This guide will walk you through the steps to add a PostgreSQL database to an existing Helm chart for your Kubernetes application. Helm charts help manage Kubernetes applications using templates, and adding PostgreSQL will involve integrating the Bitnami PostgreSQL Helm chart as a dependency and configuring it appropriately.

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Steps

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1. Add PostgreSQL Helm Repository

First, add the Bitnami repository, which hosts the PostgreSQL Helm chart, 
helm repo add bitnami https://charts.bitnami.com/bitnami
helm repo update

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2. Update Chart.yaml

Open your existing Helm chart directory and edit the Chart.yaml file to add PostgreSQL as a dependency,
Chart.yaml
apiVersion: v2
name: your-app
description: A Helm chart for your Kubernetes application
version: 0.1.0
appVersion: '1.0'

dependencies:
  - name: postgresql
    version: 12.1.9
    repository: https://charts.bitnami.com/bitnami

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3. Update values.yaml

Create or update the values.yaml file to include configuration values for PostgreSQL. You can customize the values based on your requirements. Refer to https://github.com/bitnami/charts/blob/main/bitnami/postgresql/README.md#parameters for all configuration options provided by PostgresSql helm chart.
values.yaml
postgresql:
  enabled: true
  global:
    postgresql:
      auth:
        postgresqlPassword: mysecretpassword
  primary:
    persistence:
      enabled: true
      size: 50Gi
Over-provisioning on disk size (primary.persistence.size) isn’t a bad idea considering that disks are cheap in any cloud. It is also slightly trickier to increase volume sizes at later point in time due to some Kubernetes limitations. So we recommend you pick a good number here from the start.

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4. Install/Update Dependencies

Navigate to your Helm chart directory and run the following command to update dependencies, 
helm dependency update

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5. Update Your Application Deployment

Modify your application’s deployment files to include the PostgreSQL service. Ensure your application can connect to PostgreSQL using the service name and credentials specified in the values.yaml file. For example, if using a ConfigMap to store environment variables for your application, you might update it like this,
templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: your-app-config
data:
  DATABASE_HOST: {{ .Release.Name }}-postgresql
  DATABASE_PORT: '5432'
  DATABASE_USER: 'postgres'
  DATABASE_PASSWORD: 'mysecretpassword'
  DATABASE_NAME: 'yourdbname'

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6. Deploy Your Application

Finally, deploy or upgrade your application using Helm, 
helm install your-app ./your-app-chart
# or if upgrading an existing release
helm upgrade your-app ./your-app-chart

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7. Verify Deployment

Check the status of your Helm release to ensure everything is running correctly, 
helm status your-app
Also, verify that the PostgreSQL pod is running,
kubectl get pods
You should see a pod for PostgreSQL, something like your-app-postgresql-0. Congratulations! You have successfully added PostgreSQL to your existing Helm chart. This integration allows your application to use PostgreSQL as its database, managed efficiently with Helm. You can further customize the PostgreSQL configuration by modifying the values.yaml file as needed.