Viewing Application Logs on Virtual Nodes

Find out how to view the logs of applications running on virtual nodes in a Kubernetes cluster you've created using Kubernetes Engine (OKE).

Having created a cluster with virtual nodes using Kubernetes Engine, you have two options to access the logs of applications running on the virtual nodes:

View the stdout and stderr logs from application containers using the kubectl logs command.
Send application logs to a persistent log server for viewing.

Viewing stdout and stderr logs using the kubectl logs command

To view the stdout and stderr logs from an application's containers using the kubectl logs command:

If you haven't already done so, follow the steps to set up the cluster's kubeconfig configuration file and (if necessary) set the KUBECONFIG environment variable to point to the file. Note that you must set up your own kubeconfig file. You cannot access a cluster using a kubeconfig file that a different user set up. See Setting Up Cluster Access.
In a terminal window, run the kubectl logs command by entering:
```
kubectl logs <pod-name>
```
For example:
```
kubectl logs nginx
```
When the pod contains more than one container, specify the container for which you want to view the logs. For example, if the myapp pod has two containers called nginx and logging and you want to display the logs of the logging container, enter:
```
kubectl logs myapp logging
```

Sending logs to a persistent log server

You can send the logs of applications running on virtual nodes to a remote log server such as OpenSearch, and view the logs on that server. Since virtual nodes do not support the Kubernetes daemonset, you cannot run a logging agent at the node level. Instead, you can run a lightweight logging agent as a sidecar for each of the application's pods. You can use any logging agent that can run as a sidecar on a Kubernetes pod, such as Fluent Bit.

For example, to run Fluent Bit as a sidecar on a pod to send application logs to a remote OpenSearch log server:

Create a Kubernetes ConfigMap per cluster or per namespace, specifying the Fluent Bit configuration settings to use. In particular, note that you must specify the remote log server's IP address, port number, and authentication parameters.
For example:
```
apiVersion: v1
kind: ConfigMap
metadata:
  name: fluentbit-config
data:
  fluent-bit.conf: |
    [SERVICE]
        flush        1
        daemon       Off
 
    [INPUT]
        Name              tail
        Path              /mnt/log/*
 
    [OUTPUT]
        name  opensearch
        match *
        host <ip-address>
        port <port-number>
        tls On
        HTTP_User <username>
        HTTP_Passwd <example-password>
        Index fluent-bit
        Suppress_Type_Name On
```
where:
- host <ip-address> specifies the IP address of the Opensearch server.
- port <port-number> specifies the port of the Opensearch server (9200 by default).
- HTTP_User <username> specifies the username for the Opensearch server.
- HTTP_Passwd <example-password> specifies the password for the Opensearch server.

Add a Fluent Bit sidecar to each pod. Note that you must configure all pods with a Fluent Bit sidecar to ensure that application logs are sent to the persistent log destination.

In the volumes section of each pod spec, specify:
- An emptyDir where application logs are written, as follows:
```
- name: logs
  emptyDir: {}
```
- A reference to the ConfigMap you created previously, as follows:
```
- name: fluentbit-config
  configMap:
    name: fluentbit-config
```

In the containers section of each pod spec, you must specify that the logging container:

Uses the fluent/fluent-bit image.
Mounts the logs emptyDir and the fluentbit-config ConfigMap.

For example:

- name: logging
  image: fluent/fluent-bit
  resources:
    requests:
      cpu: 100m
      memory: 50Mi
  volumeMounts:
  - name: logs
    mountPath: /mnt/log
  - name: fluentbit-config
    mountPath: /fluent-bit/etc

Here's an example of a complete pod specification:

apiVersion: v1
kind: Pod
metadata:
  name: date-logging
spec:
  containers:
  - name: date
    image: busybox
    resources:
      requests:
        cpu: 500m
        memory: 200Mi
    command: ["sh", "-c"]
    args:
      - while [ 1 ]; do
        echo "Writing the date in /mnt/log/date.log";
        date >> /mnt/log/date.log;
        sleep 10;
        done
    volumeMounts:
    - name: logs
      mountPath: /mnt/log
  - name: logging
    image:
     resources:
      requests:
        cpu: 100m
        memory: 50Mi
    volumeMounts:
    - name: logs
      mountPath: /mnt/log
    - name: fluentbit-config
      mountPath: /fluent-bit/etc
  volumes:
  - name: logs
    emptyDir: {}
  - name: fluentbit-config
    configMap:
      name: fluentbit-config

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Viewing Application Logs on Virtual Nodes

Viewing stdout and stderr logs using the kubectl logs command

Sending logs to a persistent log server

Oracle Cloud Infrastructure Documentation
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