Logging Using Stackdriver
Before reading this page, it’s highly recommended to familiarize yourself with the overview of logging in Kubernetes.
This article assumes that you have created a Kubernetes cluster with cluster-level logging support for sending logs to Stackdriver Logging. You can do this either by selecting the Enable Stackdriver Logging checkbox in the create cluster dialogue in GKE, or by setting the KUBE_LOGGING_DESTINATION flag to gcp when manually starting a cluster using kube-up.sh.
The following guide describes gathering a container’s standard output and standard error. To gather logs written by an application to a file, you can use a sidecar approach.
Overview
After creation, you can discover logging agent pods in the kube-system namespace,
one per node, by running the following command:
$ kubectl get pods --namespace=kube-system
NAME READY STATUS RESTARTS AGE
...
fluentd-gcp-v1.30-50gnc 1/1 Running 0 5d
fluentd-gcp-v1.30-v255c 1/1 Running 0 5d
fluentd-gcp-v1.30-f02l5 1/1 Running 0 5d
...
To understand how logging with Stackdriver works, consider the following synthetic log generator pod specification counter-pod.yaml:
counter-pod.yaml
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This pod specification has one container that runs a bash script that writes out the value of a counter and the date once per second, and runs indefinitely. Let’s create this pod in the default namespace.
$ kubectl create -f http://k8s.io/docs/user-guide/logging/examples/counter-pod.yaml
pod "counter" created
You can observe the running pod:
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
counter 1/1 Running 0 5m
For a short period of time you can observe the ‘Pending’ pod status, because the kubelet
has to download the container image first. When the pod status changes to Running
you can use the kubectl logs command to view the output of this counter pod.
$ 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
...
As described in the logging overview, this command fetches log entries from the container log file. If the container is killed and then restarted by Kubernetes, you can still access logs from the previous container. However, if the pod is evicted from the node, log files are lost. Let’s demonstrate this by deleting the currently running counter container:
$ kubectl delete pod counter
pod "counter" deleted
and then recreating it:
$ kubectl create -f http://k8s.io/docs/user-guide/logging/examples/counter-pod.yaml
pod "counter" created
After some time, you can access logs from the counter pod again:
$ kubectl logs counter
0: Mon Jan 1 00:01:00 UTC 2001
1: Mon Jan 1 00:01:01 UTC 2001
2: Mon Jan 1 00:01:02 UTC 2001
...
As expected, only recent log lines are present. However, for a real-world application you will likely want to be able to access logs from all containers, especially for the debug purposes. This is exactly when the previously enabled Stackdriver Logging can help.
Viewing logs
Stackdriver Logging agent attaches metadata to each log entry, for you to use later in queries to select only the messages you’re interested in: for example, the messages from a particular pod.
The most important pieces of metadata are the resource type and log name.
The resource type of a container log is container, which is named
GKE Containers in the UI (even if the Kubernetes cluster is not on GKE).
The log name is the name of the container, so that if you have a pod with
two containers, named container_1 and container_2 in the spec, their logs
will have log names container_1 and container_2 respectively.
System components have resource type compute, which is named
GCE VM Instance in the interface. Log names for system components are fixed.
For a GKE node, every log entry from a system component has one the following
log names:
- docker
- kubelet
- kube-proxy
You can learn more about viewing logs on the dedicated Stackdriver page.
One of the possible ways to view logs is using the
gcloud logging
command line interface from the Google Cloud SDK.
It uses Stackdriver Logging filtering syntax
to query specific logs. For example, you can run the following command:
$ gcloud beta logging read 'logName="projects/$YOUR_PROJECT_ID/logs/count"' --format json | jq '.[].textPayload'
...
"2: Mon Jan 1 00:01:02 UTC 2001\n"
"1: Mon Jan 1 00:01:01 UTC 2001\n"
"0: Mon Jan 1 00:01:00 UTC 2001\n"
...
"2: Mon Jan 1 00:00:02 UTC 2001\n"
"1: Mon Jan 1 00:00:01 UTC 2001\n"
"0: Mon Jan 1 00:00:00 UTC 2001\n"
As you can see, it outputs messages for the count container from both the first and second runs, despite the fact that the kubelet already deleted the logs for the first container.
Exporting logs
You can export logs to Google Cloud Storage or to BigQuery to run further analysis. Stackdriver Logging offers the concept of sinks, where you can specify the destination of log entries. More information is available on the Stackdriver Exporting Logs page.