Pods and Services - Core Concepts | Architecture

Pods
Services
Labels

Pods

OpenShift Enterprise leverages the Kubernetes concept of a pod, which is one or more containers deployed together on one host, and the smallest compute unit that can be defined, deployed, and managed.

Pods are the rough equivalent of OpenShift Enterprise v2 gears, with containers the rough equivalent of v2 cartridge instances. Each pod is allocated its own internal IP address, therefore owning its entire port space, and containers within pods can share their local storage and networking.

Pods have a lifecycle; they are defined, then they are assigned to run on a node, then they run until their container(s) exit or they are removed for some other reason. Pods, depending on policy and exit code, may be removed after exiting, or may be retained to enable access to the logs of their containers.

OpenShift Enterprise treats pods as largely immutable; changes cannot be made to a pod definition while it is running. OpenShift Enterprise implements changes by terminating an existing pod and recreating it with modified configuration, base image(s), or both. Pods are also treated as expendable, and do not maintain state when recreated. Therefore manage pods with higher-level controllers, rather than directly by users.

The following example definition of a pod provides a long-running service, which is actually a part of the OpenShift Enterprise infrastructure: the private Docker registry. It demonstrates many features of pods, most of which are discussed in other topics and thus only briefly mentioned here.

Pod object definition example

apiVersion: v1
kind: Pod
metadata:
  annotations: { ... }
  labels:                                (1)
    deployment: docker-registry-1
    deploymentconfig: docker-registry
    docker-registry: default
  generateName: docker-registry-1-       (2)
spec:
  containers:                            (3)
  - env:                                 (4)
    - name: OPENSHIFT_CA_DATA
      value: ...
    - name: OPENSHIFT_CERT_DATA
      value: ...
    - name: OPENSHIFT_INSECURE
      value: "false"
    - name: OPENSHIFT_KEY_DATA
      value: ...
    - name: OPENSHIFT_MASTER
      value: https://master.example.com:8443
    image: openshift/origin-docker-registry:v0.6.2 (5)
    imagePullPolicy: IfNotPresent
    name: registry
    ports:                              (6)
    - containerPort: 5000
      protocol: TCP
    resources: {}
    securityContext: { ... }            (7)
    volumeMounts:                       (8)
    - mountPath: /registry
      name: registry-storage
    - mountPath: /var/run/secrets/kubernetes.io/serviceaccount
      name: default-token-br6yz
      readOnly: true
  dnsPolicy: ClusterFirst
  imagePullsecrets:
  - name: default-dockercfg-at06w
  restartPolicy: Always
  serviceAccount: default               (9)
  volumes:                              (10)
  - emptyDir: {}
    name: registry-storage
  - name: default-token-br6yz
    secret:
      secretName: default-token-br6yz

1	Pods can be "tagged" with one or more labels, which can then be used to select and manage groups of pods in a single operation. The labels are stored in key-value format in the `metadata` hash. One label in this example is docker-registry=default.
2	Pods must have a unique name within their namespace. A pod definition may specify the basis of a name with the `generateName` attribute and random characters will be added automatically to generate a unique name.
3	`containers` specifies an array of container definitions; in this case (as with most), just one.
4	Environment variables can be specified to pass necessary values to each container.
5	Each container in the pod is instantiated from its own Docker image.
6	The container can bind to ports which will be made available on the pod’s IP.
7	OpenShift Enterprise defines a security context for containers which specifies whether they are allowed to run as privileged containers, run as a user of their choice, and more. The default context is very restrictive but administrators can modify this as needed.
8	The container specifies where external storage volumes should be mounted within the container. In this case, there is a volume for storing the registry’s data, and one for access to credentials the registry needs for making requests against the OpenShift Enterprise API.
9	Pods making requests against the OpenShift Enterprise API is a common enough pattern that there is a `serviceAccount` field for specifying which service account user the pod should authenticate as when making the requests. This enables fine-grained access control for custom infrastructure components.
10	The pod defines storage volumes that are available to its container(s) to use. In this case, it provides an ephemeral volume for the registry storage and a `secret` volume containing the service account credentials.

This pod definition does not include attributes that are filled by OpenShift Enterprise automatically after the pod is created and its lifecycle begins. The Kubernetes API documentation has complete details of the pod REST API object attributes, and the Kubernetes pod documentation has details about the functionality and purpose of pods.

Services

A Kubernetes service serves as an internal load balancer. It identifies a set of replicated pods in order to proxy the connections it receives to them. Backing pods can be added to or removed from a service arbitrarily while the service remains consistently available, enabling anything that depends on the service to refer to it at a consistent internal address.

Services are assigned an IP address and port pair that, when accessed, proxy to an appropriate backing pod. A service uses a label selector to find all the containers running that provide a certain network service on a certain port.

Like pods, services are REST objects. The following example shows the definition of a service for the pod defined above:

Service object definition example

apiVersion: v1
kind: Service
metadata:
  name: docker-registry      (1)
spec:
  selector:                  (2)
    docker-registry: default
  portalIP: 172.30.136.123   (3)
  ports:
  - nodePort: 0
    port: 5000               (4)
    protocol: TCP
    targetPort: 5000         (5)

1	The service name docker-registry is also used to construct an environment variable with the service IP that is inserted into other pods in the same namespace. The maximum name length is 63 characters.
2	The label selector identifies all pods with the docker-registry=default label attached as its backing pods.
3	Virtual IP of the service, allocated automatically at creation from a pool of internal IPs.
4	Port the service listens on.
5	Port on the backing pods to which the service forwards connections.

The Kubernetes service documentation has more information about services.

Labels

Labels are used to organize, group, or select API objects. For example, pods are "tagged" with labels, and then services use label selectors to identify the pods they proxy to. This makes it possible for services to reference groups of pods, even treating pods with potentially different Docker containers as related entities.

Most objects can include labels in their metadata. Labels can be used to group arbitrarily-related objects; for example, all of the pods, services, replication controllers, and deployment configurations of a particular application can be grouped.

Labels are simple key-value pairs, as in the following example:

Key-value pairs example

labels:
  key1: value1
  key2: value2

Consider:

A pod consisting of an nginx Docker container, with the label role=webserver.
A pod consisting of an Apache httpd Docker container, with the same label role=webserver.

A service or replication controller that is defined to use pods with the role=webserver label treats both of these pods as part of the same group.

The Kubernetes labels documentation has more information about labels.