Deployments and Releases¶

After an image is built, it can be scheduled on a cluster as a workload. An image is a static collection of application layers that have been packaged according to a standardized specification, but when an image is running, it is called a container.

Images only have information about themselves; they can't describe to the cluster how they should run. So when we are deploying an image to a cluster, we need to use configuration files to specify to the cluster how to run the workload. One of the most popular Kubernetes resources that specifies how to run an image is called a Deployment.

This page describes the general steps involved in creating a workload deployment, whether it's manually through the command line or using a CI/CD pipeline.

Definitions

Image: An application packaged in layers according to the Open Container Initiative (OCI) specification
Container: A running image
Deployment: A type of Kubernetes resource that specifies how to run an image on a cluster
Workload (also known as a Pod): A deployment that is scheduled and running on a cluster
Release: A deployment that receives public traffic

Deploying a Workload¶

When an image is scheduled on a Kubernetes cluster, configuration files define how the workload should run. Together, these files - and the image they refer to - are considered a deployment. When the deployment is scheduled and running on a cluster, it's called a workload.

deployment image

A typical deployment package has the following Kubernetes Resource Manifest (KRM) files:

ingress.yaml
service.yaml
deployment.yaml

Kubernetes resources called Deployments tell the cluster how to run an image, and resources like Services, and Ingresses define how the cluster should connect to the workload.

Beware! Confusing Terminology

The terms "deployment" and "deployment package" can be used to describe a couple of different things:

the process of getting the image to the cluster and configuring it to run (usually via a CI/CD pipeline)
a Kubernetes resource that specifies how an image should run
a workload

The context will help you determine which meaning is relevant.

Workload Configuration Files¶

Configuration files are YAML formatted and they define the types of Kubernetes resources to create. Although they can be named anything, the names in the example files correspond to the types of resources that are created.

deployment.yaml: A resource that defines how an image should be configured to run.
service.yaml: A resource that abstracts the networking connection details away from a workload to create a persistent, reliable endpoint.
ingress.yaml: A resource that creates an endpoint to receive traffic from outside the cluster and route it to an internal resource, like a workload.

An example Deployment Kubernetes resource manifest file

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - containerPort: 80