k8s-mongo-labeler-sidecar

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k8s-mongo-labeler-sidecar

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Kubernetes MongoDB sidecar that watches replica instances and sets label on primary node.

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# Kubernetes MongoDB Primary Pod Labeler

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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

```yaml
apiVersion: v1
kind: Service
metadata:
  name: mongo
spec:
  selector:
    role: mongo
    primary: "true"
  ports:
  - name: mongo
    port: 27017
```

## 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. |
| `K8S_REQUEST_TIMEOUT` | no | `10s` | Timeout for Kubernetes list/patch API requests (Go duration format, for example `5s`, `1m`). |
| `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. `K8S_REQUEST_TIMEOUT` is parsed as a Go duration. Invalid values fail startup.

## Published image

Container images are published to GHCR at:

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

```bash
docker pull ghcr.io/combor/k8s-mongo-labeler-sidecar:0.7.2
```

## Deployment

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

> **Note:** the example runs MongoDB **without authentication or TLS** and is intended for demonstration only. The bundled `NetworkPolicy` limits access to port 27017 to pods in the same namespace, but it is only enforced by CNIs that implement NetworkPolicy. Before production use, enable MongoDB authentication (keyFile/SCRAM) and TLS, and review the resource limits and security contexts.

## Integration test (kind)

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

Prerequisites:

- [kind](https://kind.sigs.k8s.io/)
- [kubectl](https://kubernetes.io/docs/reference/kubectl/)
- [Docker](https://www.docker.com/)
- [BuildKit](https://github.com/moby/buildkit)
- [Buildx](https://github.com/docker/buildx)

Run:

```bash
./test/integration/run.sh
```

Optional overrides:

- `CLUSTER_NAME` (default `kind-mongo-labeler`)
- `LABELER_IMAGE` (default `mongo-labeler:local`)
- `USE_PREBUILT_IMAGE` (default `false`) — skip building and use an existing image
- `TIMEOUT` (default `240s`)
- `KEEP_CLUSTER=true` (keep cluster for debugging)

The script creates a temporary kind cluster, deploys a 3-pod Mongo StatefulSet and verifies that exactly one pod has `primary=true` while non-primary pods have `primary=false`. It also verifies that the `mongo` Service routes to the primary pod via EndpointSlice.

## Run CI locally with act

The repository uses a single workflow for GitHub and local runs: `.github/workflows/ci-release.yml`.

Prerequisites:

- [Docker Desktop](https://www.docker.com/products/docker-desktop/)
- [act](https://github.com/nektos/act)

Run the full workflow locally:

```bash
act -W .github/workflows/ci-release.yml
```

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