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Merge pull request #24296 from chrislovecnm/cassandra-seed-provider-docs 

Automatic merge from submit-queue

Intial draft on SeedProvider docs

Alsa more documentation.  We need to reference the config section in the example docs.  There are multiple PRs open in those docs, so at this point I do not want to make a mess.

Let me know if there are docs standard template that will make this more pretty.
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<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>
If you are using a released version of Kubernetes, you should
refer to the docs that go with that version.
Documentation for other releases can be found at
[releases.k8s.io](http://releases.k8s.io).
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# Cassandra on Kubernetes Custom Seed Provider: releases.k8s.io/HEAD
Within any deployment of Cassandra a Seed Provider is used to for node discovery and communication. When a Cassandra node first starts it must discover which nodes, or seeds, for the information about the Cassandra nodes in the ring / rack / datacenter.
This Java project provides a custom Seed Provider which communicates with the Kubernetes API to discover the required information. This provider is bundled with the Docker provided in this example.
# Configuring the Seed Provider
The following environment variables may be used to override the default configurations:
| ENV VAR | DEFAULT VALUE | NOTES |
| ------------- |:-------------: |:-------------:|
| KUBERNETES_PORT_443_TCP_ADDR | kubernetes.default.svc.cluster.local | The hostname of the API server |
| KUBERNETES_PORT_443_TCP_PORT | 443 | API port number |
| CASSANDRA_SERVICE | cassandra | Default service name for lookup |
| POD_NAMESPACE | default | Default pod service namespace |
# Using
If no endpoints are discovered from the API the seeds configured in the cassandra.yaml file are used.
# Provider limitations
This Cassandra Provider implements `SeedProvider`. and utilizes `SimpleSnitch`. This limits a Cassandra Ring to a single Cassandra Datacenter and ignores Rack setup. Datastax provides more documentation on the use of [_SNITCHES_](https://docs.datastax.com/en/cassandra/3.x/cassandra/architecture/archSnitchesAbout.html). Further development is planned to
expand this capability.
This in affect makes every node a seed provider, which is not a recommended best practice. This increases maintenance and reduces gossip performance.
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