Building a Continuous Deployment Pipeline Using LKE (Part 4): Kubernetes Review

Slide deck: Cloud Native Continuous Deployment with GitLab, Helm, and Linode Kubernetes Engine: Quick Kubernetes review (Slide #62)

Kubernetes Review

This is an optional part of the series that provides an introduction to Kubernetes. It covers deploying a simple application on Kubernetes and each of the components that are involved (such as pods, controllers, and services).

Navigate the Series

• Main guide: Building a Continuous Deployment Pipeline Using LKE
• Previous section: Part 3: Deploying the LKE Cluster
• Next section: Part 5: Accessing Internal Services

Presentation Text

Here’s a copy of the text contained within this section of the presentation. A link to the source file can be found within each slide of the presentation. Some formatting may have been changed.

Quick Kubernetes review

• Let’s deploy a simple HTTP server
• And expose it to the outside world!
• Feel free to skip this section if you’re familiar with Kubernetes

Creating a container

• On Kubernetes, one doesn’t simply run a container
• We need to create a “Pod”
• A Pod will be a group of containers running together (often, it will be a group of one container)
• We can create a standalone Pod, but generally, we’ll use a controller (for instance: Deployment, Replica Set, Daemon Set, Job, Stateful Set…)
• The controller will take care of scaling and recreating the Pod if needed (note that within a Pod, containers can also be restarted automatically if needed)

A controller, you said?

• We’re going to use one of the most common controllers: a Deployment
• Deployments…
  • can be scaled (will create the requested number of Pods)
  • will recreate Pods if e.g. they get evicted or their Node is down
• handle rolling updates
• Deployments actually delegate a lot of these tasks to Replica Sets
• We will generally have the following hierarchy: Deployment → Replica Set → Pod

Creating a Deployment

• Without further ado: kubectl create deployment web --image=nginx
• Check what happened: kubectl get all
• Wait until the NGINX Pod is “Running”!
• Note: kubectl create deployment is great when getting started…
• … But later, we will probably write YAML instead!

Exposing the Deployment

• We need to create a Service

• We can use kubectl expose for that (but, again, we will probably use YAML later!)

• For internal use, we can use the default Service type, ClusterIP:

  kubectl expose deployment web --port=80
  

• For external use, we can use a Service of type LoadBalancer:

  kubectl expose deployment web --port=80 --type=LoadBalancer
  

Changing the Service type

• We can kubectl delete service web and recreate it
• Or, kubectl edit service web and dive into the YAML
• Or, kubectl patch service web --patch '{"spec": {"type": "LoadBalancer"}}'
• … These are just a few “classic” methods; there are many ways to do this!

Deployment → Pod

• Can we check exactly what’s going on when the Pod is created?
• Option 1: watch kubectl get all
  • displays all object types
  • refreshes every 2 seconds
  • puts a high load on the API server when there are many objects
• Option 2: kubectl get pods --watch --output-watch-events
  • can only display one type of object
  • will show all modifications happening (à la tail -f)
  • doesn’t put a high load on the API server (except for initial display)

Recreating the Deployment

• Let’s delete our Deployment: kubectl delete deployment web
• Watch Pod updates: kubectl get pods --watch --output-watch-events
• Recreate the Deployment and see what Pods do: kubectl create deployment web --image=nginx

Service stability

• Our Service still works even though we deleted and re-created the Deployment
• It wouldn’t have worked while the Deployment was deleted, though
• A Service is a stable endpoint