One node in a three-node Qdrant cluster crashes. What happens to reads and writes for shards that had a replica on that node?

What happens depends on the replication factor of each shard. If a shard had a replica on the crashed node and at least one other replica elsewhere, the surviving replica takes over serving reads and writes. For a shard whose primary was on the crashed node, the cluster promotes one of the surviving replicas to primary, and operations continue. For a shard whose replica (not primary) was on the crashed node, the primary continues as before and the cluster simply has one fewer replica until the node recovers. In both cases, there is a brief window during which the cluster detects the failure and, if necessary, elects a new primary, but reads and writes resume without manual intervention. If a shard had only one copy and that copy was on the crashed node, there is no surviving copy, and operations on that shard fail until the node recovers or the shard is restored from a snapshot.

The mechanism behind failover is that the cluster metadata, maintained through Raft, includes the shard-to-node placement and the primary/replica roles. When a node becomes unreachable, the cluster marks it as down, and the placement logic triggers a promotion for any shard whose primary was on that node. The surviving replica is promoted, and it continues to accept writes and serve reads. If the cluster cannot achieve a quorum for metadata operations, no further metadata changes can be committed, but existing shards can continue to serve traffic according to their current placement. This is the distinction between the metadata plane and the data plane: the metadata plane needs quorum, the data plane does not, and a cluster with a downed node can continue serving traffic for any shard that has a surviving replica. The availability of the collection as a whole therefore depends on the replication factor and on which specific shard's data was on the crashed node. With replication_factor=1, a node crash takes down every shard whose primary was on that node; with replication_factor=2, a single node crash leaves at least one copy of every shard; with replication_factor=3, the cluster can tolerate two simultaneous node failures.