MySQL connection failed between 2 pods in kubernetes - mysql

Question

I am a newbie on Kubernetes and try to generate 2 pods including front-end application and back-end mysql. First I make a yaml file which contains both application and mysql server like below,
apiVersion: v1
kind: Pod
metadata:
name: blog-system
spec:
containers:
- name: blog-app
image: blog-app:latest
imagePullPolicy: Never
ports:
- containerPort: 8080
args: ["-t", "-i"]
link: blog-mysql
- name: blog-mysql
image: mysql:latest
env:
- name: MYSQL_ROOT_PASSWORD
value: password
- name: MYSQL_PASSWORD
value: password
- name: MYSQL_DATABASE
value: test
ports:
- containerPort: 3306
The mysql jdbc url of front-end application is jdbc:mysql://localhost:3306/test. And pod generation is successful. The application and mysql are connected without errors. And this time I seperate application pod and mysql pod into 2 yaml files.
== pod-app.yaml
apiVersion: v1
kind: Pod
metadata:
name: blog-app
spec:
selector:
app: blog-mysql
containers:
- name: blog-app
image: app:latest
imagePullPolicy: Never
ports:
- containerPort: 8080
args: ["-t", "-i"]
link: blog-mysql
== pod-db.yaml
apiVersion: v1
kind: Pod
metadata:
name: blog-mysql
labels:
app: blog-mysql
spec:
containers:
- name: blog-mysql
image: mysql:latest
env:
- name: MYSQL_ROOT_PASSWORD
value: password
- name: MYSQL_PASSWORD
value: password
- name: MYSQL_DATABASE
value: test
ports:
- containerPort: 3306
But the front-end application can not connect to mysql pod. It throws the connection exceptions. I am afraid the mysql jdbc url has some incorrect values or the yaml value has inappropriate values. I hope any advices.

Answer 1

In the working case since same pod has two containers they are able to talk using localhost but in the second case since you have two pods you can not use localhost anymore. In this case you need to use the pod IP of the mysql pod in the frontend application. But problem with using POD IP is that it may change. Better is to expose mysql pod as service and use service name instead of IP in the frontend application. Check this guide

Answer 2

For this you need to write service for exposing the db pod.
There are 4 types of services.
ClusterIP
NodePort
LoadBalancer
ExternalName
Now you need only inside the cluster then use ClusterIP
For reference use following yaml file.
kind: Service
apiVersion: v1
metadata:
name: mysql-svc
spec:
type: ClusterIP
ports:
- port: 3306
targetPort: 3306
selector:
app: blog-mysql
Now you will be access this pod using mysql-svc:3306
Refer this in blog-app yaml with
env:
- name: MYSQL_URL
value: mysql-svc
- name: MYSQL_PORT
value: 3306
For more info use Url :https://kubernetes.io/docs/concepts/services-networking/service/

Answer 3

Pods created will have dns configured in the following manner
pod_name.namespace.svc.cluster.local
In your case assuming these pods are in default namespace your jdbc connection string will be
jdbc:mysql://blog-mysql.default.svc.cluster.local:3306/test
Refer: https://kubernetes.io/docs/concepts/services-networking/dns-pod-service/#pods
Like Arghya Sadhu and Sachin Arote suggested you can always create a service and deployment. Service and deployment helps you in the cases where you have more than one replicas of pods and service takes care of the load-balancing.