
| /createPod | POST | - | Pod JSON | HTTP status code | Create a new pod |
| /updatePod | PUT | - | Pod JSON | HTTP status code | Update pod spec |
| /deletePod | DELETE | - | Pod JSON | HTTP status code | Delete an existing pod |
| /getPod | GET | namespace, name | - | Pod JSON | Given a pod namespace and name, return the pod JSON |
| /getContainerLogs | GET | namespace, podName, containerName, tail | - | Container logs | Given the namespace, pod name and container name, return tail log lines |
| /getPodStatus | GET | namespace, name | - | Pod status JSON | Given a pod namespace and name, return the pod's status JSON |
| /getPods | GET | - | - | Array of pod JSON strings | Fetch list of created pods |
| /capacity | GET | - | - | JSON map containing resource name and values | Fetch resource capacity values |
| /nodeConditions | GET | - | - | Array of node condition JSON strings | Get list of node conditions (Ready, OutOfDisk etc) |
| /nodeAddresses | GET | - | - | Array of node address values (type/address pairs) | Fetch a list of addresses for the node status |
A typical deployment configuration for this setup would be to have the provider implementation be deployed as a container in the same pod as the virtual kubelet itself (as a "sidecar").
A sample web provider implementation is included in this repo in order to
showcase what this enables. The sample has been implemented in
Rust. The easiest way to get this up and running is
to use the Helm chart available at providers/web/charts/web-rust. Open a
terminal and install the chart like so:
$ cd providers/web/charts
$ helm install -n web-provider ./web-rust
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
web-provider-virtual-kubelet-web-6b5b7446f6-279xl 2/2 Running 0 3h
If you list the nodes in the cluster after this you should see something that looks like this:
$ kubectl get nodes
NAME STATUS ROLES AGE VERSION
aks-nodepool1-35187879-0 Ready agent 37d v1.8.2
aks-nodepool1-35187879-1 Ready agent 37d v1.8.2
aks-nodepool1-35187879-3 Ready agent 37d v1.8.2
virtual-kubelet-web Ready agent 3h v1.8.3
In case the name of the node didn't give it away, the last entry in the output above is the virtual kubelet. If you try to list the containers in the pod that represents the virtual kubelet you should be able to see the sidecar Rust container:
$ kubectl get pods -o=custom-columns=NAME:.metadata.name,CONTAINERS:.spec.containers[*].name
NAME CONTAINERS
web-provider-virtual-kubelet-web-6b5b7446f6-279xl webrust,virtualkubelet
In the output above, webrust is the sidecar container and virtualkubelet is
the broker that forwards requests to webrust. You can run a query on the
/getPods HTTP endpoint on the webrust container to see a list of the pods
that it has been asked to create. To do this we first use kubectl to setup a
port forwarding server like so:
$ kubectl port-forward web-provider-virtual-kubelet-web-6b5b7446f6-279xl 3000:3000
Now if we run curl on the http://localhost:3000/getPods URL you should see
the pod JSON getting dumped to the terminal. I ran my test on a Kubernetes
cluster deployed on Azure which
happens to deploy a daemonset with some, what I imagine are "system" pods to
every node in the cluster. You can filter the output to see just the pod names
using the jq tool like so:
$ curl -s http://localhost:3000/getPods | jq '.[] | { name: .metadata.name }'
{
"name": "kube-proxy-czz57"
}
{
"name": "kube-svc-redirect-7qlpd"
}
You can deploy workloads to the virtual kubelet as you normally do. Here's a
sample pod spec that uses nodeSelector to cause the deployment to be scheduled
on the virtual kubelet.
apiVersion: v1
kind: Pod
metadata:
name: vk-pod
labels:
foo: bar
spec:
containers:
- name: web1
image: nginx
nodeSelector:
type: virtual-kubelet
Let's go ahead and deploy the pod and run our /getPods query again:
$ kubectl apply -f ~/tmp/pod1.yaml
pod "vk-pod" created
$ curl -s http://localhost:3000/getPods | jq '.[] | { name: .metadata.name }'
{
"name": "kube-proxy-czz57"
}
{
"name": "kube-svc-redirect-7qlpd"
}
{
"name": "vk-pod"
}
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
vk-pod 0/1 Running 0 1m
web-provider-virtual-kubelet-web-6b5b7446f6-279xl 2/2 Running 6 4h
As you can tell, a new pod has been scheduled to run on our virtual kubelet instance. Deleting pods works as one would expect:
$ kubectl delete -f ~/tmp/pod1.yaml
pod "vk-pod" deleted
$ curl -s http://localhost:3000/getPods | jq '.[] | { name: .metadata.name }'
{
"name": "kube-proxy-czz57"
}
{
"name": "kube-svc-redirect-7qlpd"
}