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11_debugging_containers.md

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Debugging Containers

Video Walkthrough Thumbnail

Accessing Local Logs

Logs of a running pod can be accessed from the Web Console or from the oc cli:

  • The Logs tab of any running pod can be used to view active logs for the current pod

  • The oc command can be used to view or tail the logs:
oc -n [-dev] logs -f <pod name>

If there is more than one container in a given pod, the -c <container-name> switch is used to specify the desired container logs.

Using a Debug Container

Objective: Create a problem with our app to start debugging: In this lab, we will test the communication between our pods. A debug pod creates a new version of your pod, but without running the usual init: command. This can allow for troubleshooting of issues during the pod's startup. The debug pod also allows you to run commands from within your pod, which we can use to troubleshoot. In this section, we'll shut down our rocketchat pods, and then try to ping them from our database debug pod.

  • Find the name of your mongodb pod that's running

    oc -n [-dev] get pods

  • In a new terminal window, start a debug pod for your mongodb deployment.

    oc -n [-dev] debug [mongodb-pod-name]

  • In openshift, local hostnames follow this format

[service-name].[namespace].svc.cluster.local

You can also find the hostname listed in the web console in the Adminstrator->Networking->Services menu.

  • Curl the rocketchat service from your mongodb debug pod. Our rocketchat service has the name rocketchat-[username]

    curl [servicename].[-dev].svc.cluster.local:3000

  • From our original terminal window (not the debug pod), let's scale down the rocketchat deployment:

    oc -n [-dev] scale deployment/rocketchat-[username] --replicas=0

  • Now switching terminals again, to the mongdb debug pod, we'll try to curl the rokcetchat service:

    curl [servicename].[-dev].svc.cluster.local:3000

  • As expected, since we shut down the rocketchat deployment, we get an error when trying to connect to it. If we were experiencing a problem with our pods in OpenShift, we could use this method to test that they can communicate properly.

  • In your debug pod terminal and run the exit command. This will remove the debug pod.

    sh-4.2$ exit
exit

Removing debug pod ...

RSH and RSYNC

RSH (Remote SHell) is available to all normal pods through the web console under the Terminal tab, as well as through the oc rsh command. This allows you to remotely execute commands from the pod. For more information, you can view the RedHat documentation

  • With your choice of access, rsh into one of the application pods and test access within the namespace. First, get a list of pods in the dev namespace.
oc -n [dev] get pods
  • Take note of the name of your currently running, ready rocketchat pod. Then, use rsh command to start a remote shell from the pod using your pod's name from the previous step's output.
oc -n [dev] rsh [podname]
  • Explore your userid
 whoami
  • This command identifies the userid of your rsh session. You can then try other commands to explore the pod.
  • Now we'll try some other commands from the pod's shell. Try the 'print working directory' command to see the path of the directory you're currently in.
pwd
  • Get a list of files in the directory
ls
  • Let's use the client URL command (cURL) to see if our pod can connect to external and internal resources. Let's test first test if our pod can get data from Google.
curl -L http://www.google.com

The output may look jumbled as we're seeing a html file as plain text rather than being displayed graphically as it would in a web browser.

Now let's try connecting to an internal resource from the pod. We'll get a similar html output here.

curl -L localhost:3000

Exit the rsh session.

exit

Remote Sync(RSYNC) is also available in many pods, available through the oc rsync command and can be used to get files from the pod, or to move them from your local to the pod. To get help with this command, you can use oc rsync -h. We're going to use it to download the data file from the mongodb pod.

  • Find the name of your running, ready mongodb pod
oc -n [-dev] get pods
  • We want to use rsync to get the mongodb data files, and put them onto our local machine. The mongo pod will store files in the directory var/lib/mongodb/data. First, I'll make a new local directory called 'lab'.
mkdir lab

To confirm the directory exists, I'll switch to it with

cd lab

Then, check that this new folder is empty with

ls

Then check the path to this directory with

pwd

Next, let's synchronise this new empty folder with the data folder on our mongodb pod. For me, this local path is /users/matt/lab. Add the podname and path to your lab folder in the command below.

oc -n [-dev] rsync [mongopodname]:/var/lib/mongodb/data [localpath]

Now let's confirm that these files have been copied locally. Let's get a list of folders in the current directory.

ls

Notice the new data folder. Let's switch to it.

cd data

Let's list the files in this folder.

ls

Note that the data files from your mongo pod have been copied locally.

Port Forwarding

The oc port-forward command enables users to forward remote ports running in the cluster into a local development machine.

  • Find your pod and use the port forward command
oc -n [-dev] get pods  | grep rocketchat-[username]
oc -n [-dev] port-forward [pod name from above] 8000:3000

Next page - Logging and Visualizations