A unique ID generator that generates unique¹ eight byte identifiers in a distributed context.

1. Unique within the confines of the chosen computing environment.

When you want to assign unique identifiers to database records in a distributed computing environment that are short and guaranteed to be unique (within your data realm), this library can provide them.

If you have a computing environment where you know exactly which processes may generate IDs. The simple LocalUniqueIDGenerator can be used. This generator assumes that you know which process may use which generator-ID at any time.

For example, if you have just one process that handles the creation of new IDs (perhaps a single server that creates database records using these IDs), a generator can be used like this:

final int generatorID = 0;
final int clusterID = 0;
IDGenerator generator = LocalUniqueIDGeneratorFactory.generatorFor(generatorID, clusterID);

// Generate IDs
byte[] id = generator.generate();

The LocalUniqueIDGeneratorFactory assumes that you can guarantee that it is the only generator with the specific generator-ID and cluster-ID combination you chose, during its lifetime.

If there is a fixed number of processes that may generate IDs, you can assign one of the 256 possible generator-IDs to each one. For a more in-depth explanation of generator-IDs and cluster-IDs, see eight byte ID structure.

For a cluster of Tomcat servers in a high-availability setup, you could configure a system property on each server with a unique generator ID.

For local usage the uniqueid-core module can be used:

<dependency>
  <groupId>org.lable.oss.uniqueid</groupId>
  <artifactId>uniqueid-core</artifactId>
  <version>${uniqueid.version}</version>
</dependency>

If you need to generate unique IDs within a distributed environment, automatic coordination of the generator-ID is also a possibility. The acquisition of a generator-ID can be handled by a SynchronizedGeneratorIdentity instance, which uses Apache ZooKeeper to claim its generator-ID — for a short while, or as long as it maintains a connection to the ZooKeeper quorum.

For this functionality the uniqueid-zookeeper module is used:

<dependency>
  <groupId>org.lable.oss.uniqueid</groupId>
  <artifactId>uniqueid-core</artifactId>
  <version>${uniqueid.version}</version>
</dependency>

To use this method of generator-ID acquisition, a node on the ZooKeeper quorum must be chosen to hold the queue and resource pool used by SynchronizedGeneratorIdentity.

For example, if you choose /unique-id-generator as the node, these child nodes will be created:

/unique-id-generator/
 ├─ queue/
 ├─ pool/
 └─ cluster-id

Note that if you do not create the cluster-id node yourself (recommended), the default value of 0 will be used. To use a different cluster ID, set the content of this znode to one of the 16 permissible values (i.e., 0..15).

If you have access to the zkcli (or hbase zkcli) command line utility you can set the cluster-ID like so:

create /unique-id-generator/cluster-id 1

Or if the node already exists:

set /unique-id-generator/cluster-id 1

To use an IDGenerator with a negotiated generator-Id, create a new instance like this:

// Change the values of zookeeperQuorum and znode as needed:
final String zookeeperQuorum = "zookeeper1,zookeeper2,zookeeper3";
final String znode = "/unique-id-generator";
IDGenerator generator = SynchronizedUniqueIDGeneratorFactory.generatorFor(zookeeperQuorum, znode);
// ...
byte[] id = generator.generate()
// ...

If you expect that you will be using dozens of IDs in a single process, it is more efficient to generate IDs in batches:

Deque<byte[]> ids = generator.batch(500);
// ...
byte[] id = ids.pop();
// etc.

If you intend to generate more than a few IDs at a time, you can also wrap the generator in an AutoRefillStack, and simply call generate() on that whenever you need a new ID. It will grab IDs in batches from the wrapped IDGenerator instance for you. This is probably the simplest and safest way to use an IDGenerator.

final String zookeeperQuorum = "zookeeper1,zookeeper2,zookeeper3";
final String znode = "/unique-id-generator";
IDGenerator generator = new AutoRefillStack(
    SynchronizedUniqueIDGeneratorFactory.generatorFor(zookeeperQuorum, znode)
);
// ...
byte[] id = generator.generate()
// ...