i'm trying to get my head around 2nd Level Caching in Fluent NHibernate. So far i have done the following:
Added a reference to the caching dll
Added the following when i create my session factory:
c.SetProperty("cache.provider_class", "NHibernate.Caches.SysCache.SysCacheProvider, NHibernate.Caches.SysCache");
c.SetProperty("cache.use_second_level_cache", "true");
c.SetProperty("cache.use_query_cache", "true");
Added Cache.ReadWrite(); against all entities i wish to cache in ClassMap file e.g.
public class CountryMap : ClassMap {
public CountryMap() {
Table("Countries");
Id(x => x.CountryID);
Map(x => x.CountryName);
Cache.ReadWrite();
}
}
Now i assumed that anytime i try to get an entity which has Cache.ReadWrite() in the mapping it would cache it for the duration of the session factory (singleton). However it appears that this is not the case as the database is hit on every request. Here's a couple questions i have:
What does the CacheMode property on the session do? My session lasts for the duration of a web request. Should i set this when i create the session and if so what should i set it to?
I read somewhere that the cache is not updated unless a commit is made. I only commit data when i'm inserting/updating information in the database. For example if i have a simple page with a list of countries i would grab all the data needed but wouldn't commit the data as i don't need to make any changes. Does this mean this data won't be cached? I'm sure i've read this wrong as this would mean data is only cached once it is added the database.
I'd appreciate it if you could help. Thanks
Here is a very good explanation for second level cache with nhibernate.
You have to use transactions for the 2nd level cache to be used.
Related
I'm relatively new to NHibernate and I've got a question about it.
I use this code snippet in my MVC project in Controller's method:
MyClass entity = new MyClass
{
Foo = "bar"
};
_myRepository.Save(entity);
....
entity.Foo = "bar2";
_myRepository.Save(entity);
The first time entity saved in database succesfully. But the second time not a single request doesnt go to database. My method save in repository just does:
public void Save(T entity)
{
_session.SaveOrUpdate(entity);
}
What should I do to be able to save and then update this entity during one request? If I add _session.Flush(); after saving entity to database it works, but I'm not sure, if it's the right thing to do.
Thanks
This is the expected behavior.
Changes are only saved on Flush
Flush may be called explicitly or implicitly (see 9.6. Flush)
When using an identity generator (not recommended), inserts are sent immediately, because that's the only way to return the ID.
you should be using transactions.
a couple of good sources: here and here.
also, summer of nHibernate is how I first started with nHibernate. it's a very good resource for learning the basics.
In the TestFixtureTearDown-part of an NUnit test I try to delete some test-entities created in the TestFixtureSetUp-part. I use the following code
sessionFactory = NHibernateHelper.CreateSessionFactory(cssc["DefaultTestConnectionString"].ConnectionString);
uow = new NHibernateUnitOfWork(sessionFactory);
var g = reposGebruiker.GetByName(gebruiker.GebruikerNaam);
reposGebruiker.Delete(g);
var k = reposKlant.GetByName(klant.Naam);
reposKlant.Delete(k);
// Commit changes to persistant storage
uow.Commit();
However, after the commit, the two entities were still in the database. After searching on I came across this page on SO and so I added:
uow.Session.Flush();
However, still the entities remain in the DB. Does anyone have an idea as to why this is?
I've never used the UoW class you're using, but my projects are implemented using ISession.BeginTransaction and ISession.Transaction.Commit in a helper like this:
public void CreateContext(Action logic)
{
ISession.BeginTransaction();
logic();
ISession.Transaction.Commit();
}
And then:
CreateContext(() =>
Session.Delete(someObject));
This should work.
I want to mention that this is an example, and you'd want to make some abstractions.
How are the repositories created? In for the delete to succeed, the objects must be loaded in the same UoW (ISession) in which the Delete command is issued. The Delete method makes the objects non-persistent and marks them for deletion.
I have the following code:
public class A
{
private ISessionFactory _sf;
A(ISessionFactory sf)
{
_sf = sf;
}
public void SomeFunc()
{
using (var session = _sf.OpenSession())
using (var transaction = session.BeginTransaction())
{
// query for a object
// change its properties
// save the object
transaction.commit();
}
}
}
Its used as follows in a unit test
_session.CreateCriteria ... // some setting up values for this test
var objectA = new A(_sessionFactory);
objectA.SomeFunc();
// _session.Clear();
var someVal = _session.CreateCriteria ... // retrieve value from db to
//check if it was set to the
//proper value
//it uses a restriction on a property
//and a uniqueresult to get the object.
//it doesnt use get or load.
Assert.That(someVal, Is.EqualTo(someOtherValue)); // this is false as long
//as the _session.Clear() is commented.
//If uncommented, the test passes
I am testing against a sqlite file database. In my tests I make some changes to the db to setup it up properly. I then call SomeFunc(). It makes the required modifications. Once I am back in my test, the session however doesnt get the updated values. It still returns the value as was before calling SomeFunc(). I have to execute _session.Clear() to have the changes reflect in my assertion in the test.
Why is this needed?
Edit: cache.use_second_level_cache and cache.use_query_cache are both set to false
Edit2: Read the following statements in the NH Documentation.
From time to time the ISession will
execute the SQL statements needed to
synchronize the ADO.NET connection's
state with the state of objects held
in memory. This process, flush, occurs
by default at the following points
* from some invocations of Find() or Enumerable()
* from NHibernate.ITransaction.Commit()
* from ISession.Flush()
And in section 10.1 it says,
Ensure you understand the semantics of
Flush(). Flushing synchronizes the
persistent store with in-memory
changes but not vice-versa.
So, how do I get the in memory objects to get updated? I understand that objects are cached per session. But executing a UniqueResult() or a List() should sync with the db and invalidate the cache, right?
What I cannot understand is why is the session reporting stale data?
It depens on what king of operations do you make. NHibernate has first level cache by default. It uses cache to get entities by ID and so on.
The in memory view of objects (the level 1 cache) is per session.
A takes an ISessionFactory and opens its own session with its own transaction scope.
Even if the contents of the ISession used in SomeFunc are flushed to the database, _session will not see those changes until its level 1 cache is cleared.
You have two sessions. One is in A.SomeFunc, and the other is in your unit test. Each session has it's own instance of the entities in the session-cache (1st level cache). The sessions do not communicate or coordinate with each other. When one session writes its changes, the other session isn't notified. It still has it's own outdated instance in its session cache.
When you call _session.Clear(), you make the session "forget" everything by clearing the session cache. When you re-query, you are reading fresh data from the database, which includes the changes from the other session.
This question is a bit of a dupe, but I still don't understand the best way to handle flushing.
I am migrating an existing code base, which contains a lot of code like the following:
private void btnSave_Click()
{
SaveForm();
ReloadList();
}
private void SaveForm()
{
var foo = FooRepository.Get(_editingFooId);
foo.Name = txtName.Text;
FooRepository.Save(foo);
}
private void ReloadList()
{
fooRepeater.DataSource = FooRepository.LoadAll();
fooRepeater.DataBind();
}
Now that I am changing the FooRepository to Nhibernate, what should I use for the FooRepository.Save method? Should the FooRepository always flush the session when the entity is saved?
I'm not sure if I understand your question, but here is what I think:
Think in "putting objects to the session" instead of "getting and storing data". NH will store all new and changed objects in the session without any special call to it.
Consider this scenarios:
Data change:
Get data from the database with any query. The entities are now in the NH session
Change entities by just changing property values
Commit the transaction. Changes are flushed and stored to the database.
Create a new object:
Call a constructor to create a new object
Store it to the database by calling "Save". It is in the session now.
You still can change the object after Save
Commit the changes. The latest state will be stored to the database.
If you work with detached entities, you also need Update or SaveOrUpdate to put detached entities to the session.
Of course you can configure NH to behave differently. But it works best if you follow this default behaviour.
It doesn't matter whether or not you explicitly flush the session between modifying a Foo entity and loading all Foos from the repository. NHibernate is smart enough to auto-flush itself if you have made changes in the session that may affect the results of the query you are trying to run.
Ideally I try to use one session per "unit of work". This means one cohesive piece of work which may involve several smaller steps. If you feel that you do not have a seam in your architecture where you can achieve this, then managing the session inside the repository will also work. Just be aware that you are missing out on some of the power that NHibernate provides you.
I'd vote up Stefan Moser's answer if I could - I'm still getting to grips with Nh myself but I think it's nice to be able to write code like this:
private void SaveForm()
{
using (var unitofwork = UnitOfWork.Start())
{
var foo = FooRepository.Get(_editingFooId);
var bar = BarRepository.Get(_barId);
foo.Name = txtName.Text;
bar.SomeOtherProperty = txtBlah.Text;
FooRepository.Save(foo);
BarRepository.Save(bar);
UnitOfWork.CommitChanges();
}
}
so this way either the whole action succeeds or it fails and rolls back, keeping flushing/transaction management outside of the Repositories.
I am performing a standard update in NHibernate to a single property. However on commit of the transaction the sql update seems to set all fields I have mapped on the table even though they have not changed. Surely this can't be normal behaviour in Nhibernate? Am I doing something wrong? Thanks
using (var session = sessionFactory.OpenSession())
{
using (var transaction = session.BeginTransaction())
{
var singleMeeting = session.Load<Meeting>(10193);
singleMeeting.Subject = "This is a test 2";
transaction.Commit();
}
}
This is the normal behavior. You can try adding dynamic-update="true" to your class definition to override this behavior.
Well. yes this is normal behaviour for NHibernate. You can use generated attribute for your properties to change the behaviour. Details on Ayende's blog.
Why is this default is because with dynamics you don't get your query plan cached. And usually you don't mind that you send few more bytes over high speed network connection between your application server and database. Unless you are saving long strings where this setting is perfectly appropriate.