please explain NHibernate HiLo

Question

I'm struggling to get my head round how the HiLo generator works in NHibernate. I've read the explanation here which made things a little clearer.

My understanding is that each SessionFactory retrieves the high value from the database. This improves performance because we have access to IDs without hitting the database.

The explanation from the above link also states:

For instance, supposing you have a "high" sequence with a current value of 35, and the "low" number is in the range 0-1023. Then the client can increment the sequence to 36 (for other clients to be able to generate keys while it's using 35) and know that keys 35/0, 35/1, 35/2, 35/3... 35/1023 are all available.

How does this work in a web application as don't I only have one SessionFactory and therefore one hi value. Does this mean that in a disconnected application you can end up with duplicate (low) ids in your entity table?

In my tests I used these settings:

<id name="Id" unsaved-value="0">
  <generator class="hilo"/>
</id>

I ran a test to save 100 objects. The IDs in my table went from 32768 - 32868. The next hi value was incremented to 2. Then I ran my test again and the Ids were in the range 65536 - 65636.

First off, why start at 32768 and not 1, and secondly why the jump from 32868 to 65536?

Now I know that my surrogate keys shouldn't have any meaning but we do use them in our application. Why can't I just have them increment nicely like a SQL Server identity field would.

Finally can someone give me an explanation of how the max_lo parameter works? Is this the maximum number of low values (entity ids in my head) that can be created against the high value?

This is one topic in NHibernate that I have struggled to find documentation for. I read the entire NHibernate in action book and it still doesn't go into how this works in any detail.

Thanks Ben

Answer 1

I believe your understanding is more or less correct. The max_lo parameter is simply used to determine the number of Ids available for any given Hi value.

My best guess is that NHibernate's default max_lo value is 32768. Thus a Hi value of 1 would start your Ids at 32768 and run you right up to 65535. A Hi value of 2 would start at 65536 and run up another max_lo Ids.

Basically you use the max_lo value to control Id fragmentation. 32768 is likely not the optimal value for every situation.

It is important to note however that this only works within the scope of a SessionFactory. If you are stopping/starting your application and reinitializing the SessionFactory a whole bunch, it's going to increment the Hi value upon startup anyway and you're going to see your Ids jump pretty quickly.