Hello, I’ve been working extensively with Apache Solr, specifically with large-scale Solr Cloud collections, and I've encountered an issue that I’d like to share with you.
In Solr Cloud mode, when a collection has two or more replicas in a shard, I noticed a significant problem when restoring from a backup to the shard's leader. After the restore, the follower replica fails to start replication. This is a critical issue, as it means that the Follower will not have the updated data, rendering it Essentially useless. In the worst case, if the Leader shard becomes unavailable for any reason, the outdated Follower could be promoted to leader. This would result in a situation where the previously outdated Follower replica becomes the Leader and the Leader may even start replicating from what used to be its own replica - which will Cause a Full Loss of restoration data. I tested this behavior in a multi-sharded Solr collection with two replicas per shard, and the issue was consistently reproducible. The restore is a shard restore. I am more than willing to contribute to the necessary development to resolve this if needed.
