Kubernetes Mutating Webhook

This mutating webhook was developed to inject Haystack's agent as a sidecar to a Kubernetes pod so applications can ship trace data to Haystack server.

Though this was primarily written to inject haystack-agent as a sidecar, one can use this to inject any container as a sidecar in a pod.

Developing

If one is interested in contributing to this codebase, please read the developer documentation on how to build and test this codebase.

Using this webhook

We have provided two ways to deploy this webhook. Using Helm and using kubectl. Deployment files are in deployment/helm and deployment/kubectl respectively.

How to enable sidecar injection using this webhook

One can simply deploy this mutating webhook by cloning this repository and running the following command (needs kubectl installed and configured to point to the kubernets cluster or minikube)
```
./deployment/kubectl/deploy.sh
```
or using helm
```
helm init
helm install --name kubernetes-sidecar-injector-webhook ./deployment/helm
```
The command above installs the webhook and a map of named sidecars to be injected. One can find the map in this config map file in kubectl folder or this configmap in helm folder. In these files only one sidecar named haystack-agenthas been configured.
Apply the label kubernetes-sidecar-injector: enabled in the namespaces where the sidecar injection should be considered. This sample file applies the label mentioned to default namespace
Add an annotation sidecar-injector.expedia.com/inject with name of the sidecar to inject in pod spec where sidecar needs to be injected. This sample spec shows such an annotation added to a pod spec to inject haystack-agent.

Kubectl deployment files

Lets go over the files in the deployment/kubectl folder.

sidecar-configmap.yaml: This file contains two configmap entries. First one, kubernetes-sidecars-configmap contains a map of named sidecar containers to be injected. In this case, we have only one named sidecar called hatrack-agent. Second one haystack-agent-conf-configmap contains a configuration file that is used by haystack-agent sidecar.

Though this file carries only haystack-agent, one can replace this or add more sidecars with to be injected.
sidecar-injector-deployment.yaml: This file deploys kubernetes-sidecar-injector pod and kubernetes-sidecar-injector-svc service. This is the mutating webhook admission controller service. This is invoked by kebernetes while creating a new pod with the pod spec that is being created. That allows this webhook to inspect and make a decision on whether to inject the sidecar or not. This webhook checks for two conditions to determine whether to inject a sidecar or not
1. Namespace check: Sidecar injection will be attempted only if the the pod is being created in a namespace with the label kubernetes-sidecar-injector: enabled and the namespace is NOT kube-system or kube-public
2. Annotation check: Sidecar inkection will be attempted only if the pod being created carries an annotation sidecar-injector.expedia.com/inject. Value of this annotation will be used to locate the sidecar to be injected from the configmap in sidecar-configmap.yaml.
  
  Note: One can have a comma separated list of sidecar names if more than one sidecar needs to be injected
create-server-cert.sh: Mutating webhook admission controllers need to listen on https (TLS). This script generates a key, a certificate request and gets that request signed by Kubernetes CA. i.e., produces a signed certificate and deploys it as a kubernets secret to be used by the service defined in #2
mutatingwebhook-template.yaml: This file registers the mutating webhook admission controller. This spec carries the CA file that will validate the server certificate used by the service. This file is a template and the caBundle field in it is populated by the script replace-ca-token.sh file
deploy.sh: This is a simple bash script that deploys the webhook by executing the scripts / deployment specifications mentioned above.

Helm deployment files

Files in deployment/helm/templates are the same as the files in kubectl folder and provide the same functionality.

Addendum

Injecting env variables in the sidecar

At times one may have to pass additional information to the sidecar from the pod spec. For example, a pod specific api-key to be used by a sidecar. To allow that, this webhook looks for special annotations with prefix sidecar-injector.expedia.com in the pod spec and adds the annotation key-value as environment variables to the sidecar.

For example, this sample pod specification has the following annotation

sidecar-injector.expedia.com/some-api-key: "6feab492-fc9b-4c38-b50d-3791718c8203"

and this will cause this webhook to inject

some-api-key: "6feab492-fc9b-4c38-b50d-3791718c8203"

as an environment variable in all the sidecars injected.

mchandramouli/kubernetes-sidecar-injector