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k3s/docs/pods.md

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Pods

A pod (as in a pod of whales or pea pod) is a relatively tightly coupled group of containers that are scheduled onto the same host. It models an application-specific "virtual host" in a containerized environment. Pods serve as units of scheduling, deployment, and horizontal scaling/replication, and share fate.

Why doesn't Kubernetes just support an affinity mechanism for co-scheduling containers instead? While pods have a number of benefits (e.g., simplifying the scheduler), the primary motivation is resource sharing.

In addition to defining the containers that run in the pod, the pod specifies a set of shared storage volumes. Pods facilitate data sharing and IPC among their constituents. In the future, they may share CPU and/or memory (LPC2013).

The containers in the pod also all use the same network namespace/IP (and port space). The goal is for each pod to have an IP address in a flat shared networking namespace that has full communication with other physical computers and containers across the network. More details on networking.

While pods can be used to host vertically integrated application stacks, their primary motivation is to support co-located, co-managed helper programs, such as:

  • content management systems, file and data loaders, local cache managers, etc.
  • log and checkpoint backup, compression, rotation, snapshotting, etc.
  • data change watchers, log tailers, logging and monitoring adapters, event publishers, etc.
  • proxies, bridges, and adapters
  • controllers, managers, configurators, and updaters

Individual pods are not intended to run multiple instances of the same application, in general.

Why not just run multiple programs in a single Docker container?

  1. Transparency. Making the containers within the pod visible to the infrastructure enables the infrastructure to provide services to those containers, such as process management and resource monitoring. This facilitates a number of conveniences for users.
  2. Decoupling software dependencies. The individual containers may be rebuilt and redeployed independently. Kubernetes may even support live updates of individual containers someday.