# kubectl apply -f side-car-pod.yaml

pod/sidecar-pod created

once the pod is running

Ambassador

The ambassador pattern derives its name from an Ambassador, who is an envoy and a person a country chooses to represent their country and connect with the rest of the world. Similarly, in the Kubernetes perspective, an Ambassador pattern implements a proxy to the external world. Let me give you an example — If you build an application that needs to connect with a database server, the server configuration, etc, changes with the environment.

Now, the official recommendation to handle these is to use Config Maps, but what if you have legacy code that is already using another way of connecting to the database. Maybe, a properties file, or even worse, a hardcoded set of values. What if you want to communicate with localhost, and you can leave the rest to the admin? You can use the Ambassador pattern for these kinds of scenarios.

So, what we can do is create another container that can act as a TCP Proxy to the database, and you can connect to the proxy via localhost. The sysadmin can then use config maps and secrets with the proxy container to inject the correct connection and auth information.

#cat ambassador-pod.yaml

apiVersion: v1

kind: Pod

metadata:

  name: ambassador-pod

  labels:

    app: ambassador-app

spec:

  volumes:

  - name: shared

    emptyDir: {}

  containers:

  - name: app-container-poller

    image: yauritux/busybox-curl

    command: ["/bin/sh"]

    args: ["-c", "while true; do curl 127.0.0.1:81 > /usr/share/nginx/html/index.html; sleep 10; done"]

    volumeMounts:

    - name: shared

      mountPath: /usr/share/nginx/html

  - name: app-container-server

    image: nginx

    ports:

      - containerPort: 80

    volumeMounts:

    - name: shared

      mountPath: /usr/share/nginx/html

  - name: ambassador-container

    image: bharamicrosystems/nginx-forward-proxy

    ports:

      - containerPort: 81

save and exit the file

If you look carefully in the manifest YAML, you will find there are three containers. The app-container-poller continuously calls http://localhost:81 and sends the content to /usr/share/nginx/html/index.html.

The app-container-server runs and listens on port 80 to handle external requests. Both these containers share a common mountPath. That is similar to the sidecar approach.

There is an ambassador-container running within the pod that listens on localhost:81 and proxies the connection to example.com, so when we curl the app-container-server the endpoint on port 80, we get a response from example.com.

Adapter

The Adapter is another pattern that you can implement with multiple containers. The adapter pattern helps you standardize something heterogeneous in nature. For example, you’re running multiple applications within separate containers, but every application has a different way of outputting log files.

Now, you have a centralised logging system that accepts logs in a particular format only. What can you do in such a situation? Well, you can either change the source code of each application to output a standard log format or use an adapter to standardise the logs before sending it to your central server. That’s where the adapter pattern comes in.

#cat adapter-pod.yaml

apiVersion: v1

kind: Pod

metadata:

  name: adapter-pod

  labels:

    app: adapter-app

spec:

  volumes:

  - name: logs

    emptyDir: {}

  containers:

  - name: app-container

    image: alpine

    command: ["/bin/sh"]

    args: ["-c", "while true; do date >> /var/log/app.log; sleep 2;done"]

    volumeMounts:

    - name: logs

      mountPath: /var/log

  - name: log-adapter

    image: alpine

    command: ["/bin/sh"]

    args: ["-c", "tail -f /var/log/app.log|sed -e 's/^/Date /' > /var/log/out.log"]

    volumeMounts:

    - name: logs

      mountPath: /var/log

In the manifest, we have an app-container that outputs a stream of dates to a log file. The log-adapter container adds a Date prefix in front. Yes, it’s a very rudimentary example, but enough to get how the adapter works.