Pod: the Smallest Scheduling Unit in Kubernetes

Pod

A pod is the smallest and simplest unit in the Kubernetes object model that you create or deploy. A pod encapsulates one or more containers, storage volumes, a unique network IP address, and options that govern how the containers should run.

Pods can be used in either of the following ways:

• A container is running in a pod. This is the most common usage of pods in Kubernetes. You can view the pod as a single encapsulated container, but Kubernetes directly manages pods instead of containers.
• Multiple containers that need to be coupled and share resources run in a pod. In this scenario, an application contains a main container and several sidecar containers, as shown in Figure 1. For example, the main container is a web server that provides file services from a fixed directory, and a sidecar container periodically downloads files to the directory.
  Figure 1 Pod
  

In Kubernetes, pods are rarely created directly. Instead, controllers such as Deployments and jobs, are used to manage pods. Controllers can create and manage multiple pods, and provide replica management, rolling upgrade, and self-healing capabilities. A controller generally uses a pod template to create corresponding pods.

Creating a Pod

Kubernetes resources can be described using YAML or JSON files. The following example describes a pod named nginx. This pod contains a container named container-0 and uses the nginx:alpine image, 100m CPU, and 200 MiB memory.

apiVersion: v1                      # Kubernetes API version
kind: Pod                           # Kubernetes resource type
metadata:
  name: nginx                       # Pod name
spec:                               # Pod specifications
  containers:
  - image: nginx:alpine             # The image used is nginx:alpine.
    name: container-0               # Container name
    resources:                      # Resources required for a container
      limits:
        cpu: 100m
        memory: 200Mi
      requests:
        cpu: 100m
        memory: 200Mi
  imagePullSecrets:                 # Secret used to pull the image, which must be default-secret on CCE
  - name: default-secret

As shown in the annotation of YAML, the YAML description file includes:

• metadata: information such as name, label, and namespace
• spec: pod specification such as image and volume used

If you query a Kubernetes resource, you can see the status field. This field indicates the status of the Kubernetes resource, and does not need to be set when the resource is created. This example is a minimum set. Other parameter definition will be described later.

After the pod is defined, you can create it using kubectl. Assume that the preceding YAML file is named nginx.yaml, run the following command to create the file. -f indicates that it is created in the form of a file.

$ kubectl create -f nginx.yaml
pod/nginx created

Environment Variables

Environment variables are set in the container running environment.

Environment variables add flexibility to workload configuration. The environment variables for which you have assigned values during container creation will take effect when the container is running. This saves you the trouble of rebuilding the container image.

The following shows how to use an environment variable. You only need to configure the spec.containers.env field.

apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
    containers:
    - image: nginx:alpine
      name: container-0
      resources:
        limits:
          cpu: 100m
          memory: 200Mi
        requests:
          cpu: 100m
          memory: 200Mi
      env::                            # Environment variable
      - name: env_key
        value: env_value
    imagePullSecrets:
    - name: default-secret

Run the following command to check the environment variables in the container. The value of the env_key environment variable is env_value.

$ kubectl exec -it nginx -- env
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=nginx
TERM=xterm
env_key=env_value

Environment variables can also reference ConfigMap and secret. For details, see Referencing a ConfigMap as an Environment Variable and Referencing a Secret as an Environment Variable.

Setting Container Startup Commands

Starting a container is to start the main process. Some preparations must be made before the main process is started. For example, you may configure or initialize MySQL databases before running MySQL servers. You can set ENTRYPOINT or CMD in the Dockerfile when creating an image. As shown in the following example, the ENTRYPOINT ["top", "-b"] command is set in the Dockerfile. This command will be executed during container startup.

FROM ubuntu
ENTRYPOINT ["top", "-b"]

When calling an API, you only need to configure the containers.command field of the pod. This field is of the list type. The first parameter in the field is the command to be executed, and the subsequent parameters are the command arguments.

apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
  - image: nginx:alpine
    name: container-0
    resources:
      limits:
        cpu: 100m
        memory: 200Mi
      requests:
        cpu: 100m
        memory: 200Mi
    command:                     # Startup command
    - top
    - "-b"
  imagePullSecrets:
   - name: default-secret

Container Lifecycle

Kubernetes provides container lifecycle hooks. The hooks enable containers to run code triggered by events during their management lifecycle. For example, if you want a container to perform a certain operation before it is stopped, you can register a hook. The following lifecycle hooks are provided:

• postStart: triggered immediately after the workload is started
• preStop: triggered immediately before the workload is stopped

You only need to set the lifecycle.postStart or lifecycle.preStop parameter of the pod, as shown in the following:

apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
  - image: nginx:alpine
    name: container-0
    resources:
      limits:
        cpu: 100m
        memory: 200Mi
      requests:
        cpu: 100m
        memory: 200Mi
    lifecycle:
      postStart:                 # Post-start processing
        exec:
          command:
          - "/postStart.sh"
      preStop:                   # Pre-stop processing
        exec:
          command:
          - "/preStop.sh"
  imagePullSecrets:
   - name: default-secret