com.hazelcast.core

Interface IdGenerator

All Superinterfaces:

DistributedObject

All Known Subinterfaces:

FlakeIdGenerator

All Known Implementing Classes:

ClientFlakeIdGeneratorProxy, ClientIdGeneratorProxy, IdGeneratorImpl, IdGeneratorProxy

Deprecated.

The implementation can produce duplicate IDs in case of network split, even with split-brain protection enabled (during short window while split-brain is detected). Use HazelcastInstance.getFlakeIdGenerator(String) for an alternative implementation which does not suffer from this problem.

@Deprecated
public interface IdGenerator
extends DistributedObject

The IdGenerator is responsible for creating unique long IDs in a cluster, under normal operating scenarios.

In theory a IAtomicLong.incrementAndGet() could be used to provide the same functionality. The big difference is that the incrementAndGet requires one or more remote calls for every invocation and therefore is a performance and scalability bottleneck. The IdGenerator uses an IAtomicLong under the hood, but instead of doing a remote call for every call to newId(), it does it less frequently. It checks out a chunk of 10,000 IDs and as long as it has not yet consumed all the IDs in its chunk, no remote call is done.

It can be that IDs generated by different cluster members will get out of order because each member will get its own chunk. It can be that member 1 has chunk 1..10000 and member 2 has 10001..20000. Therefore, member 2 will automatically have IDs that are out of order with the IDs generated by member 1. Next, the chunk is kept until all IDs are used.

It should be noted that IdGenerator does not guarantee unique IDs in a split brain situation. During a split brain, different partitions of the cluster may have the primary or back-up, and will therefore continue in their respective clusters from the last known value. Other partitions of the cluster may not have the original primary nor the back-up, in which case the IdGenerator will be created as new and initialised as per the provided init(long)

When a cluster heals after a split brain, a default large side merge takes place for the IAtomicLong that keeps track of the last issued block of IDs, therefore it is possible that already issued IDs could be issued again after the cluster is repaired. As a precaution when in a new record insert situation try to detect if the key already exists and then grab another, possibly executing this logic only after a known split brain merge has occurred.

Care should be taken if you are using the IdGenerator to provide keys for IMap or ICache, as after split brain it could be possible for two unique values to share the same key. When a MapMergePolicy is applied, one of these unique values will have to be discarded. Unless configured, the default merge policy is PutIfAbsentMapMergePolicy.

It is NOT RECOMMENDED to use IdGenerator to provide keys that are also used as unique identifiers in underlying persistent storage, for example in an IMap that writes to a relational database using MapStore.

Method Summary

Modifier and Type	Method and Description
boolean	init(long id) Deprecated. Tries to initialize this IdGenerator instance with the given ID.
long	newId() Deprecated. Generates and returns a cluster-wide unique ID.

Methods inherited from interface com.hazelcast.core.DistributedObject

destroy, getName, getPartitionKey, getServiceName

Method Detail

init

boolean init(long id)

Deprecated.

Tries to initialize this IdGenerator instance with the given ID.

The next generated ID will be 1 greater than the supplied ID.

Returns:

true if initialization succeeded, false otherwise.

newId

long newId()

Deprecated.

Generates and returns a cluster-wide unique ID.

Returns:

new cluster-wide unique ID