Kubernetes - mount container stdout stderr logs

Question

I use "chentex/random-logger" image, which write stdout/stderr to the container. I want to make deployment yaml, which run the chentex's image, and put it's logs in file inside shared volume. Can I do it without modifying the image?

This is the simple deployment of the image:

apiVersion: v1
kind: Deployment
metadata:
  name: random-logger
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: random-logger
    spec:
      containers:
      - name: random-logger
        image: chentex/random-logger:latest

Answer 1

It is best practice to send log message to stdout for applications running in a container. The chentex/random-logger just follows this approach without any option to configure this, but we can bring up a hack like this:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: random-logger
spec:
  selector:
    matchLabels:
      app: random-logger
  template:
    metadata:
      labels:
        app: random-logger
    spec:
      containers:
      - name: random-logger
        image: chentex/random-logger:latest
        command: ["sh", "-c",  "./entrypoint.sh &> /logfile"]

When requesting the logs from the running pod there is nothing to see:

$ kubectl logs random-logger-76c6fd98d5-8d5fm

The application logs are written to logfile within the container:

$ kubectl exec random-logger-76c6fd98d5-8d5fm cat /logfile
2019-02-28T00:23:23+0000 DEBUG first loop completed.
2019-02-28T00:23:25+0000 ERROR something happened in this execution.
2019-02-28T00:23:29+0000 INFO takes the value and converts it to string.
2019-02-28T00:23:31+0000 WARN variable not in use.
2019-02-28T00:23:37+0000 INFO takes the value and converts it to string.

Although this is possible, it is in general not advised. See the Kubernetes documentation about Logging Architecture for more background information.