Kubernetes Pod communication through services

Question

In the case that an app and database are in different containers/pods (each a separate deployment yaml) how do you get them to communicate?

For example, a Django app requires the host name of the database in it's config/environment variables (along with the database name and a few other things) in order to connect to the database.

You should be able to specify the service as follows (assuming the database has a service called db-service in the default namespace):

Inside Django app demployment.yaml file:

    env:
    - name: SQL_HOST
      value: "db-service.default"

or you could do some hackery with the HOSTNAME for the database container (if it's similar to the app name) for example:

Inside Django app demployment.yaml file:

env:
- name: POD_NAME
  valueFrom:
    fieldRef:
      fieldPath: metadata.name

- name: SQL_HOST
  value: $(POD_NAME)-db

Inside Postgres demployment.yaml file:

spec:
  replicas: 1
  selector:
    matchLabels:
      app: sitename-db-container
  template:
    metadata:
      labels:
        app: sitename-db-container
    spec:
      hostname: sitename-db

But what happens when you have multiple deployments inside a service for the same app (each having their app - database container pair)? How will the service know which app pod will communicate with what database pod? Does there now need to be a separate service for every app and database deployment?

You would create a service with a unique name for each pod/db you want to access. — Matt
A templating tool like Helm can help you create the matched pair of pairs of application and database deployments and services, and fill in the environment variable value in the YAML file for you. — David Maze

Unknown Unknown · Accepted Answer · 2020-01-29T13:30:30

But what happens when you have multiple deployments inside a service for the same app (each having their app - database container pair)? How will the service know which app pod will communicate with what database pod? Does there now need to be a separate service for every app and database deployment?

What do you mean by "multiple deployments inside a service" ? In a Service definition you are supposed to select only one set of Pods, let's say managed by one specific Deployment. As @Matt suggested, you should always create a service with a unique name for each pod/db you want to access. If you have Pods dedicated to do specific tasks you deploy them separately (as separate Deployments). They can even consist of just one Pod if you don't need any redundancy. And basically you will always create a separate Service ( obviously with unique name as you cannot create more Services using the same name ) for exposing different microservice ( represented by unique Deployment). Note that if you don't create a Deployment but a simple Pod it won't be managed by any controller so if it crashes, nothing will take care of recreating it. So definitely you should always use Deployment even to run a single Pod representing your microservice.

Have you read this topic in official kubernetes documentation ? If you don't specify Service type, by default it creates so called ClusterIP Service which is basically what you need to expose your app components internally (make them available for other app components in your cluster).

Kubernetes Pod communication through services

2 Answers