DDD: Entity identity before being persisted

Question

In Domain Driven Design, one of the defining characteristic of an Entity is that it has an identity.

Problem:

I am not able to provide a uni

Answer 1

You can use a sequence generator to generate unique int/long identifiers when you instantiate an entity object.

The interface looks like:

interface SequenceGenerator {
    long getNextSequence();
}

A typical implementation of a sequence generator uses a sequence table in the database. The sequence table contains two columns: sequenceName and allocatedSequence.

When getNextSequence is called first time, it writes a large value (say 100) to the allocatedSequence column and return 1. The next call will return 2 without need to access the database. When the 100 sequences runs out, it reads and increments the allocatedSequence by 100 again.

Have a look at the SequenceHiLoGenerator in Hibernate source code. It basically does what I described above.