Kubernetes: Application Coupling

Demonstrates different application coupling options in Kubernetes.

Read also: https://blog.openshift.com/kubernetes-application-coupling/

The application

Let's assume our application comprises two programs, a generator program, appending random numbers to a file data like so:

$ docker run --rm -v "$PWD":/tmp/out -w /tmp/out alpine:3.6 sh -c 'tmpf=data && touch $tmpf && while true ; do echo $RANDOM >> $tmpf ; sleep 2 ; done ;'

As well as a simple echo program, reading the generated numbers and serving it via HTTP on port 8000:

$ docker run --rm -p 8000:8000 -v "$PWD":/tmp/in -w /tmp/in python:2.7 python -m SimpleHTTPServer

The application can then be consumed like so:

$ curl http://localhost:8000/data
4646
25543
3259
15121
...

Coupling via Dockerfile

The tightest coupling is to put both programs into the same Dockerfile like so:

FROM python:2.7-alpine
ADD ./generator/generator.sh /app/generator.sh
ADD ./echo/echo.sh /app/echo.sh
ADD app.sh /app/app.sh
WORKDIR /app
CMD ["sh", "app.sh"]

You can take this Dockerfile, build the container image, push it to a registry and deploy it using a pod.

Coupling via a pod

Putting each of the application parts into a container image and launching them in a pod indeed allows us to have colocation as well as enable local communication. Let's start with the container images using the OpenShift build process which will result in images available in the internal registry at 172.30.1.1:5000.

For the generator, using a dedicated Dockerfile:

$ cd generator/
$ oc new-build --strategy=docker --name='generator' .
$ oc start-build generator --from-dir .

And for the echo program, also using a dedicated Dockerfile:

$ cd echo/
$ oc new-build --strategy=docker --name='echo' .
$ oc start-build echo --from-dir .

Next, I launch the two containers in a pod called appcoup, defined in app-pod.yaml:

$ oc create -f app-pod.yaml