7.2. Distribution Mode's Consistent Hash Algorithm

The hashing algorithm in Red Hat JBoss Data Grid is based on consistent hashing. The term consistent hashing is still used for this implementation, despite some divergence from a traditional consistent hash.

Distribution mode uses a consistent hash algorithm to select a node from the cluster to store entries upon. The consistent hash algorithm is configured with the number of copies of each cache entry to be maintained within the cluster. Unlike generic consistent hashing, the implementation used in JBoss Data Grid splits the key space into fixed segments. The number of segments is configurable using numSegments and cannot be changed without restarting the cluster. The mapping of keys to segments is also fixed — a key maps to the same segment, regardless of how the topology of the cluster changes.

The number of copies set for each data item requires balancing performance and fault tolerance. Creating too many copies of the entry can impair performance and too few copies can result in data loss in case of node failure.

Each hash segment is mapped to a list of nodes called owners. The order is important because the first owner (also known as the primary owner) has a special role in many cache operations (for example, locking). The other owners are called backup owners. There is no rule about mapping segments to owners, although the hashing algorithms simultaneously balance the number of segments allocated to each node and minimize the number of segments that have to move after a node joins or leaves the cluster.