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3.2.5.2. Implementing a ConsistentHashFactory

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.
In addition to these, a ConsistentHashFactory can be implemented manually. The custom ConsistentHashFactory must implement the following methods: 
			
create(Hash hashFunction,
          int numOwners,
          int numSegments,
          List<Address> members)
// create a new consistent hash instance

updateMembers(ConsistentHash baseCH,
          List<Address> newMembers)
// update the list of members of an existing consistent hash instance; the implementation should not assign new segments to a node, unless that segments doesn't have any owners

rebalance(ConsistentHash baseCH)
// rebalance the segments between existing members (so a node joining requires both updateMembers and rebalance)

union(ConsistentHash ch1, ConsistentHash ch2)
// create a consistent hash instance which, for each segment, has the same owners as both consistent hash parameters. The result consistent hash is used for write operations during state transfer.
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Consistent hash instances can be created with the following parameters:
  • The hashFunction parameter is used on top of the keys' own hashCode() implementation.
  • The numOwners parameter is the ideal number of owners for each key. The created consistent hash can have a greater or fewer number of owners, however each key will have at least one owner.
  • The numSegments defines the number of hash-space arguments. The implementation may either round up the number of segments for performance, or it may ignore the parameter altogether.
  • The members parameter provides a list of addresses representing the new cache members.
Currently it is not possible to pass custom parameters to ConsistentHashFactory implementations.
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