Question

I have one application which uses NHibernate to save entities to a database, and it uses the HiLo generate of NHibernate to create the id’s. I have another application which I need to save data to the same table, however in this application, I’m not using NHibernate. Is there a simple way to use HiLo without adding reference to NHibernate and mapping files?

<id name="EntityId" unsaved-value="0" >
    <column name="EntityId" sql-type="int" not-null="true" />
    <generator class="hilo">
    <param name="table">hibernate_unique_key</param>
        <param name="column">next_hi</param>
        <param name="max_lo">1000</param>
    </generator>
</id>

Update: I created a new method to get the next id which I believe has the same behavior as the NHibernate HiLo sequence. I’m not putting any locks on the database, so if this were a high frequency table that I was inserting to, it could be an issue, but for very low frequency, there is a very low possibility that there may be a concurrency issue.

/// <summary>
/// Gets the next entity id.
/// </summary>
/// <returns></returns>
public static int GetNextAvailableEntityId()
{
    int max_lo = 1000;
    int nextHi = DefaultContext.Database.SqlQuery<int>("select next_hi from hibernate_unique_key").Single();
    int nextRangeStart = max_lo * nextHi;
    int currentMax = DefaultContext.Database.SqlQuery<int>("SELECT MAX(entityid) FROM Entities").Single();

    if (currentMax < nextRangeStart)
    {
        return currentMax + 1;
    }
    else
    {
        DefaultContext.Database.ExecuteSqlCommand("update hibernate_unique_key set next_hi = next_hi + 1");
        return nextHi;
    }
}

Answer 1

If in doubt, take a look at NH’s code to be sure that you implement it correctly.

It is something like this:

• open a separate transaction, increment and read the next high value, commit
• create 1000 ids by next_high * 1000 + x
• when you run out of ids, start again

You can create a single instance of an id generator for all transaction of the same process. Make sure that your id generator is thread safe.