Jump right into Kubernetes development with career-focused lessons and hands-on practice.

DevOps for Developers

Kubernetes is a powerful container management tool that's taking the world by storm. This detailed course will help you master it.

In this course, you'll start with the fundamentals of Kubernetes and what the main components of a cluster look like. You'll then learn how to use those components to build, test, deploy, and upgrade applications, as well as how to achieve state persistence once your application is deployed. You'll also learn how to secure your deployments and manage resources, which are crucial DevOps skills.

By the time you're done, you'll have a firm grasp on what Kubernetes is, as well as the skills to deploy your own clusters and applications with confidence.

A Practical Guide to Kubernetes

# Step-by-step Pod scheduling 
Let's discuss some of the details of Kubernetes components and understand how Pod scheduling works.

**Three** major components are involved in the process:

## 1. API server
The ***API server*** is the central component of a Kubernetes cluster, and it runs on the primary node. Since we are using `k3d`, both primary and worker nodes are baked into the same virtual machine. However, a more serious Kubernetes cluster should separate the two on different hosts.

All other components interact with the API server and keep watch for changes. Most of the coordination in Kubernetes consists of a component writing to the API Server resource that another component is watching. The second component will then react to changes almost immediately.
## 2. Scheduler
The ***scheduler*** is also running on the primary node. Its job is to watch for unassigned Pods and assign them to a node with available resources (CPU and memory) matching Pod requirements. Since we're running a single-node cluster, specifying resources would not provide much insight into their usage, so we'll leave them for later.
## 3. Kubelet
***Kubelet*** runs on each node. Its primary function is to make sure that assigned Pods are running on the node. It watches for any new Pod assignments for the node. If a Pod is assigned to the node that Kubelet is running on, it will pull the Pod definition and use it to create containers through Docker or any other supported container engine.
# Sequential breakdown of events

The sequence of events with the `kubectl create -f db.yml` command is as follows:

1. The Kubernetes client (`kubectl`) sends a request to the API server, requesting creation of a Pod defined in the `db.yml` file.

2. Since the scheduler is watching the API server for new events, it detects that there is an unassigned Pod.
3. The scheduler decides which node to assign the Pod to and sends that information to the API server.
4. Kubelet also checks the API server. It detects that the Pod was assigned to the node it is running on.
5. Kubelet sends a request to Docker requesting the creation of the containers that form the Pod. In our case, the Pod defines a single container based on the `mongo` image.
6. Finally, Kubelet sends a request to the API server, notifying it that the Pod is created successfully.

The process might not make much sense right now because we're running a single-node cluster. If we had more VMs, scheduling might have happened somewhere else, and the complexity of the process would be easier to grasp. We'll get there in due time.


The following illustration shows a Pod scheduling sequence.

Learn about the different stages involved in Pod's creation. 

Components and Stages Involved in Pod Scheduling

Learn about the different stages involved in Pod's creation.

Components and Stages involved in a Pod's Scheduling

Step-by-step Pod scheduling