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Troubleshoot Applications

This guide is to help users debug applications that are deployed into Kubernetes and not behaving correctly. This is not a guide for people who want to debug their cluster. For that you should check out this guide.

Diagnosing the problem

The first step in troubleshooting is triage. What is the problem? Is it your Pods, your Replication Controller or your Service?

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:

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:

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, but this should generally not be necessary given tools in the Kubernetes API. Therefore, if you find yourself needing to ssh into a machine, please file a feature request on GitHub describing your use case and why these tools are insufficient.

My pod is running but not doing what I told it to do

If your pod is not behaving as you expected, it may be that there was an error in your pod description (e.g. mypod.yaml file on your local machine), and that the error was silently ignored when you created the pod. Often a section of the pod description is nested incorrectly, or a key name is typed incorrectly, and so the key is ignored. For example, if you misspelled command as commnd then the pod will be created but will not use the command line you intended it to use.

The first thing to do is to delete your pod and try creating it again with the --validate option. For example, run kubectl create --validate -f mypod.yaml. If you misspelled command as commnd then will give an error like this:

I0805 10:43:25.129850   46757 schema.go:126] unknown field: commnd
I0805 10:43:25.129973   46757 schema.go:129] this may be a false alarm, see https://github.com/kubernetes/kubernetes/issues/6842
pods/mypod

The next thing to check is whether the pod on the apiserver matches the pod you meant to create (e.g. in a yaml file on your local machine). For example, run kubectl get pods/mypod -o yaml > mypod-on-apiserver.yaml and then manually compare the original pod description, mypod.yaml with the one you got back from apiserver, mypod-on-apiserver.yaml. There will typically be some lines on the “apiserver” version that are not on the original version. This is expected. However, if there are lines on the original that are not on the apiserver version, then this may indicate a problem with your pod spec.

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 introspect events related to the replication controller.

Debugging Services

Services provide load balancing across a set of pods. There are several common problems that can make Services not work properly. The following instructions should help debug Service problems.

First, verify that there are endpoints for the service. For every Service object, the apiserver makes an endpoints resource available.

You can view this resource with:

$ kubectl get endpoints ${SERVICE_NAME}

Make sure that the endpoints match up with the number of containers that you expect to be a member of your service. For example, if your Service is for an nginx container with 3 replicas, you would expect to see three different IP addresses in the Service’s endpoints.

My service is missing endpoints

If you are missing endpoints, try listing pods using the labels that Service uses. Imagine that you have a Service where the labels are:

...
spec:
  - selector:
     name: nginx
     type: frontend

You can use:

$ kubectl get pods --selector=name=nginx,type=frontend

to list pods that match this selector. Verify that the list matches the Pods that you expect to provide your Service.

If the list of pods matches expectations, but your endpoints are still empty, it’s possible that you don’t have the right ports exposed. If your service has a containerPort specified, but the Pods that are selected don’t have that port listed, then they won’t be added to the endpoints list.

Verify that the pod’s containerPort matches up with the Service’s containerPort

Network traffic is not forwarded

If you can connect to the service, but the connection is immediately dropped, and there are endpoints in the endpoints list, it’s likely that the proxy can’t contact your pods.

There are three things to check:

What's next

If none of the above solves your problem, follow the instructions in Debugging Service document to make sure that your Service is running, has Endpoints, and your Pods are actually serving; you have DNS working, iptables rules installed, and kube-proxy does not seem to be misbehaving.

You may also visit troubleshooting document for more information.

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