Fixed Partitions
Also known as:
Logical Partitions
Fixed Partitioning
Hash Slots
Definition
Fixed Partitions is a data partitioning pattern that keeps the number of partitions constant for the life of the cluster. Keys map to a fixed set of logical partitions with a hash, and those partitions are then assigned to physical nodes through a separate mapping. When nodes join or leave, whole partitions move between nodes but keys never rehash, so cluster resizing moves only a small slice of data instead of almost everything.
Key Takeaways
- Mapping keys straight to nodes with hash(key) % nodeCount remaps almost all data whenever the node count changes.
- Fixed Partitions adds a stable middle layer: keys map to a fixed number of partitions, and partitions map to nodes.
- Because the partition count never changes, only whole partitions move on a resize, not individual keys.
- Pick a partition count much larger than your expected node count up front, because changing it later is expensive.
How It Works
- At cluster creation, fix the number of partitions (for example 1024 or Redis Cluster’s 16384 slots).
- Locate a key with partition = hash(key) % partitionCount, a mapping that never changes.
- Keep a separate partition-to-node assignment table, usually in a consistent core like ZooKeeper or etcd.
- When a node is added or removed, reassign some partitions to it and move only those partitions’ data.