Consistent Hashing in Ringpop

Ringpop uses consistent hashing to minimize reshuffling when nodes join or leave the cluster. This allows automatic rebalancing with evenly distributed load.

What is Consistent Hashing?

Consistent hashing maps data to physical nodes in a way that minimizes reorganization when nodes are added or removed. This is done by mapping data to points on a hash ring.

• Nodes are assigned ranges on the ring
• Data is hashed to a point on the ring
• Data is owned by the node responsible for the range containing the hash

When a node joins:

• It takes ownership of a portion of the ranges of other nodes
• A minimal amount of keys need to be relocated

When a node leaves:

• Its ranges are split up and assigned to other nodes
• Again only a minimal redistribution of keys is needed

This allows nodes to dynamically join and leave with minimal disruption to data mapping.

Implementation Details

Ringpop uses the following specific techniques:

• FarmHash for hashing - Provides very fast hashes needed for large clusters
• Red-black trees for range lookup - O(log n) operations make lookups fast
• Consistent hash ring to track nodes and ranges - Ranges shifted cleanly on changes

The consistent hash ring maintains the mapping of ranges to nodes. When a node joins or fails, the ring is updated to redistribute the ranges with minimal changes to existing mappings.

This provides efficient scaling and resiliency as nodes come and go. Failed nodes cause a graceful redistribution of load without widespread impact.

Benefits of Consistent Hashing

• Minimal redistribution of keys when nodes join or leave
• Auto-rebalancing of load across nodes
• Decentralized and resilient architecture
• Efficient lookups/inserts/deletes in O(log n) time

Consistent hashing is a great fit for dynamically scaling clusters like Ringpop. The efficient distribution and minimal reorganization make it easy to elastically grow and shrink the cluster.