gutsofpratik/horizon-skills/horizontal-scaling

What to Do When Qdrant Needs More Capacity

Vertical first: simpler operations, no network overhead, good up to ~100M vectors per node depending on dimensions and quantization. Horizontal when: data exceeds single node capacity, need fault tolerance, need to isolate tenants, or IOPS-bound (more nodes = more independent IOPS).

Most basic distributed configuration

• 3 nodes with replication_factor: 2 for zero-downtime scaling, and at least 2 shards per node (6 shards) so you can add nodes later without resharding Sharding

Minimum of 3 nodes is important for consensus and fault tolerance. With 3 nodes, you can lose 1 node without downtime. With 2 nodes, losing 1 node causes downtime for collection operations. Replication factor of 2 means each shard has 1 replica, so you have 2 copies of data. This allows for zero-downtime scaling and maintenance. With replication_factor: 1, zero-downtime is not guaranteed even for point-level operations, and cluster maintenance requires downtime.

Choosing number of shards

Shards are the unit of data distribution. More shards allows more nodes and better distribution, but adds overhead. Fewer shards reduces overhead but limits horizontal scaling.

For cluster of 3-6 nodes the recommended shard count is 6-12. This allows for 2-4 shards per node, which balances distribution and overhead.

Changing number of shards

Use when: shard count isn't evenly divisible by node count, causing uneven distribution, or need to rebalance.

Resharding is expensive and time-consuming, so use it as a last resort if regular data distribution is not possible. The collection stays available during resharding, with slightly reduced performance, but a lot of data moves between nodes Resharding.

• Available in Qdrant Cloud Resharding
• Resharding is not available for self-hosted deployments.

Better alternatives: over-provision shards initially, or spin up new cluster with correct config and migrate data.

What NOT to Do

• Do not jump to horizontal before exhausting vertical (adds complexity for no gain)
• Do not set shard_number that isn't a multiple of node count (uneven distribution)
• Do not use replication_factor: 1 in production if you need fault tolerance
• Do not add nodes to a self-hosted cluster without rebalancing shards (use the shard move API to redistribute); Qdrant Cloud rebalances shards automatically Cluster scaling
• Do not scale down RAM without load testing (cache eviction causes days-long latency incidents)
• Do not hit the collection limit by using one collection per tenant (use payload partitioning)