<p>What's the Hi/Lo algorithm?</p>

<p>I've found this in the <a href="http://en.wikipedia.org/wiki/NHibernate" rel="noreferrer">NHibernate</a> documentation (it's one method to generate unique keys, section 5.1.4.2), but I haven't found a good explanation of how it works.</p>

<p>I know that Nhibernate handles it, and I don't need to know the inside, but I'm just curious.</p>

The basic idea is that you have two numbers to make up a primary key- a "high" number and a "low" number. A client can basically increment the "high" sequence, knowing that it can then safely generate keys from the entire range of the previous "high" value with the variety of "low" values.
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.
It can be very useful (particularly with ORMs) to be able to set the primary keys on the client side, instead of inserting values without primary keys and then fetching them back onto the client. Aside from anything else, it means you can easily make parent/child relationships and have the keys all in place before you do
any
inserts, which makes batching them simpler.