I was testing out CH-Ops - an admin GUI for self-hosted ClickHouse - on a simple setup: 1 shard, 2 replicas. Stumbled onto the replication queue almost by accident.

Here's what I did: I stopped one of the nodes (let's call it Node B), then inserted some data through the other one (Node A). Just wanted to see what would happen.

Then, while Node B was still down, I checked it in CH-Ops. It had stuff sitting in its replication queue.

My first assumption was: okay, this must be showing what's left to replicate across the cluster - the total pending replication work.

So I switched over and checked Node A, the one that was actually up and had just received the insert.