whentimeslows/website
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Revise Pod concept (#22603)

Browse Source 
* Revise Pod concept

Adapt the existing Pod documentation to suit the Docsy theme, by
promoting the Pod concept itself to /docs/concepts/workloads/pods/

Following on from this, update the Pod Lifecycle page to cover the
lifecycle of a Pod and follow on directly from the Pod concept,
for readers keen to understand things in detail.

This change also removes the automatic contents list from the Pod
overview page. Instead, the new page links to all the pages
inside the Pod section.

* Update links to Pod concept

Link to updated content

* Incorporate Pod concept suggestions

Co-authored-by: Celeste Horgan <celeste@cncf.io>

* Revise StatefulSet suggestion for Pod concept

Co-authored-by: Celeste Horgan <celeste@cncf.io>

Co-authored-by: Celeste Horgan <celeste@cncf.io>
This commit is contained in:
Tim Bannister
2020-07-27 04:18:16 +01:00
committed by GitHub
parent c80c9c40c1
commit 49eee8fd3d
31 changed files with 704 additions and 736 deletions
Download Patch File Download Diff File Expand all files Collapse all files
content/en/docs/concepts/workloads/pods/_index.md
Executable → Regular
+267 -1
View File
@@ -1,5 +1,271 @@
---
title: "Pods"
reviewers:
- erictune
title: Pods
content_type: concept
weight: 10
no_list: true
card:
name: concepts
weight: 60
---
<!-- overview -->
_Pods_ are the smallest deployable units of computing that you can create and manage in Kubernetes.
A _Pod_ (as in a pod of whales or pea pod) is a group of one or more
{{< glossary_tooltip text="containers" term_id="container" >}}, with shared storage/network resources, and a specification
for how to run the containers. A Pod's contents are always co-located and
co-scheduled, and run in a shared context. A Pod models an
application-specific "logical host": it contains one or more application
containers which are relatively tightly coupled.
In non-cloud contexts, applications executed on the same physical or virtual machine are analogous to cloud applications executed on the same logical host.
As well as application containers, a Pod can contain
[init containers](/docs/concepts/workloads/pods/init-containers/) that run
during Pod startup. You can also inject
[ephemeral containers](/docs/concepts/workloads/pods/ephemeral-containers/)
for debugging if your cluster offers this.
<!-- body -->
## What is a Pod?
{{< note >}}
While Kubernetes supports more
{{< glossary_tooltip text="container runtimes" term_id="container-runtime" >}}
than just Docker, [Docker](https://www.docker.com/) is the most commonly known
runtime, and it helps to describe Pods using some terminology from Docker.
{{< /note >}}
The shared context of a Pod is a set of Linux namespaces, cgroups, and
potentially other facets of isolation - the same things that isolate a Docker
container. Within a Pod's context, the individual applications may have
further sub-isolations applied.
In terms of Docker concepts, a Pod is similar to a group of Docker containers
with shared namespaces and shared filesystem volumes.
## Using Pods
Usually you don't need to create Pods directly, even singleton Pods. Instead, create them using workload resources such as {{< glossary_tooltip text="Deployment"
term_id="deployment" >}} or {{< glossary_tooltip text="Job" term_id="job" >}}.
If your Pods need to track state, consider the
{{< glossary_tooltip text="StatefulSet" term_id="statefulset" >}} resource.
Pods in a Kubernetes cluster are used in two main ways:
* **Pods that run a single container**. The "one-container-per-Pod" model is the
most common Kubernetes use case; in this case, you can think of a Pod as a
wrapper around a single container; Kubernetes manages Pods rather than managing
the containers directly.
* **Pods that run multiple containers that need to work together**. A Pod can
encapsulate an application composed of multiple co-located containers that are
tightly coupled and need to share resources. These co-located containers
form a single cohesive unit of service—for example, one container serving data
stored in a shared volume to the public, while a separate _sidecar_ container
refreshes or updates those files.
The Pod wraps these containers, storage resources, and an ephemeral network
identity together as a single unit.
{{< note >}}
Grouping multiple co-located and co-managed containers in a single Pod is a
relatively advanced use case. You should use this pattern only in specific
instances in which your containers are tightly coupled.
{{< /note >}}
Each Pod is meant to run a single instance of a given application. If you want to
scale your application horizontally (to provide more overall resources by running
more instances), you should use multiple Pods, one for each instance. In
Kubernetes, this is typically referred to as _replication_.
Replicated Pods are usually created and managed as a group by a workload resource
and its {{< glossary_tooltip text="controller" term_id="controller" >}}.
See [Pods and controllers](#pods-and-controllers) for more information on how
Kubernetes uses workload resources, and their controllers, to implement application
scaling and auto-healing.
### How Pods manage multiple containers
Pods are designed to support multiple cooperating processes (as containers) that form
a cohesive unit of service. The containers in a Pod are automatically co-located and
co-scheduled on the same physical or virtual machine in the cluster. The containers
can share resources and dependencies, communicate with one another, and coordinate
when and how they are terminated.
For example, you might have a container that
acts as a web server for files in a shared volume, and a separate "sidecar" container
that updates those files from a remote source, as in the following diagram:
{{< figure src="/images/docs/pod.svg" alt="example pod diagram" width="50%" >}}
Some Pods have {{< glossary_tooltip text="init containers" term_id="init-container" >}} as well as {{< glossary_tooltip text="app containers" term_id="app-container" >}}. Init containers run and complete before the app containers are started.
Pods natively provide two kinds of shared resources for their constituent containers:
[networking](#pod-networking) and [storage](#pod-storage).
## Working with Pods
You'll rarely create individual Pods directly in Kubernetes—even singleton Pods. This
is because Pods are designed as relatively ephemeral, disposable entities. When
a Pod gets created (directly by you, or indirectly by a
{{< glossary_tooltip text="controller" term_id="controller" >}}), the new Pod is
scheduled to run on a {{< glossary_tooltip term_id="node" >}} in your cluster.
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.
{{< note >}}
Restarting a container in a Pod should not be confused with restarting a Pod. A Pod
is not a process, but an environment for running container(s). A Pod persists until
it is deleted.
{{< /note >}}
When you create the manifest for a Pod object, make sure the name specified is a valid
[DNS subdomain name](/docs/concepts/overview/working-with-objects/names#dns-subdomain-names).
### Pods and controllers
You can use workload resources to create and manage multiple Pods for you. A controller
for the resource handles replication and rollout and automatic healing in case of
Pod failure. For example, if a Node fails, a controller notices that Pods on that
Node have stopped working and creates a replacement Pod. The scheduler places the
replacement Pod onto a healthy Node.
Here are some examples of workload resources that manage one or more Pods:
* {{< glossary_tooltip text="Deployment" term_id="deployment" >}}
* {{< glossary_tooltip text="StatefulSet" term_id="statefulset" >}}
* {{< glossary_tooltip text="DaemonSet" term_id="daemonset" >}}
### Pod templates
Controllers for {{< glossary_tooltip text="workload" term_id="workload" >}} resources create Pods
from a _pod template_ and manage those Pods on your behalf.
PodTemplates are specifications for creating Pods, and are included in workload resources such as
[Deployments](/docs/concepts/workloads/controllers/deployment/),
[Jobs](/docs/concepts/jobs/run-to-completion-finite-workloads/), and
[DaemonSets](/docs/concepts/workloads/controllers/daemonset/).
Each controller for a workload resource uses the `PodTemplate` inside the workload
object to make actual Pods. The `PodTemplate` is part of the desired state of whatever
workload resource you used to run your app.
The sample below is a manifest for a simple Job with a `template` that starts one
container. The container in that Pod prints a message then pauses.
```yaml
apiVersion: batch/v1
kind: Job
metadata:
name: hello
spec:
template:
# This is the pod template
spec:
containers:
- name: hello
image: busybox
command: ['sh', '-c', 'echo "Hello, Kubernetes!" && sleep 3600']
restartPolicy: OnFailure
# The pod template ends here
```
Modifying the pod template or switching to a new pod template has no effect on the
Pods that already exist. Pods do not receive template updates directly. Instead,
a new Pod is created to match the revised pod template.
For example, the deployment controller ensures that the running Pods match the current
pod template for each Deployment object. If the template is updated, the Deployment has
to remove the existing Pods and create new Pods based on the updated template. Each workload
resource implements its own rules for handling changes to the Pod template.
On Nodes, the {{< glossary_tooltip term_id="kubelet" text="kubelet" >}} does not
directly observe or manage any of the details around pod templates and updates; those
details are abstracted away. That abstraction and separation of concerns simplifies
system semantics, and makes it feasible to extend the cluster's behavior without
changing existing code.
## Resource sharing and communication
Pods enable data sharing and communication among their constituent
containters.
### Storage in Pods {#pod-storage}
A Pod can specify a set of shared storage
{{< glossary_tooltip text="volumes" term_id="volume" >}}. All containers
in the Pod can access the shared volumes, allowing those containers to
share data. Volumes also allow persistent data in a Pod to survive
in case one of the containers within needs to be restarted. See
[Storage](/docs/concepts/storage/) for more information on how
Kubernetes implements shared storage and makes it available to Pods.
### Pod networking
Each Pod is assigned a unique IP address for each address family. Every
container in a Pod shares the network namespace, including the IP address and
network ports. Inside a Pod (and **only** then), the containers that belong to the Pod
can communicate with one another using `localhost`. When containers in a Pod communicate
with entities *outside the Pod*,
they must coordinate how they use the shared network resources (such as ports).
Within a Pod, containers share an IP address and port space, and
can find each other via `localhost`. The containers in a Pod can also communicate
with each other using standard inter-process communications like SystemV semaphores
or POSIX shared memory. Containers in different Pods have distinct IP addresses
and can not communicate by IPC without
[special configuration](/docs/concepts/policy/pod-security-policy/).
Containers that want to interact with a container running in a different Pod can
use IP networking to comunicate.
Containers within the Pod see the system hostname as being the same as the configured
`name` for the Pod. There's more about this in the [networking](/docs/concepts/cluster-administration/networking/)
section.
## Privileged mode for containers
Any container in a Pod can enable privileged mode, using the `privileged` flag on the [security context](/docs/tasks/configure-pod-container/security-context/) of the container spec. This is useful for containers that want to use operating system administrative capabilities such as manipulating the network stack or accessing hardware devices.
Processes within a privileged container get almost the same privileges that are available to processes outside a container.
{{< note >}}
Your {{< glossary_tooltip text="container runtime" term_id="container-runtime" >}} must support the concept of a privileged container for this setting to be relevant.
{{< /note >}}
## Static Pods
_Static Pods_ are managed directly by the kubelet daemon on a specific node,
without the {{< glossary_tooltip text="API server" term_id="kube-apiserver" >}}
observing them.
Whereas most Pods are managed by the control plane (for example, a
{{< glossary_tooltip text="Deployment" term_id="deployment" >}}), for static
Pods, the kubelet directly supervises each static Pod (and restarts it if it fails).
Static Pods are always bound to one {{< glossary_tooltip term_id="kubelet" >}} on a specific node.
The main use for static Pods is to run a self-hosted control plane: in other words,
using the kubelet to supervise the individual [control plane components](/docs/concepts/overview/components/#control-plane-components).
The kubelet automatically tries to create a {{< glossary_tooltip text="mirror Pod" term_id="mirror-pod" >}}
on the Kubernetes API server for each static Pod.
This means that the Pods running on a node are visible on the API server,
but cannot be controlled from there.
## {{% heading "whatsnext" %}}
* Learn about the [lifecycle of a Pod](/docs/concepts/workloads/pods/pod-lifecycle/).
* Learn about [PodPresets](/docs/concepts/workloads/pods/podpreset/).
* Lean about [RuntimeClass](/docs/concepts/containers/runtime-class/) and how you can use it to
configure different Pods with different container runtime configurations.
* Read about [Pod topology spread constraints](/docs/concepts/workloads/pods/pod-topology-spread-constraints/).
* Read about [PodDisruptionBudget](https://kubernetes.io/docs/concepts/workloads/pods/disruptions/) and how you can use it to manage application availability during disruptions.
* Pod is a top-level resource in the Kubernetes REST API.
The [Pod](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#pod-v1-core)
object definition describes the object in detail.
* [The Distributed System Toolkit: Patterns for Composite Containers](https://kubernetes.io/blog/2015/06/the-distributed-system-toolkit-patterns) explains common layouts for Pods with more than one container.
To understand the context for why Kubernetes wraps a common Pod API in other resources (such as {{< glossary_tooltip text="StatefulSets" term_id="statefulset" >}} or {{< glossary_tooltip text="Deployments" term_id="deployment" >}}, you can read about the prior art, including:
* [Aurora](http://aurora.apache.org/documentation/latest/reference/configuration/#job-schema)
* [Borg](https://research.google.com/pubs/pub43438.html)
* [Marathon](https://mesosphere.github.io/marathon/docs/rest-api.html)
* [Omega](https://research.google/pubs/pub41684/)
* [Tupperware](https://engineering.fb.com/data-center-engineering/tupperware/).