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Concepts

Detailed explanations of Kubernetes system concepts and abstractions.

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Logging Architecture

Application and systems logs can help you understand what is happening inside your cluster. The logs are particularly useful for debugging problems and monitoring cluster activity. Most modern applications have some kind of logging mechanism; as such, most container engines are likewise designed to support some kind of logging. The easiest and most embraced logging method for containerized applications is to write to the standard output and standard error streams.

However, the native functionality provided by a container engine or runtime is usually not enough for a complete logging solution. For example, if a container crashes, a pod is evicted, or a node dies, you’ll usually still want to access your application’s logs. As such, logs should have a separate storage and lifecycle independent of nodes, pods, or containers. This concept is called cluster-level-logging. Cluster-level logging requires a separate backend to store, analyze, and query logs. Kubernetes provides no native storage solution for log data, but you can integrate many existing logging solutions into your Kubernetes cluster.

Cluster-level logging architectures are described in assumption that a logging backend is present inside or outside of your cluster. If you’re not interested in having cluster-level logging, you might still find the description of how logs are stored and handled on the node to be useful.

Basic logging in Kubernetes

In this section, you can see an example of basic logging in Kubernetes that outputs data to the standard output stream. This demonstration uses a pod specification with a container that writes some text to standard output once per second.

counter-pod.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: counter
spec:
  containers:
  - name: count
    image: busybox
    args: [/bin/sh, -c,
            'i=0; while true; do echo "$i: $(date)"; i=$((i+1)); sleep 1; done']

To run this pod, use the following command:

$ kubectl create -f https://k8s.io/docs/tasks/debug-application-cluster/counter-pod.yaml
pod "counter" created

To fetch the logs, use the kubectl logs command, as follows:

$ kubectl logs counter
0: Mon Jan  1 00:00:00 UTC 2001
1: Mon Jan  1 00:00:01 UTC 2001
2: Mon Jan  1 00:00:02 UTC 2001
...

You can use kubectl logs to retrieve logs from a previous instantiation of a container with --previous flag, in case the container has crashed. If your pod has multiple containers, you should specify which container’s logs you want to access by appending a container name to the command. See the kubectl logs documentation for more details.

Logging at the node level

Node level logging

Everything a containerized application writes to stdout and stderr is handled and redirected somewhere by a container engine. For example, the Docker container engine redirects those two streams to a logging driver, which is configured in Kubernetes to write to a file in json format.

Note: The Docker json logging driver treats each line as a separate message. When using the Docker logging driver, there is no direct support for multi-line messages. You need to handle multi-line messages at the logging agent level or higher.

By default, if a container restarts, the kubelet keeps one terminated container with its logs. If a pod is evicted from the node, all corresponding containers are also evicted, along with their logs.

An important consideration in node-level logging is implementing log rotation, so that logs don’t consume all available storage on the node. Kubernetes currently is not responsible for rotating logs, but rather a deployment tool should set up a solution to address that. For example, in Kubernetes clusters, deployed by the kube-up.sh script, there is a logrotate tool configured to run each hour. You can also set up a container runtime to rotate application’s logs automatically, e.g. by using Docker’s log-opt. In the kube-up.sh script, the latter approach is used for COS image on GCP, and the former approach is used in any other environment. In both cases, by default rotation is configured to take place when log file exceeds 10MB.

As an example, you can find detailed information about how kube-up.sh sets up logging for COS image on GCP in the corresponding script.

When you run kubectl logs as in the basic logging example, the kubelet on the node handles the request and reads directly from the log file, returning the contents in the response.

Note: currently, if some external system has performed the rotation, only the contents of the latest log file will be available through kubectl logs. E.g. if there’s a 10MB file, logrotate performs the rotation and there are two files, one 10MB in size and one empty, kubectl logs will return an empty response.

System component logs

There are two types of system components: those that run in a container and those that do not run in a container. For example:

On machines with systemd, the kubelet and container runtime write to journald. If systemd is not present, they write to .log files in the /var/log directory. System components inside containers always write to the /var/log directory, bypassing the default logging mechanism. They use the glog logging library. You can find the conventions for logging severity for those components in the development docs on logging.

Similarly to the container logs, system component logs in the /var/log directory should be rotated. In Kubernetes clusters brought up by the kube-up.sh script, those logs are configured to be rotated by the logrotate tool daily or once the size exceeds 100MB.

Cluster-level logging architectures

While Kubernetes does not provide a native solution for cluster-level logging, there are several common approaches you can consider. Here are some options:

Using a node logging agent

Using a node level logging agent

You can implement cluster-level logging by including a node-level logging agent on each node. The logging agent is a dedicated tool that exposes logs or pushes logs to a backend. Commonly, the logging agent is a container that has access to a directory with log files from all of the application containers on that node.

Because the logging agent must run on every node, it’s common to implement it as either a DaemonSet replica, a manifest pod, or a dedicated native process on the node. However the latter two approaches are deprecated and highly discouraged.

Using a node-level logging agent is the most common and encouraged approach for a Kubernetes cluster, because it creates only one agent per node, and it doesn’t require any changes to the applications running on the node. However, node-level logging only works for applications’ standard output and standard error.

Kubernetes doesn’t specify a logging agent, but two optional logging agents are packaged with the Kubernetes release: Stackdriver Logging for use with Google Cloud Platform, and Elasticsearch. You can find more information and instructions in the dedicated documents. Both use fluentd with custom configuration as an agent on the node.

Using a sidecar container with the logging agent

You can use a sidecar container in one of the following ways:

Streaming sidecar container

Sidecar container with a streaming container

By having your sidecar containers stream to their own stdout and stderr streams, you can take advantage of the kubelet and the logging agent that already run on each node. The sidecar containers read logs from a file, a socket, or the journald. Each individual sidecar container prints log to its own stdout or stderr stream.

This approach allows you to separate several log streams from different parts of your application, some of which can lack support for writing to stdout or stderr. The logic behind redirecting logs is minimal, so it’s hardly a significant overhead. Additionally, because stdout and stderr are handled by the kubelet, you can use built-in tools like kubectl logs.

Consider the following example. A pod runs a single container, and the container writes to two different log files, using two different formats. Here’s a configuration file for the Pod:

two-files-counter-pod.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: counter
spec:
  containers:
  - name: count
    image: busybox
    args:
    - /bin/sh
    - -c
    - >
      i=0;
      while true;
      do
        echo "$i: $(date)" >> /var/log/1.log;
        echo "$(date) INFO $i" >> /var/log/2.log;
        i=$((i+1));
        sleep 1;
      done
    volumeMounts:
    - name: varlog
      mountPath: /var/log
  volumes:
  - name: varlog
    emptyDir: {}

It would be a mess to have log entries of different formats in the same log stream, even if you managed to redirect both components to the stdout stream of the container. Instead, you could introduce two sidecar containers. Each sidecar container could tail a particular log file from a shared volume and then redirect the logs to its own stdout stream.

Here’s a configuration file for a pod that has two sidecar containers:

two-files-counter-pod-streaming-sidecar.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: counter
spec:
  containers:
  - name: count
    image: busybox
    args:
    - /bin/sh
    - -c
    - >
      i=0;
      while true;
      do
        echo "$i: $(date)" >> /var/log/1.log;
        echo "$(date) INFO $i" >> /var/log/2.log;
        i=$((i+1));
        sleep 1;
      done
    volumeMounts:
    - name: varlog
      mountPath: /var/log
  - name: count-log-1
    image: busybox
    args: [/bin/sh, -c, 'tail -n+1 -f /var/log/1.log']
    volumeMounts:
    - name: varlog
      mountPath: /var/log
  - name: count-log-2
    image: busybox
    args: [/bin/sh, -c, 'tail -n+1 -f /var/log/2.log']
    volumeMounts:
    - name: varlog
      mountPath: /var/log
  volumes:
  - name: varlog
    emptyDir: {}

Now when you run this pod, you can access each log stream separately by running the following commands:

$ kubectl logs counter count-log-1
0: Mon Jan  1 00:00:00 UTC 2001
1: Mon Jan  1 00:00:01 UTC 2001
2: Mon Jan  1 00:00:02 UTC 2001
...

$ kubectl logs counter count-log-2
Mon Jan  1 00:00:00 UTC 2001 INFO 0
Mon Jan  1 00:00:01 UTC 2001 INFO 1
Mon Jan  1 00:00:02 UTC 2001 INFO 2
...

The node-level agent installed in your cluster picks up those log streams automatically without any further configuration. If you like, you can configure the agent to parse log lines depending on the source container.

Note, that despite low CPU and memory usage (order of couple of millicores for cpu and order of several megabytes for memory), writing logs to a file and then streaming them to stdout can double disk usage. If you have an application that writes to a single file, it’s generally better to set /dev/stdout as destination rather than implementing the streaming sidecar container approach.

Sidecar containers can also be used to rotate log files that cannot be rotated by the application itself. An example of this approach is a small container running logrotate periodically. However, it’s recommended to use stdout and stderr directly and leave rotation and retention policies to the kubelet.

Sidecar container with a logging agent

Sidecar container with a logging agent

If the node-level logging agent is not flexible enough for your situation, you can create a sidecar container with a separate logging agent that you have configured specifically to run with your application.

Note: Using a logging agent in a sidecar container can lead to significant resource consumption. Moreover, you won’t be able to access those logs using kubectl logs command, because they are not controlled by the kubelet.

As an example, you could use Stackdriver, which uses fluentd as a logging agent. Here are two configuration files that you can use to implement this approach. The first file contains a ConfigMap to configure fluentd.

fluentd-sidecar-config.yaml 
apiVersion: v1
data:
  fluentd.conf: |
    <source>
      type tail
      format none
      path /var/log/1.log
      pos_file /var/log/1.log.pos
      tag count.format1
    </source>

    <source>
      type tail
      format none
      path /var/log/2.log
      pos_file /var/log/2.log.pos
      tag count.format2
    </source>

    <match **>
      type google_cloud
    </match>
kind: ConfigMap
metadata:
  name: fluentd-config

Note: The configuration of fluentd is beyond the scope of this article. For information about configuring fluentd, see the official fluentd documentation.

The second file describes a pod that has a sidecar container running fluentd. The pod mounts a volume where fluentd can pick up its configuration data.

two-files-counter-pod-agent-sidecar.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: counter
spec:
  containers:
  - name: count
    image: busybox
    args:
    - /bin/sh
    - -c
    - >
      i=0;
      while true;
      do
        echo "$i: $(date)" >> /var/log/1.log;
        echo "$(date) INFO $i" >> /var/log/2.log;
        i=$((i+1));
        sleep 1;
      done
    volumeMounts:
    - name: varlog
      mountPath: /var/log
  - name: count-agent
    image: k8s.gcr.io/fluentd-gcp:1.30
    env:
    - name: FLUENTD_ARGS
      value: -c /etc/fluentd-config/fluentd.conf
    volumeMounts:
    - name: varlog
      mountPath: /var/log
    - name: config-volume
      mountPath: /etc/fluentd-config
  volumes:
  - name: varlog
    emptyDir: {}
  - name: config-volume
    configMap:
      name: fluentd-config

After some time you can find log messages in the Stackdriver interface.

Remember, that this is just an example and you can actually replace fluentd with any logging agent, reading from any source inside an application container.

Exposing logs directly from the application

Exposing logs directly from the application

You can implement cluster-level logging by exposing or pushing logs directly from every application; however, the implementation for such a logging mechanism is outside the scope of Kubernetes.

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