Merge pull request #1611 from devin-donnelly/concepts-root

Adds Concepts root page and starts to fill out the underlying section.
2017-01-18 15:25:04 -08:00
parent cd49c123cd b0092fb70d
commit b4eb739023
8 changed files with 233 additions and 7 deletions
@@ -2,9 +2,15 @@ bigheader: "Concepts"
 abstract: "Detailed explanations of Kubernetes system concepts and abstractions."
 toc:
 - docs/concepts/index.md
+
+- title: Kubernetes Objects
+  section:
+  - docs/concepts/abstractions/overview.md
+  - docs/concepts/abstractions/pod.md
+  - title: Controllers
+    section:
+    - docs/concepts/abstractions/controllers/statefulsets.md
+
 - title: Object Metadata
  section:
  - docs/concepts/object-metadata/annotations.md
- title: Controllers
-  section:
-  - docs/concepts/abstractions/controllers/statefulsets.md
@@ -0,0 +1,16 @@
+apiVersion: extensions/v1beta1
+kind: Deployment
+metadata:
+  name: nginx-deployment
+spec:
+  replicas: 3
+  template:
+    metadata:
+      labels:
+        app: nginx
+    spec:
+      containers:
+      - name: nginx
+        image: nginx:1.7.9
+        ports:
+        - containerPort: 80
@@ -0,0 +1,66 @@
+---
+title: Kubernetes Objects
+---
+
+{% capture overview %}
+This page explains how Kubernetes objects are represented in the Kubernetes API, and how you can express them in `.yaml` format.
+{% endcapture %}
+
+{% capture body %}
+### Understanding Kubernetes Objects
+
+*Kubernetes Objects* are persistent entities in the Kubernetes system. Kubenetes uses these entities to represent the state of your cluster. Specifically, they can describe:
+
+* What containerized applications are running (and on which nodes)
+* The resources available to those applications
+* The policies around how those applications behave, such as restart policies, upgrades, and fault-tolerance
+
+A Kubernetes object is a "record of intent"--once you create the object, the Kubernetes system will constantly work to ensure that that object exists. By creating an object, you're effectively telling the Kubernetes system what you want your cluster's workload to look like; this is your cluster's **desired state**.
+
+To work with Kubernetes objects--whether to create, modify, or delete them--you'll need to use the [Kubernetes API](https://github.com/kubernetes/kubernetes/blob/master/docs/devel/api-conventions.md). When you use the `kubectl` comamnd-line interface, for example, the CLI makes the necessary Kubernetes API calls for you; you can also use the Kubernetes API directly in your own programs. Kubernetes currently provides a `golang` [client library](https://github.com/kubernetes/client-go) for this purpose, and other language libraries (such as [Python](https://github.com/kubernetes-incubator/client-python)) are being developed.
+
+#### Object Spec and Status
+
+Every Kubernetes object includes two nested object fields that govern the object's configuration: the object *spec* and the object *status*. The *spec*, which you must provide, describes your *desired state* for the object--the characteristics that you want the object to have. The *status* describes the *actual state* for the object, and is supplied and updated by the Kubernetes system. At any given time, the Kubernetes Control Plane actively manages an object's actual state to match the desired state you supplied.
+
+For example, a Kubernetes Deployment is an object that can represent an application running on your cluster. When you create the Deployment, you might set the Deployment spec to specify that you want three replicas of the application to be running. The Kubernetes system reads the Deployment spec and starts three instances of your desired application--updating the status to match your spec. If any of those instances should fail (a status change), the Kubernetes system responds to the difference between spec and status by making a correction--in this case, starting a replacement instance.
+
+For more information on the object spec, status, and metadata, see the [Kubernetes API Conventions](https://github.com/kubernetes/kubernetes/blob/master/docs/devel/api-conventions.md#spec-and-status).
+
+#### Describing a Kubernetes Object
+
+When you create an object in Kubernetes, you must provide the object spec that describes its desired state, as well as some basic information about the object (such as a name). When you use the Kubernetes API to create the object (either directly or via `kubectl`), that API request must include that information as JSON in the request body. **Most often, you provide the information to `kubectl` in a .yaml file.** `kubectl` converts the information to JSON when making the API request.
+
+Here's an example `.yaml` file that shows the required fields and object spec for a Kubernetes Deployment:
+
+{% include code.html language="yaml" file="nginx-deployment.yaml" ghlink="/docs/concepts/abstractions/nginx-deployment.yaml" %}
+
+One way to create a Deployment using a `.yaml` file like the one above is to use the []`kubectl create`]() command in the `kubectl` command-line interface, passing the `.yaml` file as an argument. Here's an example:
+
+```shell
+$ kubectl create -f docs/user-guide/nginx-deployment.yaml --record
+```
+
+The output is similar to this:
+
+```shell
+deployment "nginx-deployment" created
+```
+
+#### Required Fields
+
+In the `.yaml` file for the Kubernetes object you want to create, you'll need to set values for the following fields:
+
+* `apiVersion` - Which version of the Kubernetes API you're using to create this object
+* `kind` - What kind of object you want to create
+* `metadata` - Data that helps uniquely identify the object, including a `name` string, UID, and optional `namespace`
+
+You'll also need to provide the object `spec` field. The precise format of the object `spec` is different for every Kubernetes object, and contains nested fields specific to that object. The [Kubernetes API reference](/docs/api/) can help you find the spec format for all of the objects you can create using Kubernetes.
+
+{% endcapture %}
+
+{% capture whatsnext %}
+* Learn about the most important basic Kubernetes objects, such as [Pod](/docs/concepts/abstractions/pod/).
+{% endcapture %}
+
+{% include templates/concept.md %}
@@ -0,0 +1,15 @@
+---
+---
+
+#### <a name="pod-termination"></a> Pod Termination
+
+Since Pods represent processes running on your cluster, Kubernetes provides for *graceful termination* when Pods are no longer needed. Kubernetes implements graceful termination by applying a default *grace period* of 30 seconds from the time that you issue a termination request. A typical Pod termination in Kubernetes involves the following steps:
+
+1. You send a command or API call to terminate the Pod.
+1. Kubernetes updates the Pod status to reflect the time after which the Pod is to be considered "dead" (the time of the termination request plus the grace period).
+1. Kubernetes marks the Pod state as "Terminating" and stops sending traffic to the Pod.
+1. Kubernetes send a `TERM` signal to the Pod, indicating that the Pod should shut down.
+1. When the grace period expires, Kubernetes issues a `SIGKILL` to any processes still running in the Pod.
+1. Kubernetes removes the Pod from the API server on the Kubernetes Master.
+
+> **Note:** The grace period is configurable; you can set your own grace period when interacting with the cluster to request termination, such as using the `kubectl delete` command. See the [Terminating a Pod]() tutorial for more information.
@@ -0,0 +1,78 @@
+---
+title: Pods
+---
+
+{% capture overview %}
+This page provides an overview of `Pod`, the smallest deployable object in the Kubernetes object model.
+{% endcapture %}
+
+{:toc}
+
+{% capture body %}
+### Understanding Pods
+
+A *Pod* is the basic building block of Kubernetes--the smallest and simplest unit in the Kubernetes object model that you create or deploy. A Pod represents a running process on your cluster.
+
+A Pod encapsulates an application container (or, in some cases, multiple containers), storage resources, a unique network IP, and options that govern how the container(s) should run. A Pod represents a unit of deployment: *a single instance of an application in Kubernetes*, which might consist of either a single container or a small number of containers that are tightly coupled and that share resources.
+
+> [Docker](https://www.docker.com) is the most common container runtime used in a Kubernetes Pod, but Pods support other container runtimes as well.
+
+Pods are employed a number of ways in a Kubernetes cluster, including:
+
+* **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, and Kubernetes manages the Pods rather than the containers directly.
+* **Pods that run multiple containers that need to work together**. A Pod might encapsulate an application composed multiple co-located containers that are tightly coupled and need to share resources. These co-located containers might form a single cohesive unit of service--one container serving files from a shared volume to the public, while a separate "sidecar" container refreshes or updates those files. The Pod wraps these containers and storage resources together as a single managable entity.
+
+The [Kubernetes Blog](http://blog.kubernetes.io) has some additional information on Pod use cases. For more information, see:
+
+* [The Distributed System Toolkit: Patterns for Composite Containers](http://blog.kubernetes.io/2015/06/the-distributed-system-toolkit-patterns.html)
+* [Container Design Patterns](http://blog.kubernetes.io/2016/06/container-design-patterns.html)
+
+Each Pod is meant to run a single instance of a given application. If you want to scale your application horizontally (e.g., run muliple instances), you should use multiple Pods, one for each instance. In Kubernetes, this is generally referred to as _replication_. Replicated Pods are usually created and managed as a group by an abstraction called a Controller. See [Pods and Controllers](#pods-and-controllers) for more information.
+
+#### 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 phyiscal 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.
+
+Note that 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. 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:
+
+![pod diagram](/images/docs/pod.svg){: style="max-width: 50%" }
+
+Pods provide two kinds of shared resources for their constituent containers: *networking* and *storage*.
+
+##### Networking
+
+Each Pod is assigned a unique IP address. Every the container in a Pod shares the network namespace, including the IP address and network ports. Containers *inside a 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).
+
+##### Storage
+
+A Pod can specify a set of shared storage *volumes*. 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 Volumes for more information on how Kubernetes implements shared storage in a Pod.
+
+### 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 Controller), it is scheduled to run on a Node in your your cluster. The Pod remains on that Node until the process is terminated, the pod object is deleted, or 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 the Pod. The Pod itself does not run, but is an environment the containers run in and persists until it is deleted.
+
+Pods do not, by themselves, self-heal. If a Pod is scheduled to a Node that fails, or if the scheduling operation itself fails, the Pod is deleted; likewise, a Pod won't survive an eviction due to a lack of resources or Node maintenance. Kubernetes uses a higher-level abstraction, called a *Controller*, that handles the work of managing the relatively disposable Pod instances. Thus, while it is possible to use Pod directly, it's far more common in Kubernetes to manage your pods using a Controller. See [Pods and Controllers](#pods-and-controllers) for more information on how Kubernetes uses Controllers to implement Pod scaling and healing.
+
+#### Pods and Controllers
+
+A Controller can create and manage multiple Pods for you, handling replication and rollout and providing self-healing capabilities at cluster scope. For example, if a Node fails, the Controller might automatically replace the Pod by scheduling an identical replacement on a different Node). 
+
+Some examples of Controllers that contain one or more pods include:
+
+* Deployment
+* [StatefulSet](/docs/concepts/abstractions/controllers/statefulsets/)
+* DaemonSet
+
+In general, Controllers use a Pod Template that you provide to create the Pods for which it is responsible.
+
+{% endcapture %}
+
+{% capture whatsnext %}
+* Learn more about Pod behavior:
+  * Pod Termination
+  * Other Pod Topics
+{% endcapture %}
+
+{% include templates/concept.md %}
@@ -2,16 +2,59 @@
 title: Concepts
 ---

-The Concepts section of the Kubernetes documentation is a work in progress.
+The Concepts section helps you learn about the parts of the Kubernetes system and the abstractions Kubernetes uses to represent your cluster, and helps you obtain a deeper understanding of how Kubernetes works.
+
+## Overview
+
+To work with Kubernetes, you use *Kubernetes API objects* to describe your cluster's *desired state*: what applications or other workloads you want to run, what container images they use, the number of replicas, what network and disk resources you want to make available, and more. You set your desired state by creating objects using the Kubernetes API, typically via the command-line interface, `kubectl`. You can also use the Kubernetes API directly to interact with the cluster and set or modify your desired state.
+
+Once you've set your desired state, the *Kubernetes Control Plane* works to make the cluster's current state match the desired state. To do so, Kuberentes performs a variety of tasks automatically--such as starting or restarting containers, scaling the number of replicas of a given application, and more. The Kubernetes Control Plane consists of a collection processes running on your cluster: 
+
+* The **Kubernetes Master** is a collection of four processes that run on a single node in your cluster, which is designated as the master node.
+* Each individual non-master node in your cluster runs two processes:
+  * **kubelet**, which communicates with the Kubernetes Master.
+  * **kube-proxy**, a network proxy which reflects Kubernetes networking services on each node.
+
+## Kubernetes Objects
+
+Kubernetes contains a number of abstractions that represent your the state of your system: deployed containerized applications and workloads, their associated network and disk resources, and other information about what your cluster is doing. These abstractions are represented by objects in the Kubernetes API; see the [Kubernetes Objects overview](/docs/concepts/abstractions/overview/) for more details. 
+
+The basic Kubernetes objects include:
+
+* [Pod](/docs/concepts/abstractions/pod/)
+* Service
+* Volume
+* Namespace
+
+In addition, Kubernetes contains a number of higher-level abstractions called Controllers. Controllers build upon the basic objects, and provide additional functionality and convenience features. They include:
+
+* ReplicaSet
+* Deployment
+* [StatefulSet](/docs/concepts/abstractions/controllers/statefulsets/)
+* DaemonSet
+* Job
+
+## Kubernetes Control Plane
+
+The various parts of the Kubernetes Control Plane, such as the Kubernetes Master and kubelet processes, govern how Kubernetes communicates with your cluster. The Control Plane maintains a record of all of the Kubernetes Objects in the system, and runs continuous control loops to manage those objects' state. At any given time, the Control Plane's control loops will respond to changes in the cluster and work to make the actual state of all the objects in the system to the desired state that you provided.
+
+For example, when you use the Kubernetes API to create a Deployment object, you provide a new desired state for the system. The Kubernetes Control Plane records that object creation, and carries out your instructions by starting the required applications and scheduling them to cluster nodes--thus making the cluster's actual state match the desired state.
+
+### Kubernetes Master
+
+The Kubernetes master is responsible for maintaining the desired state for your cluster. When you interact with Kubernetes, such as by using the `kubectl` command-line interface, you're communicating with your cluster's Kubernetes master.
+
+> The "master" refers to a collection of processes managing the cluster state.  Typically these processes are all run on a single node in the cluster, and this node is also referred to as the master. The master can also be replicated for availability and redundnacy.
+
+### Kubernetes Nodes
+
+The nodes in a cluster are the machines (VMs, physical servers, etc) that run your applications and cloud workflows. The Kubernetes master controls each node; you'll rarely interact with nodes directly.

 #### Object Metadata


 * [Annotations](/docs/concepts/object-metadata/annotations/)

-#### Controllers
-* [StatefulSets](/docs/concepts/abstractions/controllers/statefulsets/)
-

 ### What's next

@@ -9,3 +9,4 @@ docs/api-reference/v1/definitions.md
 docs/api-reference/v1/operations.md
 docs/user-guide/pods/_viewing-a-pod.md
 docs/user-guide/simple-yaml.md
+docs/concepts/abstractions/pod-termination.md
@@ -8,3 +8,4 @@ docs/user-guide/walkthrough/k8s201.md
 docs/user-guide/logging-demo/README.md
 docs/user-guide/downward-api/README.md
 docs/user-guide/configmap/README.md
+docs/concepts/abstractions/pod-termination.md