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Describing Containers

Services and container communication

Permitting communication between your containers (more specifically - your Pods) is handled via the ports property of an application in container. Under the hood, we use a Kubernetes Service.

Creating a service

Let's say we want to give other Pods running on the cluster the ability to communicate with our database container. Here's the simplest way to do it:

sql-container:
  type: tosca.nodes.MiCADO.Container.Application.Docker
  properties:
    image: mysql
    ports:
    - port: 3306
  interfaces:
    Kubernetes:
      create:

If you're familiar with Kubernetes, the one little snippet above generates the two manifests in the box below.

Kubernetes manifests 
apiVersion: v1
kind: Service
metadata:
  name: sql-container
  labels:
    app.kubernetes.io/name: sql-container
    app.kubernetes.io/instance: service-ex
    app.kubernetes.io/managed-by: micado
spec:
  selector:
    app.kubernetes.io/name: sql-container
    app.kubernetes.io/instance: service-ex
    app.kubernetes.io/managed-by: micado
    app.kubernetes.io/version: latest
  ports:
  - name: 3306-tcp
    port: 3306
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: sql-container
  labels:
    app.kubernetes.io/name: sql-container
    app.kubernetes.io/instance: service-ex
    app.kubernetes.io/managed-by: micado
    app.kubernetes.io/version: latest
spec:
  template:
    metadata:
      labels:
        app.kubernetes.io/name: sql-container
        app.kubernetes.io/instance: service-ex
        app.kubernetes.io/managed-by: micado
        app.kubernetes.io/version: latest
    spec:
      containers:
      - image: mysql
  selector:
    matchLabels:
      app.kubernetes.io/name: sql-container
      app.kubernetes.io/instance: service-ex
      app.kubernetes.io/managed-by: micado
      app.kubernetes.io/version: latest

Now, any other container can access port 3306 on this container at sql-container:3306. You can see the hostname defaults to the name of the TOSCA node you've defined.

If you have multiple ports to expose using the same hostname, you can simply add to the list of ports, like so:

...
  properties:
    image: mysql
    ports:
    - port: 3306
    - port: 33062
...

Other containers can reach both sql-container:3306 and sql-container:33062

Protocols and TargetPort

Ports default to TCP. For UDP connections, simply:

...
  properties:
    image: flannel
    ports:
    - port: 8285
      protocol: UDP
...

Kubernetes also supports the SCTP protocol

Sometimes, you may want/need to expose your container on a different port than that configured by the container by default. Consider an NGINX container listening on port 80, that we intend to expose to other containers using port 8080:

webserver:
  type: tosca.nodes.MiCADO.Container.Application.Docker
  properties:
    image: nginx
    ports:
    - port: 8080
      targetPort: 80
...

Other containers on the cluster can point at webserver:8080 to hit :80 inside the container

Hostnames and Port names

As we've seen, the name of the generated Service (also the hostname for ingress to the container) defaults to the name of the parent container. You can change this by overriding the Service metadata like so:

sql-container:
  type: tosca.nodes.MiCADO.Container.Application.Docker
  properties:
    image: mysql
    ports:
    - port: 3306
      metadata:
        name: database
...

Using the above snippet, you would now reach your container at database:3306

It is important not to confuse this with the name of the port, which is generated automatically based on the port number and protocol (see the previously generated manifests for an example of this).

If you have the urge to specify your own port names, you can: 

...
  properties:
    image: mysql
    ports:
    - name: myport
      port: 3306
      metadata:
        name: database
...

The port name has changed, but has no effect on our route to this container. It is still reachable at database:3306

ClusterIPs

When we create a service, Kubernetes randomly assigns a ClusterIP to it, restricted by default to the range 10.96.0.0/12.

If you have a requirement for IP addressing over using hostnames, you can specify a ClusterIP like so:

sql-container:
  type: tosca.nodes.MiCADO.Container.Application.Docker
  properties:
    image: mysql
    ports:
    - port: 3306
      clusterIP: 10.97.101.98
...

Fun fact: you can specify a clusterIP of None

We can now hit this container at 10.97.101.98:3306. It is also still available at sql-container:3306.

Next up: Exposing containers externally