Understanding the Replication Queue in ClickHouse

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. Its queue was empty. That didn't match what I expected at all. If the queue was a cluster-wide "here's what still needs to replicate" view, Node A should've shown something too - it was the one that had the fresh data now waiting to reach Node B. Instead it was Node B, the down one, sitting there with pending tasks. That mismatch is what sent me digging. Turns out the queue isn't cluster-wide at all - it's specific to each ClickHouse instance. Once I brought Node B back up, its queue drained in seconds and the data showed up. That whole experiment is basically the entire post in miniature. Here's the mental model I ended up with. A Queue Belongs to a Replica, Not to the Table This is the first thing to get straight. With a ReplicatedMergeTree table, you can have multiple replicas holding copies of the same data. It's tempting to think of replication as one shared pipe between them. It isn't. Each replica keeps its own local replication queue . So if you see: Replica 1 → queue_size = 0 Replica 2 → queue_size = 25 that doesn't mean 25 operations are waiting somewhere in the middle for both replicas to pick up. It means Replica 2, specifically, has 25 tasks it hasn't finished yet. Once that clicked for me, the rest of the system made a lot more sense. So Where Do These Tasks Come From? Replication in ClickHouse