High Availability for VM runner docs

Author: SushrutHarness

docs/continuous-integration/use-ci/set-up-build-infrastructure/vm-build-infrastructure/high-availability.md

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+---
+title: High Availability (HA)
+description: Set up high availability on VM build infrastructure.
+sidebar_position: 45
+---
+
+## Minimum Version
+
+In order to use this feature, please ensure that you are using [drone runner](https://github.com/drone-runners/drone-runner-aws/tree/v1.0.0-rc.187?tab=readme-ov-file#high-availablity) version of [`v1.0.0-rc.187`](https://github.com/drone-runners/drone-runner-aws/releases/tag/v1.0.0-rc.187) or greater. 
+
+## Setup
+
+When using VM build infrastructure, you can deploy multiple replicas of the runner or infrastructure to ensure high availability. Below is an example of a deployment yaml that deploys 2 replicas of the runner behind a load balancer.
+
+```yaml
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: harness-delegate-ng
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: drone-runner
+  namespace: harness-delegate-ng
+spec:
+  replicas: 2
+  selector:
+    matchLabels:
+      app: drone-runner
+  template:
+    metadata:
+      labels:
+        app: drone-runner
+    spec:
+      containers:
+        - name: drone-runner
+          image: drone/drone-runner-aws:v1.0.0-rc.187
+          args: ["delegate", "--pool", "/runner/pool.yml"]
+          env:
+            - name: DRONE_RUNNER_HA
+              value: "true"
+            - name: DRONE_DATABASE_DRIVER
+              value: postgres
+            - name: DRONE_DATABASE_DATASOURCE
+              value: host=runnerha-postgres-service.harness-delegate-ng.svc.cluster.local port=5431 user=admin password=password dbname=dlite sslmode=disable
+          ports:
+            - containerPort: 3000
+          volumeMounts:
+            - name: config-volume
+              mountPath: /runner
+              readOnly: true
+      volumes:
+        - name: config-volume
+          projected:
+            sources:
+              - configMap:
+                  name: drone-runner-config
+              - secret:
+                  name: drone-runner-secret
+---
+apiVersion: v1
+kind: Service
+metadata:
+  name: drone-runner-lb
+  namespace: harness-delegate-ng
+spec:
+  type: LoadBalancer
+  selector:
+    app: drone-runner
+  ports:
+    - port: 3000
+      targetPort: 3000
+      protocol: TCP
+```
+
+The load balancer ip obtained above can be used in delegate with the env variable `RUNNER_URL`. Populate `DRONE_DATABASE_DATASOURCE` accordingly. Note that above yaml mounts a volume config-volume with path `/runner`. Make sure your `pool.yml` file and secrets required are mapped properly. See the following GCP specific `pool.yaml` file used in the deployment above.
+
+```yaml
+version: "1"
+instances:
+  - name: linux-amd64
+    type: google
+    pool: 2
+    limit: 10
+    platform:
+      os: linux
+      arch: amd64 
+    spec:
+      hibernate: false
+      privateIP: true
+      account:
+        project_id: projectname
+        json_path: runner/gcp-secret.json
+      image: projects/projectname/global/images/hosted-vm-64
+      machine_type: e2-medium
+      zones:
+        - us-central1-a
+      disk:
+        size: 100
+```