Debugging Pods and Replication Controllers
Debugging pods
The first step in debugging a pod is taking a look at it. Check the current state of the pod and recent events with the following command:
$ kubectl describe pods ${POD_NAME}
Look at the state of the containers in the pod. Are they all Running? Have
there been recent restarts?
Continue debugging depending on the state of the pods.
My pod stays pending
If a pod is stuck in Pending it means that it can not be scheduled onto a
node. Generally this is because there are insufficient resources of one type or
another that prevent scheduling. Look at the output of the kubectl describe
... command above. There should be messages from the scheduler about why it
can not schedule your pod. Reasons include:
Insufficient resources
You may have exhausted the supply of CPU or Memory in your cluster. In this case you can try several things:
-
Add more nodes to the cluster.
-
Terminate unneeded pods to make room for pending pods.
-
Check that the pod is not larger than your nodes. For example, if all nodes have a capacity of
cpu:1, then a pod with a limit ofcpu: 1.1will never be scheduled.You can check node capacities with the
kubectl get nodes -o <format>command. Here are some example command lines that extract just the necessary information:kubectl get nodes -o yaml | grep '\sname\|cpu\|memory' kubectl get nodes -o json | jq '.items[] | {name: .metadata.name, cap: .status.capacity}'The resource quota feature can be configured to limit the total amount of resources that can be consumed. If used in conjunction with namespaces, it can prevent one team from hogging all the resources.
Using hostPort
When you bind a pod to a hostPort there are a limited number of places that
the pod can be scheduled. In most cases, hostPort is unnecessary; try using a
service object to expose your pod. If you do require hostPort then you can
only schedule as many pods as there are nodes in your container cluster.
My pod stays waiting
If a pod is stuck in the Waiting state, then it has been scheduled to a
worker node, but it can’t run on that machine. Again, the information from
kubectl describe ... should be informative. The most common cause of
Waiting pods is a failure to pull the image. There are three things to check:
- Make sure that you have the name of the image correct.
- Have you pushed the image to the repository?
- Run a manual
docker pull <image>on your machine to see if the image can be pulled.
My pod is crashing or otherwise unhealthy
First, take a look at the logs of the current container:
$ kubectl logs ${POD_NAME} ${CONTAINER_NAME}
If your container has previously crashed, you can access the previous container’s crash log with:
$ kubectl logs --previous ${POD_NAME} ${CONTAINER_NAME}
Alternately, you can run commands inside that container with exec:
$ kubectl exec ${POD_NAME} -c ${CONTAINER_NAME} -- ${CMD} ${ARG1} ${ARG2} ... ${ARGN}
Note that -c ${CONTAINER_NAME} is optional and can be omitted for pods that
only contain a single container.
As an example, to look at the logs from a running Cassandra pod, you might run:
$ kubectl exec cassandra -- cat /var/log/cassandra/system.log
If none of these approaches work, you can find the host machine that the pod is running on and SSH into that host.
Debugging Replication Controllers
Replication controllers are fairly straightforward. They can either create pods or they can’t. If they can’t create pods, then please refer to the instructions above to debug your pods.
You can also use kubectl describe rc ${CONTROLLER_NAME} to inspect events
related to the replication controller.