NHibernate HiLo Generator

Why use HiLo

HiLo identity generators let NHibernate assign identity values to child objects without hitting the database on every save. The native identity generation strategy, by contrast, makes NHibernate hit the database on every insert to obtain the next id, which hurts performance and batching.

For more background on generator strategies, see NH2.1.0 - Generators behavior explained by Fabio Maulo.

Using HiLo id generation

Add a Fluent NHibernate IIdConvention to your infrastructure/mapping project. This is the actual convention used by the TardisBank sample (Suteki.TardisBank.Infrastructure/NHibernateMaps/Conventions/PrimaryKeyConvention.cs):

public class PrimaryKeyConvention : IIdConvention
{
    public void Apply(IIdentityInstance instance)
    {
        instance.Column(instance.EntityType.Name + "Id");
        instance.GeneratedBy.HiLo("10");
    }
}

Then create the table NHibernate uses to allocate “hi” blocks of ids:

CREATE TABLE [dbo].[hibernate_unique_key](
    [next_hi] [int] NOT NULL
) ON [PRIMARY]

Seed the table with a starting value (e.g. 1) and you’re done — NHibernate will allocate ids in blocks, only round-tripping to the database once per block instead of once per insert.