container.training/slides/k8s/logs-cli.md

Accessing logs from the CLI

• The kubectl logs commands has limitations:

  • it cannot stream logs from multiple pods at a time

  • when showing logs from multiple pods, it mixes them all together

• We are going to see how to do it better

---

Doing it manually

• We could (if we were so inclined), write a program or script that would:

  • take a selector as an argument

  • enumerate all pods matching that selector (with kubectl get -l ...)

  • fork one kubectl logs --follow ... command per container

  • annotate the logs (the output of each kubectl logs ... process) with their origin

  • preserve ordering by using kubectl logs --timestamps ... and merge the output

--

• We could do it, but thankfully, others did it for us already!

---

Stern

Stern is an open source project by Wercker.

From the README:

Stern allows you to tail multiple pods on Kubernetes and multiple containers within the pod. Each result is color coded for quicker debugging.

The query is a regular expression so the pod name can easily be filtered and you don't need to specify the exact id (for instance omitting the deployment id). If a pod is deleted it gets removed from tail and if a new pod is added it automatically gets tailed.

Exactly what we need!

---

Installing Stern

• Run stern (without arguments) to check if it's installed:

  $ stern
  Tail multiple pods and containers from Kubernetes
  
  Usage:
    stern pod-query [flags]
  

• If it is not installed, the easiest method is to download a binary release

• The following commands will install Stern on a Linux Intel 64 bits machine:

  sudo curl -L -o /usr/local/bin/stern \
       https://github.com/wercker/stern/releases/download/1.8.0/stern_linux_amd64
  sudo chmod +x /usr/local/bin/stern
  

---

Using Stern

• There are two ways to specify the pods for which we want to see the logs:

  • -l followed by a selector expression (like with many kubectl commands)

  • with a "pod query", i.e. a regex used to match pod names

• These two ways can be combined if necessary

.exercise[

• View the logs for all the rng containers:

  stern rng
  

]

---

Stern convenient options

• The --tail N flag shows the last N lines for each container

  (Instead of showing the logs since the creation of the container)

• The -t / --timestamps flag shows timestamps

• The --all-namespaces flag is self-explanatory

.exercise[

• View what's up with the weave system containers:

  stern --tail 1 --timestamps --all-namespaces weave
  

]

---

Using Stern with a selector

• When specifying a selector, we can omit the value for a label

• This will match all objects having that label (regardless of the value)

• Everything created with kubectl run has a label run

• We can use that property to view the logs of all the pods created with kubectl run

.exercise[

• View the logs for all the things started with kubectl run:

  stern -l run
  

]