28.4. ConsistentHashFactories

Red Hat JBoss Data Grid offers a pluggable mechanism for selecting the consistent hashing algorithm. It is shipped with four implementations but a custom implementation can also be used.
JBoss Data Grid ships with four ConsistentHashFactory implementations:
  • DefaultConsistentHashFactory - keeps segments balanced evenly across all the nodes, however the key mapping is not guaranteed to be same across caches,as this depends on the history of each cache.
  • SyncConsistentHashFactory - guarantees that the key mapping is the same for each cache, provided the current membership is the same. This has a drawback in that a node joining the cache can cause the existing nodes to also exchange segments, resulting in either additional state transfer traffic, the distribution of the data becoming less even, or both.
  • TopologyAwareConsistentHashFactory - equivalent of DefaultConsistentHashFactory, but with server hinting enabled.
  • TopologyAwareSyncConsistentHashFactory - equivalent of SyncConsistentHashFactory, but with server hinting enabled.
The consistent hash implementation can be selected via the hash configuration: 
<hash consistentHashFactory="org.infinispan.distribution.ch.TopologyAwareSyncConsistentHashFactory"/>
This configuration guarantees caches with the same members have the same consistent hash, and if the machineId, rackId, or siteId attributes are specified in the transport configuration it also spreads backup copies across physical machines/racks/data centers.
It has a potential drawback in that it can move a greater number of segments than necessary during re-balancing. This can be mitigated by using a larger number of segments.
Another potential drawback is that the segments are not distributed as evenly as possible, and actually using a very large number of segments can make the distribution of segments worse.
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28.4.1. Implementing a ConsistentHashFactory

A custom ConsistentHashFactory must implement the org.infinispan.distribution.ch.ConsistenHashFactory interface with the following methods (all of which return an implementation of org.infinispan.distribution.ch.ConsistentHash):

Example 28.1. ConsistentHashFactory Methods

create(Hash hashFunction, int numOwners, int numSegments, List<Address>
members,Map<Address, Float> capacityFactors)
updateMembers(ConsistentHash baseCH, List<Address> newMembers, Map<Address,
Float> capacityFactors)
rebalance(ConsistentHash baseCH)
union(ConsistentHash ch1, ConsistentHash ch2)
Currently it is not possible to pass custom parameters to ConsistentHashFactory implementations.
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