Run Stateless AP Replication Controller
- Creating a replication controller
- Updating replication controller pods
- Resizing a replication controller
- Viewing replication controllers
- Deleting replication controllers
A replication controller ensures that a specified number of pod “replicas” are running at any one time. If there are too many, it will kill some. If there are too few, it will start more.
Creating a replication controller
Replication controllers are created with kubectl create:
$ kubectl create -f FILE
Where:
-f FILEor--filename FILEis a relative path to a configuration file in either JSON or YAML format.
You can use the sample file below to try a create request.
A successful create request returns the name of the replication controller. To view more details about the controller, see Viewing replication controllers below.
Replication controller configuration file
When creating a replication controller, you must point to a configuration file
as the value of the -f flag. The configuration
file can be formatted as YAML or as JSON, and supports the following fields:
{
"apiVersion": "v1",
"kind": "ReplicationController",
"metadata": {
"name": "",
"labels": "",
"namespace": ""
},
"spec": {
"replicas": int,
"selector": {
"":""
},
"template": {
"metadata": {
"labels": {
"":""
}
},
"spec": {
// See 'The spec schema' below
}
}
}
}
Required fields are:
kind: AlwaysReplicationController.apiVersion: Currentlyv1.metadata: An object containing:name: Required ifgenerateNameis not specified. The name of this replication controller. It must be an RFC1035 compatible value and be unique within the namespace.labels: Optional. Labels are arbitrary key:value pairs that can be used for grouping and targeting by other resources and services.generateName: Required ifnameis not set. A prefix to use to generate a unique name. Has the same validation rules asname.namespace: Optional. The namespace of the replication controller.annotations: Optional. A map of string keys and values that can be used by external tooling to store and retrieve arbitrary metadata about objects.
spec: The configuration for this replication controller. It must contain:replicas: The number of pods to create and maintain.selector: A map of key:value pairs assigned to the set of pods that this replication controller is responsible for managing. This must match the key:value pairs in thetemplate’slabelsfield.templatecontains:- A
metadataobject withlabelsfor the pod. - The
specschema that defines the pod configuration.
- A
The spec schema
The spec schema (that is a child of template) is described in the locations
below:
- The
specschema section of the Creating Multi-Container Pods page covers required and frequently-used fields. - The entire
specschema is documented in the Kubernetes API reference.
Sample file
The following sample file creates 2 pods, each containing a single container
using the redis image. Port 80 on each container is opened. The replication
controller is tagged with the serving label. The pods are given the label
frontend and the selector is set to frontend, to indicate that the
controller should manage pods with the frontend label.
{
"kind": "ReplicationController",
"apiVersion": "v1",
"metadata": {
"name": "frontend-controller",
"labels": {
"state": "serving"
}
},
"spec": {
"replicas": 2,
"selector": {
"app": "frontend"
},
"template": {
"metadata": {
"labels": {
"app": "frontend"
}
},
"spec": {
"volumes": null,
"containers": [
{
"name": "php-redis",
"image": "redis",
"ports": [
{
"containerPort": 80,
"protocol": "TCP"
}
],
"imagePullPolicy": "IfNotPresent"
}
],
"restartPolicy": "Always",
"dnsPolicy": "ClusterFirst"
}
}
}
}
Updating replication controller pods
See Rolling Updates.
Resizing a replication controller
To increase or decrease the number of pods under a replication controller’s
control, use the kubectl scale command:
$ kubectl scale rc NAME --replicas=COUNT \
[--current-replicas=COUNT] \
[--resource-version=VERSION]
Tip: You can use the rc alias in your commands in place of
replicationcontroller.
Required fields are:
NAME: The name of the replication controller to update.--replicas=COUNT: The desired number of replicas.
Optional fields are:
--current-replicas=COUNT: A precondition for current size. If specified, the resize will only take place if the current number of replicas matches this value.-
--resource-version=VERSION: A precondition for resource version. If specified, the resize will only take place if the current replication controller version matches this value. Versions are specified in thelabelsfield of the replication controller’s configuration file, as a key:value pair with a key ofversion. For example,--resource-version testmatches:"labels": { "version": "test" }
Viewing replication controllers
To list replication controllers on a cluster, use the kubectl get command:
$ kubectl get rc
A successful get command returns all replication controllers on the cluster in the specified or default namespace. For example:
NAME DESIRED CURRENT READY AGE
frontend 2 2 2 1h
You can also use get rc NAME to return information about a specific
replication controller.
To view detailed information about a specific replication controller, use the
kubectl describe command:
$ kubectl describe rc NAME
A successful describe request returns details about the replication controller including number and status of pods managed, and recent events:
Name: frontend
Namespace: default
Image(s): gcr.io/google_samples/gb-frontend:v3
Selector: name=frontend
Labels: name=frontend
Replicas: 2 current / 2 desired
Pods Status: 2 Running / 0 Waiting / 0 Succeeded / 0 Failed
No volumes.
Events:
FirstSeen LastSeen Count From SubobjectPath Reason Message
Fri, 06 Nov 2015 16:52:50 -0800 Fri, 06 Nov 2015 16:52:50 -0800 1 {replication-controller } SuccessfulCreate Created pod: frontend-gyx2h
Fri, 06 Nov 2015 16:52:50 -0800 Fri, 06 Nov 2015 16:52:50 -0800 1 {replication-controller } SuccessfulCreate Created pod: frontend-vc9w4
Deleting replication controllers
To delete a replication controller as well as the pods that it controls, use
kubectl delete:
$ kubectl delete rc NAME
By default, kubectl delete rc will resize the controller to zero (effectively
deleting all pods) before deleting it.
To delete a replication controller without deleting its pods, use
kubectl delete and specify --cascade=false:
$ kubectl delete rc NAME --cascade=false
A successful delete request returns the name of the deleted resource.