Kubernetes Pod and Container Concepts
External
- https://kubernetes.io/docs/concepts/workloads/pods/ (fully synced ✓)
Internal
Overview
A pod is the fundamental, atomic compute unit created and managed by Kubernetes. An application is deployed as one or more equivalent pods. There are various strategies to partition applications to pods. A pod groups together one or more containers. There are several types of containers: application containers, init containers and ephemeral containers. Pods are deployed on worker nodes. A pod has a well-defined lifecycle with several phases, and the pod's containers can only be in one of a well-defined number of states. Kubernetes learns of what happens with a container by container probes.
Pod
A pod is a group of one or more containers Kubernetes deploys and manages a compute unit, and the specification for how to run the containers. Kubernetes will not manage compute entities with smaller granularity, such as containers or processes. The containers of a pod are atomically deployed and managed as a group. A useful mental model when thinking of a pod is that of a logical host, where all its containers share a context.
Pod Operation Atomicity
Atomic Success or Failure
The deployment of a pod is an atomic operation. This means that a pod is either entirely deployed, with all its containers co-located on the same node, or not deployed at all. There will never be a situation where a partially deployed pod will be servicing application requests.
All Containers of a Pod are Scheduled on the Same Node
A pod can be scheduled on one node and one node only - regardless of many containers the pod has. All containers in the pod will be always co-located and co-scheduled on the same node. Only when all pod resources are ready the pod becomes available and application traffic is directed to it.
The containers in a pod share network resources and storage. From this perspective, a pod can be thought of as a logical host with all its processes (containers) sharing the network stack and the storage available to the host. The pod contains collocated containers that are relatively tightly coupled and run within the shared context provided by the pod. Within that context, an application may have individual cgroups isolation applied.
TODO HERE. A pod models an application-specific logical host, containing applications that in a pre-container world would have run on the same physical or virtual host, and in consequence, the pod cannot span hosts. The pod is the smallest unit that can be defined, deployed and managed by Kubernetes. Complex applications can be made of any number of pods, and Kubernetes helps with pod orchestration.
Pod Lifecycle
Pod Phases
Pods and Nodes
Pods and Containers
Container
TODO:
Container Types
Application Container
Init Container
Ephemeral Container
Container States
Container Probes
Summary of a relationship between container probe result and overall pod situation.