Flake ID Generator
Hazelcast Flake ID Generator is used to generate cluster-wide unique identifiers.
Generated identifiers are long primitive values and are k-ordered (roughly ordered).
IDs are in the range from 0 to Long.MAX_VALUE.
Generating Cluster-Wide IDs
The IDs contain timestamp component and a node ID component, which is assigned when the member joins the cluster. This allows the IDs to be ordered and unique without any coordination between the members, which makes the generator safe even in split-brain scenarios (for limitations in this case, see the Node ID assignment section below).
Timestamp component is in milliseconds since 1.1.2018, 0:00 UTC and has 41 bits. This caps the useful lifespan of the generator to little less than 70 years (until ~2088). The sequence component is 6 bits. If more than 64 IDs are requested in single millisecond, IDs gracefully overflow to the next millisecond and uniqueness is guaranteed in this case. The implementation does not allow overflowing by more than 15 seconds, if IDs are requested at higher rate, the call blocks. Note, however, that clients are able to generate even faster because each call goes to a different (random) member and the 64 IDs/ms limit is for single member.
Performance
Operation on member is always local, if the member has valid node ID, otherwise it’s remote.
On the client, the newId() method goes to a random member and gets a batch of IDs,
which is then returned locally for a limited time. The pre-fetch size and the validity
time can be configured for each client and member.
Example
Let’s write an example identifier generator.
public class ExampleFlakeIdGenerator {
public static void main(String[] args) {
HazelcastInstance hazelcast = Hazelcast.newHazelcastInstance();
ClientConfig clientConfig = new ClientConfig()
.addFlakeIdGeneratorConfig(new ClientFlakeIdGeneratorConfig("idGenerator")
.setPrefetchCount(10)
.setPrefetchValidityMillis(MINUTES.toMillis(10)));
HazelcastInstance client = HazelcastClient.newHazelcastClient(clientConfig);
FlakeIdGenerator idGenerator = client.getFlakeIdGenerator("idGenerator");
for (int i = 0; i < 10000; i++) {
sleepSeconds(1);
System.out.printf("Id: %s\n", idGenerator.newId());
}
}
}Node ID Assignment
Flake IDs require a unique node ID to be assigned to each member, from which point the member can generate unique IDs without any coordination. Hazelcast uses the member list version from the moment when the member joined the cluster as a unique node ID.
The join algorithm is specifically designed to ensure that member list join version is unique for each member in the cluster. This ensures that IDs are unique even during network splits, with one caveat: at most one member is allowed to join the cluster during a network split. If two members join different subclusters, they are likely to get the same node ID. This is resolved when the cluster heals, but until then, they can generate duplicate IDs.
Node ID Overflow
Node ID component of the ID has 16 bits. Members with the member list join version higher than 2^16 won’t be able to generate IDs, but functionality is preserved by forwarding to another member. It is possible to generate IDs on any member or client as long as there is at least one member with join version smaller than 2^16 in the cluster. The remedy is to restart the cluster: the node ID component will be reset and assigned starting from zero again. Uniqueness after the restart will be preserved thanks to the timestamp component.
Configuring Flake ID Generator
Following is an example declarative configuration snippet:
<hazelcast>
...
<flake-id-generator name="default">
<prefetch-count>100</prefetch-count>
<prefetch-validity-millis>600000</prefetch-validity-millis>
<epoch-start>1514764800000</epoch-start>
<node-id-offset>0</node-id-offset>
<bits-sequence>6</bits-sequence>
<bits-node-id>16</bits-node-id>
<allowed-future-millis>15000</allowed-future-millis>
<statistics-enabled>true</statistics-enabled>
</flake-id-generator>
...
</hazelcast>hazelcast:
flake-id-generator:
default:
prefetch-count: 100
prefetch-validity-millis: 600000
epoch-start: 1514764800000
node-id-offset: 0
bits-sequence: 6
bits-node-id: 16
allowed-future-millis: 15000
statistics-enabled: trueThe following are the descriptions of configuration elements and attributes:
-
name: Name of your Flake ID Generator. It is a required attribute. -
prefetch-count: Count of IDs which are pre-fetched on the background when one call to FlakeIdGenerator.newId() is made. Its value must be in the range 1 -100,000. Its default value is 100. This setting pertains only tonewId()calls made on the member that configured it. -
prefetch-validity-millis: Specifies for how long the pre-fetched IDs can be used. After this time elapses, a new batch of IDs are fetched. Time unit is milliseconds. Its default value is 600,000 milliseconds (10 minutes). The IDs contain a timestamp component, which ensures a rough global ordering of them. If an ID is assigned to an object that was created later, it will be out of order. If ordering is not important, set this value to 0. This setting pertains only tonewId()calls made on the member that configured it. -
epoch-start: Offset of the timestamp component. Time unit is milliseconds, default is 1514764800000 (1.1.2018 0:00 UTC) -
node-id-offset: Specifies the offset that is added to the node ID assigned to cluster member for this generator. Might be useful in A/B deployment scenarios where you have cluster A which you want to upgrade. You create cluster B and for some time both will generate IDs and you want to have them unique. In this case, configure node ID offset for generators on cluster B. -
bits-sequence: Bit-length of the sequence component. Default value is 6 bits. -
bits-node-id: Bit-length of node id component. Default value is 16 bits. -
allowed-future-millis: Sets how far to the future is the generator allowed to generate IDs without blocking. Default is 15 seconds. -
statistics-enabled: Specifies whether the statistics gathering is enabled for your Flake ID Generator. If set tofalse, you cannot collect statistics in your implementation (usinggetLocalFlakeIdGeneratorStats()) and also Hazelcast Management Center will not show them. Its default value istrue.
