Tail logfiles with a webhook

The tailer-webhook is a different approach for the same problem: parsing legacy application’s log file. As an alternative to using a host file tailer service, you can use a file tailer webhook service. While the containers of the host file tailers run in a separated pod, file tailer webhook uses a different approach: if a pod has a specific annotation, the webhook injects a sidecar container for every tailed file into the pod.

The tailer-webhook behaves differently compared to the host-tailer:

Pros:

• A simple annotation on the pod initiates the file tailing.
• There is no need to use mounted volumes, Logging operator will manage the volumes and mounts between your containers.

Cons:

• Required to start the Logging operator with webhooks service enabled. This requires additional configuration, especially on certificates since webhook services are allowed over TLS only.
• Possibly uses more resources, since every tailed file attaches a new sidecar container to the pod.

Enable webhooks in Logging operator 🔗︎

We recommend using cert-manager to manage your certificates. Since using cert-manager is not part of this article, we assume you already have valid certs.

You will require the following things:

• a valid client certificate,
• a CA certificate, and
• a custom value.yaml file for your helm chart.

The following example refers to a Kubernetes secret named webhook-tls which is a self-signed certificate generated by cert-manager.

Add the following lines to your custom values.yaml or create a new file if needed:

env:
  - name: ENABLE_WEBHOOKS
    value: "true"
volumes:
  - name: webhook-tls
    secret:
      secretName: webhook-tls
volumeMounts:
  - name: webhook-tls
    mountPath: /tmp/k8s-webhook-server/serving-certs

This will:

• Set ENABLE_WEBHOOKS environment variable to true. This is the official way to enable webhooks in Logging operator.
• Create a volume from the webhook-tls Kubernetes secret.
• Mount the webhook-tls secret volume to the /tmp/k8s-webhook-server/serving-certs path where Logging operator will search for it.

Now you are ready to install Logging operator with the new custom values:

helm upgrade --install --wait --create-namespace --namespace logging -f operator_values.yaml  logging-operator ./charts/logging-operator

Alternatively, instead of using the values.yaml file, you can run the installation from command line also by passing the values with the set and set-string parameters:

helm upgrade --install --wait --create-namespace --namespace logging --set "env[0].name=ENABLE_WEBHOOKS" --set-string "env[0].value=true" --set "volumes[0].name=webhook-tls" --set "volumes[0].secret.secretName=webhook-tls" --set "volumeMounts[0].name=webhook-tls" --set "volumeMounts[0].mountPath=/tmp/k8s-webhook-server/serving-certs"  logging-operator ./charts/logging-operator

You also need a service which points to the webhook port (9443) of Logging operator, and where the mutatingwebhookconfiuration will point to. Running the following command in shell will create the required service:

kubectl apply -f - <<EOF
apiVersion: v1
kind: Service
metadata:
  name: logging-webhooks
  namespace: logging
spec:
  ports:
    - name: logging-webhooks
      port: 443
      targetPort: 9443
      protocol: TCP
  selector:
    app.kubernetes.io/instance: logging-operator
  type: ClusterIP
EOF

Furthermore, you need to tell Kubernetes to send admission requests to our webhook service. To do that, create a mutatingwebhookconfiguration Kubernetes resource, and:

• Set the configuration to call /tailer-webhook path on your logging-webhooks service when v1.Pod is created.
• Set failurePolicy to ignore, which means that the original pod will be created on webhook errors.
• Set sideEffects to none, because we won’t cause any out-of-band changes in Kubernetes.

Unfortunately, mutatingwebhookconfiguration requires the caBundle field to be filled because we used a self-signed certificate, and the certificate cannot be validated through the system trust roots. If your certificate was generated with a system trust root CA, remove the caBundle line, because the certificate will be validated automatically. There are more sophisticated ways to load the CA into this field, but this solution requires no further components.

For example: you can inject the CA with a simple cert-manager cert-manager.io/inject-ca-from: logging/webhook-tls annotation on the mutatingwebhookconfiguration resource.

kubectl apply -f - <<EOF
apiVersion: admissionregistration.k8s.io/v1
kind: MutatingWebhookConfiguration
metadata:
  name: sample-webhook-cfg
  namespace: logging
  labels:
    app: sample-webhook
webhooks:
  - name: sample-webhook.banzaicloud.com
    clientConfig:
      service:
        name: logging-webhooks
        namespace: logging
        path: "/tailer-webhook"
      caBundle: $(kubectl get secret webhook-tls -n logging -o json | jq -r '.data["ca.crt"]')
    rules:
      - operations: [ "CREATE" ]
        apiGroups: [""]
        apiVersions: ["v1"]
        resources: ["pods"]
        scope: "*"
    failurePolicy: Ignore
    sideEffects: None
    admissionReviewVersions: [v1]
EOF

Triggering the webhook 🔗︎

File tailer webhook is based on a Mutating Admission Webhook. It is called every time when a pod starts.

To trigger the webhook, add the following annotation to the pod metadata:

• Annotation key: sidecar.logging-extensions.banzaicloud.io/tail

• Value of the annotation: the filename (including path, and optionally the container) you want to tail, for example:

  annotations: {"sidecar.logging-extensions.banzaicloud.io/tail": "/var/log/date"}
  

• To tail multiple files, add only one annotation, and separate the filenames with commas, for example:

  ...
  metadata:
      name: test-pod
      annotations: {"sidecar.logging-extensions.banzaicloud.io/tail": "/var/log/date,/var/log/mycustomfile"}
  spec:
  ...
  

• If the pod contains multiple containers, see Multi-container pods.

Note: If the pod with the sidecar annotation is in the default namespace, Logging operator handles tailer-webhook annotations clusterwide. To restrict the webhook callbacks to the current namespace, change the scope of the mutatingwebhookconfiguration to namespaced.

File tailer example 🔗︎

The following example creates a pod that is running a shell in infinite loop that appends the date command’s output to a file every second. The annotation sidecar.logging-extensions.banzaicloud.io/tail notifies Logging operator to attach a sidecar container to the pod. The sidecar tails the /legacy-logs/date.log file and sends its output to the stdout.

apiVersion: v1
kind: Pod
metadata:
    name: test-pod
    annotations: {"sidecar.logging-extensions.banzaicloud.io/tail": "/var/log/date"}
spec:
    containers:
    - image: debian
        name: sample-container
        command: ["/bin/sh", "-c"]
        args:
            - while true; do
                date >> /var/log/date;
                sleep 1;
        done
    - image: debian
        name: sample-container2
...

After you have created the pod with the required annotation, make sure that the test-pod contains two containers by running kubectl get pod

Expected output:

NAME       READY   STATUS    RESTARTS   AGE
test-pod   2/2     Running   0          29m

Check the container names in the pod to see that the Logging operator has created the sidecar container called legacy-logs-date-log. The sidecar containers’ name is always built from the path and name of the tailed file. Run the following command:

kubectl get pod test-pod -o json | jq '.spec.containers | map(.name)'

Expected output:

[
  "test",
  "legacy-logs-date-log"
]

Check the logs of the test container. Since it writes the logs into a file, it does not produce any logs on stdout.

kubectl logs test-pod test; echo $?

Expected output:

0

Check the logs of the legacy-logs-date-log container. This container exposes the logs of the test container on its stdout.

kubectl logs test-pod legacy-logs-date-log

Expected output:

Fluent Bit v1.9.5
* Copyright (C) 2015-2022 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

[2022/09/15 11:26:11] [ info] [fluent bit] version=1.9.5, commit=9ec43447b6, pid=1
[2022/09/15 11:26:11] [ info] [storage] version=1.2.0, type=memory-only, sync=normal, checksum=disabled, max_chunks_up=128
[2022/09/15 11:26:11] [ info] [cmetrics] version=0.3.4
[2022/09/15 11:26:11] [ info] [sp] stream processor started
[2022/09/15 11:26:11] [ info] [input:tail:tail.0] inotify_fs_add(): inode=938627 watch_fd=1 name=/legacy-logs/date.log
[2022/09/15 11:26:11] [ info] [output:file:file.0] worker #0 started
Thu Sep 15 11:26:11 UTC 2022
Thu Sep 15 11:26:12 UTC 2022
...

Multi-container pods 🔗︎

In some cases you have multiple containers in your pod and you want to distinguish which file annotation belongs to which container. You can order every file annotations to particular container by prefixing the annotation with a ${ContainerName}: container key. For example:

...
metadata:
    name: test-pod
    annotations: {"sidecar.logging-extensions.banzaicloud.io/tail": "sample-container:/var/log/date,sample-container2:/var/log/anotherfile,/var/log/mycustomfile,foobarbaz:/foo/bar/baz"}
spec:
...