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. From a resource footprint perspective, a pod is bigger than a container, but smaller than a Virtual Machine. 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. A pod contains one or more application containers and zero or more init containers.
The equivalent Amazon ECS construct is the task.
Pod Manifest
A pod manifest or a workload resource manifest includes a pod template.
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 a virtual network device - a unique IP -, storage, in form of filesystem volumes and access to shared memory. From this perspective, a pod can be thought of as an application-specific logical host with all its processes (containers) sharing the network stack and the storage available to the host. In a pre-container world, these processes would have run on the same physical or virtual host. In line with this analogy, the pod cannot span hosts. The pod's containers are relatively tightly coupled and run within the shared context provided by the pod. The shared context of a pod is a set of Linux namespaces and cgroups. Within a pod's contexts, individual containers may have further sub-isolations applied.
Pods enable data sharing and communication among their constituent containers.
Storage
A pod can specify a set of shared storage volumes. All containers in the pod can access shared volumes, allowing them to share data. Volumes also allow persistent data in a pod to survive in case one of its containers needs to be restarted. For more details on pod storage see:
Networking
Security Context
Pods also define the security context for each of its containers.
Single-Container Pods vs. Multi-Container Pods
Pods are used in two main ways: pods that run a single container and pods that run multiple containers that work together.
The most common case is to declare a single container in a pod. In this case the pod is an extra wrapper around one container - Kubernetes manages the pod instead of managing the container directly. Even if a pod can accommodate multiple containers, the preferred way to scale an application is to add more one-container pods, instead of adding more containers in a pod.
There are advanced use cases - for example, service meshes - that require running multiple containers inside a pod. Containers share a pod when they execute tightly-coupled workloads, provide complementary functionality and need to share resources. Configuring two or more containers in the same pod guarantees that the containers will be run on the same node. Some commonly accepted use cases for collocated containers are service meshes and logging. A typical patter for which this arrangement is common is the sidecar pattern.
Each container of a multi-container pod can be exposed externally on its individual port. The containers share the pod's network namespace, thus the TCP and UDP port ranges.
Pod State
Pods should not maintain state, they should be handled as expendable. Kubernetes treats pods as static, largely immutable - changes cannot be made to a pod definition while the pod is running - and expendable, they do not maintain state when they are destroyed and recreated. Therefore, they are managed as workload resources backed by controllers, such as deployments or jobs, not directly by users, though pods can be started and managed individually, if the user wishes so. To modify a pod configuration, the current pod must be terminated, and a new one with a modified base image and/or configuration must be created.
In case the pods maintain state, Kubernetes provides a specialized workload resource names stateful set.
Pod Lifecycle
Pods are usually created by the controllers which manage workload resources, but they can also be created individually. A pod instance is created based on a pod template, which can exist by itself in a pod manifest or it can be a part of a workload resource manifest. Once created, the pods are scheduled to run on a node. Once scheduled on the node, the pod remains on that node until the pod finishes execution, the pod object is deleted, the pod is evicted for lack of resources or the node fails.
If the template a set of pods was created based on changes, the workload resource controller that created the pods detects the change and creates new pods while the old pods are deleted, rather than updating or patching the existing pods.
It is possible to manage pods directly, by updating some of the fields of a running pod, in place with kubectl patch or kubectl replace. However, updating the pods in place has limitations. Most of pod metadata (namespace, name, uid, creationTimestamp, etc.) is immutable. generation can only be incremented. More details on in-place pod updates are available here: Pod Update and Replacement.
Pod Phases
Pods and Nodes
Pods and Containers
A pod and its containers have independent lifecycles. A pod is not a process, but an environment for running containers. Containers can be restarted in a pod, but a pod is never restarted: if a pod is gone, it is never resurrected. In the best case, another quasi-identical pod is created to take its place.
Pod Security
Pod Horizontal Scaling
Every pod is meant to run a single instance of a given application. If the application needs to scale to sustain more load, multiple pods should be started. In Kubernetes, this is typically referred to as replication. The equivalent pod instances are referred to as replicas. They are usually created and managed as a group by a workload resource and its controller.
Pods and Workload Resources
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.