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Welcome to this comprehensive guide on managing application logs in Kubernetes. In this article, we explore various logging mechanisms, starting with Docker and moving on to Kubernetes, to help you monitor and troubleshoot your applications effectively.

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Logging in Docker

Docker containers typically log events to the standard output. Consider the “event simulator” container, which generates random events simulating a web server. When you run this container, it writes log entries such as: 
docker run kodekloud/event-simulator
2018-10-06 15:57:15,937 - root - INFO - USER1 logged in
2018-10-06 15:57:16,943 - root - INFO - USER2 logged out
2018-10-06 15:57:17,944 - root - INFO - USER3 is viewing page3
2018-10-06 15:57:18,951 - root - INFO - USER4 is viewing page1
2018-10-06 15:57:19,954 - root - INFO - USER1 logged out
2018-10-06 15:57:21,956 - root - INFO - USER1 logged in
2018-10-06 15:57:22,957 - root - INFO - USER3 is viewing page2
2018-10-06 15:57:23,959 - root - INFO - USER1 logged out
2018-10-06 15:57:24,959 - root - INFO - USER2 is viewing page2
2018-10-06 15:57:25,962 - root - INFO - USER4 is viewing page3
2018-10-06 15:57:26,965 - root - INFO - USER3 is viewing page1
2018-10-06 15:57:27,965 - root - INFO - USER3 logged out
2018-10-06 15:57:29,967 - root - INFO - USER1 is viewing page2
If you run the container in detached mode using the -d flag, the logs will not appear on your terminal immediately. Instead, you can stream them later with: 
docker run -d kodekloud/event-simulator
docker logs -f <container_id>

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Logging in Kubernetes

Deploying the same Docker image within a Kubernetes pod leverages Kubernetes’ logging capabilities. To get started, create a pod using the following YAML definition: 
apiVersion: v1
kind: Pod
metadata:
  name: event-simulator-pod
spec:
  containers:
    - name: event-simulator
      image: kodekloud/event-simulator
Create the pod with this command: 
kubectl create -f event-simulator.yaml
Once the pod is running, view the live logs using: 
kubectl logs -f event-simulator-pod
This command outputs logs similar to the Docker example: 
2018-10-06 15:57:15,937 - root - INFO - USER1 logged in
2018-10-06 15:57:16,943 - root - INFO - USER2 logged out
2018-10-06 15:57:17,944 - root - INFO - USER2 is viewing page2
2018-10-06 15:57:18,951 - root - INFO - USER3 is viewing page3
2018-10-06 15:57:20,095 - root - INFO - USER4 is viewing page1
2018-10-06 15:57:21,956 - root - INFO - USER2 logged out
2018-10-06 15:57:21,956 - root - INFO - USER1 logged in
2018-10-06 15:57:23,093 - root - INFO - USER3 is viewing page2
2018-10-06 15:57:24,959 - root - INFO - USER1 logged out
2018-10-06 15:57:25,961 - root - INFO - USER2 is viewing page2
2018-10-06 15:57:25,961 - root - INFO - USER1 logged in
For more effective troubleshooting, use log filtering and analysis tools in combination with Kubernetes logs.

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Logging with Multiple Containers in a Pod

Kubernetes supports pods with multiple containers. If you update your pod definition to include an additional container named image-processor, the configuration will look like this: 
apiVersion: v1
kind: Pod
metadata:
  name: event-simulator-pod
spec:
  containers:
    - name: event-simulator
      image: kodekloud/event-simulator
    - name: image-processor
      image: some-image-processor
Create the pod using: 
kubectl create -f event-simulator.yaml
Attempting to view logs without specifying the container when multiple containers are present will result in an error. Instead, specify the container name to view its logs: 
kubectl logs -f event-simulator-pod event-simulator
This command displays log output for the event-simulator container: 
2018-10-06 15:57:15,937 - root - INFO - USER1 logged in
2018-10-06 15:57:16,943 - root - INFO - USER2 logged out
2018-10-06 15:57:17,944 - root - INFO - USER2 is viewing page2
2018-10-06 15:57:18,951 - root - INFO - USER3 is viewing page3
2018-10-06 15:57:19,954 - root - INFO - USER4 is viewing page1
2018-10-06 15:57:20,955 - root - INFO - USER2 logged out
When working with pods that contain multiple containers, always specify the container name in the kubectl logs command to avoid errors.

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Additional Resources

For further information on logging and monitoring in Kubernetes, refer to the following resources: By mastering these logging techniques, you ensure efficient monitoring and troubleshooting of your applications in both Docker and Kubernetes environments. Happy logging! Thank you.

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