How can I run NHibenate queries asynchronously?

Question

One way to increase scalability of the server application is to run IO-bound operation (reading files, sockets, web requests, database requests etc) asynchronously. This doe

Answer 1

Unfortunately, no. NHibernate does not expose the internal implementation of the command execution in the way L2S does.

You'll have to use the threadpool OR create a patch for NH to add asynchronous query support. That would be very welcome by the community and would make for a nice exercise (but it's not trivial at all)

Answer 2

Note that async database calls do NOT imply better overall scalability by themselves. I recommend reading the article "Should my database calls be Asynchronous?" for an in-depth analysis. Here's a quote from that article:

One respected DB/Web architect went so far as to say:
For database applications using async operations to reduce the number of blocked threads on the web server is almost always a complete waste of time. A small web server can easily handle way more simultaneous blocking requests than your database back-end can process concurrently. Instead make sure your service calls are cheap at the database, and limit the number of concurrently executing requests to a number that you have tested to work correctly and maximize overall transaction throughput.

Answer 3

Although there is still no support for async queries in NH, you can still partially overcome some of the undesired effects of running (long-running) db calls from request thread.

What you want is to split Threadpool between short-running and long-running operations. Of course this is not possible with actual implementation of Threadpool and TPL but you can help yourself quite eassilly by writing your own Producer/Consumer queue with awaitable items and customized concurency.

Please have a look at example i have put together : https://gist.github.com/3746240

Code is copy/pasted from great book "C# 5.0 in a Nutshell: The Definitive Reference" by Joseph Albahari and Ben Albahari with modification done by me causing the scheduler to create dedicated worker threads for items proccesing.

Answer 4

multiple async calls can be rewritten with Futures

var footask = QueryFooAsync();
var bartask = QueryBarAsync();
var baztask = QueryBazAsync();

var foos = await footask;
var bars = await bartask;
var baz = await baztask;

// do something with foos, bars, baz

can be replaced with

var foos = session.Query<Foo>().....ToFuture();
var bars = session.Query<Bar>().....ToFuture();
var baz = session.Query<Bazes>().....ToFutureValue();

await Task.Factory.StartNew(() => var ignored = baz.Value)  // await the results

// do something with foos, bars, baz

this even has the benefit over async code that the roundtrip time is only paid once instead of 3 times.

Answer 5

As of NHibernate v5, async is now fully supported!

Here are some nifty examples:

Customer customer = await session.GetAsync<Customer>(1);

List<Customer> customers = await session.Query<Customer>().ToListAsync();

Customer customer = await session.Query<Customer>()
.Where(x => x.Name.Contains("Customer 1"))
.SingleOrDefaultAsync();

Updating an entity

using (ISession session = sessionFactory.OpenSession())
using (ITransaction transaction = session.BeginTransaction())
{
    Customer customer = await session.GetAsync<Customer>(1);
    customer.Name = "Customer 3";
    await session.SaveOrUpdateAsync(customer);
    await transaction.CommitAsync();
}

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