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Thien-Thi Nguyen 2015-06-01 16:09:08 +02:00
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docs/replication-controller.md
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## What is a _replication controller_?
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. Unlike in the case where a user directly created pods, a replication controller replaces pods that are deleted or terminated for any reason, such as in the case of node failure or disruptive node maintenance, such as a kernel upgrade. For this reason, we recommend that you use a replication controller even if your application requires only a single pod. Think of it similarly to a process supervisor, only it supervises multiple pods across multiple nodes instead of individual processes on a single node. Replication controller delegates local container restarts to some agent on the node (e.g., Kubelet or Docker).
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. Unlike in the case where a user directly created pods, a replication controller replaces pods that are deleted or terminated for any reason, such as in the case of node failure or disruptive node maintenance, such as a kernel upgrade. For this reason, we recommend that you use a replication controller even if your application requires only a single pod. Think of it similarly to a process supervisor, only it supervises multiple pods across multiple nodes instead of individual processes on a single node. A replication controller delegates local container restarts to some agent on the node (e.g., Kubelet or Docker).
As discussed in [life of a pod](pod-states.md), `replicationController` is *only* appropriate for pods with `RestartPolicy = Always` (Note: If `RestartPolicy` is not set, the default value is `Always`.). `ReplicationController` should refuse to instantiate any pod that has a different restart policy. As discussed in [issue #503](https://github.com/GoogleCloudPlatform/kubernetes/issues/503#issuecomment-50169443), we expect other types of controllers to be added to Kubernetes to handle other types of workloads, such as build/test and batch workloads, in the future.