Support for GKE (k8s) Workload Identity Federation

[thomasveale]

Feature

Hey Devs! Great project, thanks for your hard work and continued maintenance.

Following the sidecar pattern, we aim to forward logs to Google StackDriver through the zipkin docker container. The issue is, we're not mounting our service account key directly, instead we use (workload identity federation)[https://cloud.google.com/kubernetes-engine/docs/how-to/workload-identity] to inject a short lived service account token directly into the docker image. Using identity federation, your workload can impersonate a service account. This lets the external workload access Google Cloud resources directly, eliminating the maintenance and security burden associated with service account keys. This is the recommended solution for authenticating with google services from GKE.

The token is mounted at /var/run/secrets/kubernetes.io/serviceaccount/token and if we specify point the GOOGLE_APPLICATION_CREDENTIALS to this path, we encounter a decoding error because the token is a base64 encoded Json Web Token.

A work around would be to run some PostStart script to decode the token and remount it somewhere else for Zipkin to use or zipkin-gcp should support workload identity. This is a messy, and unstable solution because there is no guarantee that script will execute before the entry point to zipkin.

It would be great if zipkin-gcp would support workload identity out of the box (i.e. if no GOOGLE_APPLICATION_CREDENTIALS is provided or if a specific flag/environment variable was set).

Rationale

Handling and mounting additional key material is dangerous. As workload identity is the recommended authentication mechanism for GKE (Kubernetes), this would likely be helpful to many others. Using workload identity federation, applications can access Google Cloud resources from Amazon Web Services (AWS), Microsoft Azure, or any identity provider that supports OpenID Connect (OIDC).

Example Scenario

Zero configuration startup, the google cloud project and token could be extracted by default.
A flag could be set like GKE_WORKLOAD_IDENTITY=true to enable automatic configuration.

Prior Art

https://github.com/googleapis/google-auth-library-java
https://googleapis.dev/python/google-auth/latest/user-guide.html

[jcchavezs]

Thanks for this. Zipkin is a voluneteering project so if you can come up with a PR for this we could review it.

[thomasveale]

Thanks for getting back to this and apologies for not reading that!

Sadly my java is quite weak. Working on improving that so I may be able to come back with something!

[codefromthecrypt]

@thomasveale please ping back if still interested in this, as I know it has been a while.