Pods are the smallest deployable units of computing that you can create and manage in Kubernetes.

A Deployment provides declarative updates for Pods and ReplicaSets.

You describe a desired state in a Deployment, and the Deployment Controller changes the actual state to the desired state at a controlled rate. You can define Deployments to create new ReplicaSets, or to remove existing Deployments and adopt all their resources with new Deployments.

An abstract way to expose an application running on a set of Pods as a network service.

Types of Services: ClusterIP: Exposes the Service on a cluster-internal IP. Choosing this value makes the Service only reachable from within the cluster. This is the default ServiceType.

NodePort: Exposes the Service on each Node's IP at a static port (the NodePort). A ClusterIP Service, to which the NodePort Service routes, is automatically created. You'll be able to contact the NodePort Service, from outside the cluster, by requesting :.

LoadBalancer: Exposes the Service externally using a cloud provider's load balancer. NodePort and ClusterIP Services, to which the external load balancer routes, are automatically created.

ExternalName: Maps the Service to the contents of the externalName field (e.g. foo.bar.example.com), by returning a CNAME record with its value. No proxying of any kind is set up.

Reference: https://kubernetes.io/docs/concepts/services-networking/service/

A ReplicaSet's purpose is to maintain a stable set of replica Pods running at any given time. As such, it is often used to guarantee the availability of a specified number of identical Pods.

Sometimes you don't need load-balancing and a single Service IP. In this case, you can create what are termed "headless" Services, by explicitly specifying "None" for the cluster IP (.spec.clusterIP).

You can use a headless Service to interface with other service discovery mechanisms, without being tied to Kubernetes' implementation.

Control Plane Components

kube-apiserver:

The API server is a component of the Kubernetes control plane that exposes the Kubernetes API. The API server is the front end for the Kubernetes control plane.

The main implementation of a Kubernetes API server is kube-apiserver. kube-apiserver is designed to scale horizontally—that is, it scales by deploying more instances. You can run several instances of kube-apiserver and balance traffic between those instances.

etcd Consistent and highly-available key value store used as Kubernetes' backing store for all cluster data.

kube-scheduler: Control plane component that watches for newly created Pods with no assigned node, and selects a node for them to run on.

kube-controller-manager: Control plane component that runs controller processes.

Logically, each controller is a separate process, but to reduce complexity, they are all compiled into a single binary and run in a single process.

Some types of these controllers are:

Node controller: Responsible for noticing and responding when nodes go down. Job controller: Watches for Job objects that represent one-off tasks, then creates Pods to run those tasks to completion. Endpoints controller: Populates the Endpoints object (that is, joins Services & Pods). Service Account & Token controllers: Create default accounts and API access tokens for new namespaces.

cloud-controller-manager: A Kubernetes control plane component that embeds cloud-specific control logic. The cloud controller manager lets you link your cluster into your cloud provider's API, and separates out the components that interact with that cloud platform from components that only interact with your cluster. The cloud-controller-manager only runs controllers that are specific to your cloud provider. If you are running Kubernetes on your own premises, or in a learning environment inside your own PC, the cluster does not have a cloud controller manager.

Worker Node Components

kubelet An agent that runs on each node in the cluster. It makes sure that containers are running in a Pod.

kube-proxy kube-proxy is a network proxy that runs on each node in your cluster, implementing part of the Kubernetes Service concept.

kube-proxy maintains network rules on nodes. These network rules allow network communication to your Pods from network sessions inside or outside of your cluster.

Kubernetes API

The core of Kubernetes' control plane is the API server. The API server exposes an HTTP API that lets end users, different parts of your cluster, and external components communicate with one another.

The Kubernetes API lets you query and manipulate the state of API objects in Kubernetes (for example: Pods, Namespaces, ConfigMaps, and Events).

Most operations can be performed through the kubectl command-line interface or other command-line tools, such as kubeadm, which in turn use the API. However, you can also access the API directly using REST calls.

Containers are a form of operating system virtualization. A single container might be used to run anything from a small microservice or software process to a larger application. Inside a container are all the necessary executables, binary code, libraries, and configuration files. Compared to server or machine virtualization approaches, however, containers do not contain operating system images.

This makes them more lightweight and portable, with significantly less overhead. In larger application deployments, multiple containers may be deployed as one or more container clusters. Such clusters might be managed by a container orchestrator such as Kubernetes.

Reference : https://www.netapp.com/devops-solutions/what-are-containers/

• High level of Automation
• Self-healing
• Secure by default
• Cost-efficient
• Easy-to-maintain
• Scalable

The Twelve Factors

• I. Codebase: One codebase tracked in revision control, many deploys
• II. Dependencies: Explicitly declare and isolate dependencies
• III. Config: Store config in the environment
• IV. Backing services: Treat backing services as attached resources
• V. Build, release, run: Strictly separate build and run stages
• VI. Processes: Execute the app as one or more stateless processes
• VII. Port binding: Export services via port binding
• VIII. Concurrency: Scale out via the process model
• IX. Disposability: Maximize robustness with fast startup and graceful shutdown
• X. Dev/prod parity: Keep development, staging, and production as similar as possible
• XI. Logs: Treat logs as event streams
• XII. Admin processes: Run admin/management tasks as one-off processes

Reference: https://12factor.net/