docs/installation-rediscluster.rst

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Important
^^^^^^^^^^
The redis-cluster currently is NOT part of RIC platform & hence is completely optional.
This piece of document has been created as part of delivery item for below jira ticket 
https://jira.o-ran-sc.org/browse/RIC-109 
This ticket is about assessing the feasibility of redis-cluster (with data sharding) 
supporting desired pod anti-affinity for high availability as per the ticket.

Overview
^^^^^^^^^^
This document describes the environment/conditions used to test the feasibility of Redis 
cluster set-up as detailed in the above ticket. Redis Cluster is a distributed implementation 
of Redis with high performance goals. More details at https://redis.io/topics/cluster-spec

Environment Set-Up
^^^^^^^^^^
The set up was tested with kubernetes v1.19 cluster with 
   #. Pod topology spread constraint enabled
      Reference: https://kubernetes.io/docs/concepts/workloads/pods/pod-topology-spread-constraints
   #. CEPH as the Cluster Storage Solution.
	  Reference: https://github.com/rook/rook.github.io/blob/master/docs/rook/v1.4/ceph-filesystem.md
   #. Three worker nodes in the kubernet cluster

Execution
^^^^^^^^^^
Once environment is set-up,  a redis-cluster can be set up using the helm-chart (also provided with 
this commit). Once cluster is running, any master/slave of the redis instance pods can be deleted which
will be compensated automatically by new instances

At this stage the perl utility program (included with helm-chart) can be run. The helm chart installation
output generates the requirement commands to invoke.

This utility program identifies the missing anti-affinity(as per above ticket) of redis instances required
in a redis-cluster. When executed it communicates to redis nodes to switch roles (e.g. master/slave)
such that the end-state meets the desired anti-affinity.