{"action":"created","comment":{"url":"https://api.github.com/repos/kubernetes/kubernetes/pulls/comments/46609904","id":46609904,"diff_hunk":"@@ -0,0 +1,533 @@\n+<!-- BEGIN MUNGE: UNVERSIONED_WARNING -->\n+\n+<!-- BEGIN STRIP_FOR_RELEASE -->\n+\n+<img src=\"http://kubernetes.io/img/warning.png\" alt=\"WARNING\"\n+     width=\"25\" height=\"25\">\n+<img src=\"http://kubernetes.io/img/warning.png\" alt=\"WARNING\"\n+     width=\"25\" height=\"25\">\n+<img src=\"http://kubernetes.io/img/warning.png\" alt=\"WARNING\"\n+     width=\"25\" height=\"25\">\n+<img src=\"http://kubernetes.io/img/warning.png\" alt=\"WARNING\"\n+     width=\"25\" height=\"25\">\n+<img src=\"http://kubernetes.io/img/warning.png\" alt=\"WARNING\"\n+     width=\"25\" height=\"25\">\n+\n+<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>\n+\n+If you are using a released version of Kubernetes, you should\n+refer to the docs that go with that version.\n+\n+<strong>\n+The latest release of this document can be found\n+[here](http://releases.k8s.io/release-1.1/docs/proposals/multiple-schedulers.md).\n+\n+Documentation for other releases can be found at\n+[releases.k8s.io](http://releases.k8s.io).\n+</strong>\n+--\n+\n+<!-- END STRIP_FOR_RELEASE -->\n+\n+<!-- END MUNGE: UNVERSIONED_WARNING -->\n+\n+# PetSets: Running pods which need strong identity and storage\n+\n+## Open Issues\n+\n+* Add examples\n+* Discuss failure modes for various types of clusters\n+* Provide an active-active example\n+* Templating proposals need to be argued through to reduce options\n+\n+## Motivation\n+\n+Many examples of clustered software systems require stronger guarantees per instance than are provided\n+by the Replication Controller (aka ReplicaSet). Instances of these systems typically require:\n+\n+1. Data per instance which should not be lost even if the pod is deleted, typically on a persistent volume\n+   * Some cluster instances may have tens of TB of stored data - forcing new instances to replicate data\n+     from other members over the network is onerous\n+2. A stable and unique identity associated with that instance of the storage - such as a unique member id\n+3. A consistent network identity that allows other members to locate the instance even if the pod is deleted\n+4. A predictable number of instances to ensure that systems can form a quorum\n+   * This may be necessary during initialization\n+5. Instances can migrate from machine to machine as necessary and are not tied to an instance\n+6. The ability to scale up in a controlled fashion, but are very rarely scaled down without human\n+   intervention\n+\n+Kubernetes should expose a pod controller (a PetSet) that satisfies these requirements in a flexible\n+manner. It should be easy for users to manage and reason about the behavior of this set. An administrator\n+with familiarity in a particular cluster system should be able to leverage this controller and its\n+supporting documentation to run that clustered system on Kubernetes. It is expected that some adaptation\n+is required to support each new cluster.\n+\n+\n+## Use Cases\n+\n+The software listed below forms the primary use-cases for a PetSet on the cluster - problems encountered\n+while adapting these for Kubernetes should be addressed in a final design.\n+\n+* Quorum with Leader Election\n+  * MongoDB - in replica set mode forms a quorum with an elected leader, but instances must be preconfigured\n+    and have stable network identities.\n+  * ZooKeeper - forms a quorum with an elected leader, but is sensitive to cluster membership changes and\n+    replacement instances *must* present consistent identities\n+  * etcd - forms a quorum with an elected leader, can alter cluster membership in a consistent way, and\n+    requires stable network identities\n+* Decentralized Quorum\n+  * Cassandra - allows flexible consistency and distributes data via innate hash ring sharding, is also\n+    flexible to scaling, more likely to support members that come and go. Scale down may trigger massive\n+    rebalances.\n+* Active-active\n+  * **Need active-active master example - galera? **\n+* ??? \n+\n+\n+## Background\n+\n+ReplicaSets are designed with a weak guarantee - that there should be N replicas of a particular pod template.\n+Each pod instance varies only by name, and the replication controller errs on the side of ensuring that N\n+replicas exist as quickly as possible (by creating new pods as soon as old ones begin graceful deletion, 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