Kubernetes MongoDB Primary Pod Labeler

This sidecar detects the current MongoDB replica set primary and labels Kubernetes pods so services can target the writable node.

How it works

Every 5 seconds the sidecar:

1. Connects to MongoDB (MONGO_ADDRESS, default localhost:27017).
2. Detects the primary pod name.
3. Lists pods in NAMESPACE matching LABEL_SELECTOR.
4. Patches labels:
   • primary pod: primary=true
   • other pods: primary=false when LABEL_ALL=true
   • other pods: removes primary label when LABEL_ALL=false

It uses Kubernetes Patch (strategic merge), not full-object Update.

Service selector example

apiVersion: v1
kind: Service
metadata:
  name: mongo-external
spec:
  type: LoadBalancer
  ports:
  - name: mongo
    port: 27017
  selector:
    role: mongo
    primary: "true"

Configuration

When running inside Kubernetes, in-cluster config is used automatically.
When running outside a cluster, kubeconfig defaults to ~/.kube/config and can be overridden with --kubeconfig.

Environment variables:

Variable	Required	Default	Description
LABEL_SELECTOR	yes	none	Pod label selector (for example role=mongo).
NAMESPACE	no	default	Namespace where pods are listed and patched.
MONGO_ADDRESS	no	localhost:27017	MongoDB endpoint used for primary detection.
LABEL_ALL	no	false	Boolean. If true, non-primary pods get primary=false; if false, the label is removed.
DEBUG	no	false	Boolean. If true, enables debug logging.

LABEL_ALL and DEBUG are parsed as booleans. Invalid values fail startup.

Published image

Container images are published to GHCR at:

ghcr.io/combor/k8s-mongo-labeler-sidecar

docker pull ghcr.io/combor/k8s-mongo-labeler-sidecar:latest-amd64
docker pull ghcr.io/combor/k8s-mongo-labeler-sidecar:latest-arm64

Deployment

deployment-example.yaml can be used as an example deployment manifest.

Integration test (kind)

The repository includes an end-to-end test environment in test/integration.

Prerequisites:

• kind
• kubectl
• Docker with BuildKit/Buildx enabled

Run:

./test/integration/run.sh

Optional overrides:

• NAMESPACE (default mongo-it)
• CLUSTER_NAME (default mongo-labeler-it)
• LABELER_IMAGE (default mongo-labeler-it:local)
• TIMEOUT (default 240s)
• KEEP_CLUSTER=true (keep cluster for debugging)

The script creates a temporary kind cluster, deploys Mongo + sidecar, and verifies that mongo-0 receives primary=true.