Hello, Thank you for reaching out to Cloudera !    To achieve HA in SolrCloud, there are 2 concepts -  1. Multiple Live Solr Instances  2. Replicas for the Collections   For example, if you have 2 Solr Instances live in SolrCloud, and you have one collection with multiple shards but with 1 replica. In this case, the Collection is not highly available. In event of issue on one Solr Node, the collection will not be functional .   Now, if you have 2 Solr Instances live in SolrCloud, and you have one collection with multiple shards with 2 replicas ( Replicas <= Number of Solr Instances ). In this case, Collection is highly available. In event of issue on one Solr Node, the replicas hosted on other Solr Node will be registered as Active, and will serve client query/updates.   To add replicas to an existing collection, ADDREPLICA [1] API can be used. If you have any questions, please do not hesitate to update this discussion.   [1] https://solr.apache.org/guide/8_4/replica-management.html#addreplica ... View more