Connecting Web and DB Pods in Kubernetes

Sometimes you have to run both your web app and database inside the cluster (instead of using a managed service like RDS). Here are two ways I linked them.

1. Sidecar pattern – web + DB containers in a single pod.
2. Separate services – independent pods connected via services.

1. Sidecar Pattern

Both containers share the same pod, thus the same IP/namespace, so the app can connect to MySQL via localhost.

# service
apiVersion: v1
kind: Service
metadata:
  name: sidecar-svc
spec:
  selector:
    app: sidecar
  type: LoadBalancer
  ports:
  - port: 80
    targetPort: 8000

# deployment
apiVersion: apps/v1
kind: Deployment
metadata:
  name: sidecar-deployment
spec:
  replicas: 1
  selector:
    matchLabels:
      app: sidecar
  template:
    metadata:
      labels:
        app: sidecar
    spec:
      containers:
      - name: fastapi-container
        image: fast:0.1
        ports:
        - containerPort: 8000
        env:
        - name: MYSQL_HOST
          value: "localhost"
        # other DB env vars…
      - name: mysql-container
        image: mysql:8.0.28
        ports:
        - containerPort: 3306
        env:
        - name: MYSQL_ROOT_PASSWORD
          value: "password"

This is simple but not ideal for scaling—better suited to small apps.

2. Separate Services

Run web and DB in separate deployments. The web app references the MySQL service name (mysql-svc) instead of localhost.

# services
apiVersion: v1
kind: Service
metadata:
  name: fastapi-svc
spec:
  selector:
    app: fastapi
  type: LoadBalancer
  ports:
  - port: 80
    targetPort: 8000
---
apiVersion: v1
kind: Service
metadata:
  name: mysql-svc          # note the name
spec:
  selector:
    app: mysql
  type: NodePort
  ports:
  - port: 3306

# fastapi deployment
apiVersion: apps/v1
kind: Deployment
metadata:
  name: fastapi
spec:
  replicas: 1
  selector:
    matchLabels:
      app: fastapi
  template:
    metadata:
      labels:
        app: fastapi
    spec:
      containers:
      - name: fastapi
        image: fast:0.1
        ports:
        - containerPort: 8000
        env:
        - name: MYSQL_HOST
          value: "mysql-svc"   # service DNS
        # other env vars…
---
# mysql deployment
apiVersion: apps/v1
kind: Deployment
metadata:
  name: mysql-deployment
spec:
  replicas: 1
  selector:
    matchLabels:
      app: mysql
  template:
    metadata:
      labels:
        app: mysql
    spec:
      containers:
      - name: mysql-pod
        image: mysql:8.0.28
        ports:
        - containerPort: 3306
        env:
        - name: MYSQL_ROOT_PASSWORD
          value: "password"

Kubernetes DNS resolves mysql-svc to the backing pod(s), so the FastAPI container can connect to MySQL via that hostname.

About Port Names

I wondered if naming a port would let me connect by name (similar to Docker Compose). Not quite—names help map container ports inside a Service:

# mysql container
ports:
  - name: db-port
    containerPort: 3306

# service
ports:
  - port: 8000
    targetPort: db-port

Here, the Service’s targetPort references the named port, but it doesn’t replace DNS. Use the Service name for inter-pod communication.