How do you do Versioning in Nhibernate?

Question

I can't believe it is so hard to get someone to show me a simple working example. It leads me to believe that everyone can only talk like they know how to do it but in reality they don't.

I shorten the post down to only what I want the example to do. Maybe the post was getting to long and scared people away.

To get this bounty I am looking for a WORKING EXAMPLE that I can copy in VS 2010 and run.

What the example needs to do.

1. Show what datatype should be in my domain for version as a timestamp in mssql 2008
2. Show nhibernate automatically throwing the "StaleObjectException"
3. Show me working examples of these 3 scenarios

Scenario 1

User A comes to the site and edits Row1. User B comes(note he can see Row1) and clicks to edit Row1, UserB should be denied from editing the row until User A is finished.

Scenario 2

User A comes to the site and edits Row1. User B comes 30mins later and clicks to edit Row1. User B should be able to edit this row and save. This is because User A took too long to edit the row and lost his right to edit.

Scenario 3

User A comes back from being away. He clicks the update row button and he should be greeted with StaleObjectException.

I am using asp.net mvc and fluent nhibernate. Looking for the example to be done in these.

---

What I tried

I tried to build my own but I can't get it throw the StaleObjectException nor can I get the version number to increment. I tired opening 2 separate browser and loaded up the index page. Both browsers showed the same version number.

public class Default1Controller : Controller
{
    //
    // GET: /Default1/

    public ActionResult Index()
    {
        var sessionFactory = CreateSessionFactory();

        using (var session = sessionFactory.OpenSession())
        {
            using (var transaction = session.BeginTransaction())
            {
                var firstRecord = session.Query<TableA>().FirstOrDefault();
                transaction.Commit();
                return View(firstRecord);
            }

        }

    }

    public ActionResult Save()
    {
        var sessionFactory = CreateSessionFactory();
        using (var session = sessionFactory.OpenSession())
        {
            using (var transaction = session.BeginTransaction())
            {
                var firstRecord = session.Query<TableA>().FirstOrDefault();
                firstRecord.Name = "test2";
                transaction.Commit();
                return View();
            }
        }
    }

    private static ISessionFactory CreateSessionFactory()
    {
        return Fluently.Configure()
            .Database(MsSqlConfiguration.MsSql2008
                .ConnectionString(c => c.FromConnectionStringWithKey("Test")))
            .Mappings(m => m.FluentMappings.AddFromAssemblyOf<TableA>())
                             //  .ExposeConfiguration(BuidSchema)
            .BuildSessionFactory(); 
    }


    private static void BuidSchema(NHibernate.Cfg.Configuration config)
    {
        new NHibernate.Tool.hbm2ddl.SchemaExport(config).Create(false, true);
    }

}


public class TableA
{
    public virtual Guid Id { get; set; }
    public virtual string Name { get; set; }

    // Not sure what data type this should be for timestamp.
    // To eliminate changing to much started with int version
    // but want in the end timestamp.
    public virtual int Version { get; set; } 
}

public class TableAMapping : ClassMap<TableA>
{
    public TableAMapping()
    {
        Id(x => x.Id);
        Map(x => x.Name);
        Version(x => x.Version);
    }
}

Answer 1

Will nhibernate stop the row from being retrieved?

No. Locks are only placed for the extent of a transaction, which in a web application ends when the request ends. Also, the default type of transaction isolation mode is Read committed which means that read locks are released as soon as the select statement terminates. If you are reading and making edits in the same request and transaction, you could place a read and write lock on the row at hand which would prevent other transactions from writing to or reading from that row. However, this type of concurrency control doesn't work well in a web application.

Or would the User B be able to still see the row but if he tried to save it would crash?

This would happen if [optimistic concurrency] was being used. In NHibernate, optimistic concurrency works by adding a version field . Save/update commands are issued with the version upon which the update was based. If that differs from the version in the database table, no rows are updated and NHibernate will throw.

What happens if User A say cancels and does not edit. Do I have to release the lock myself or is there a timeout can be set to release the lock?

No, the lock is released at the end of the request.

Overall, your best bet is to opt for optimistic concurrency with version fields managed by NHibernate.

Answer 2

How does it look in code? Do I setup in my fluent nhibernate to generate a timestamp(not sure if I would timespan datatype).

I would suggest using a version column. If you're using FluentNhibernate with auto mappings, then if you make a column called Version of type int/long it will use that to version by default, alternatively you can use the Version() method in the mapping to do so (it's similar for timestamp).

So now I generated somehow the timestamp and the user is editing a row(through a gui). Should I be storing the timestamp in memory or something? Then when the user submits call from memory the timestamp and id of the row and check?

When the user starts editing a row, you retrieve it and store the current version (the value of the version property). I would recommend putting the current version in a hidden field in the form. When the user saves his changes, you can either do a manual check against the version in the database (check that it's the same as the version in the hidden field), or you can set the version property to the value from the hidden field (if you are using databinding, you could do this automatically). If you set the version property, then when you try to save the entity, NHibernate will check that the version you're saving matches the version in the database, and throws an exception if it doesn't.

NHibernate will issue an update query something like:

UPDATE xyz SET , Version = 16 WHERE Id = 1234 AND Version = 15

(assuming your version was 15) - in the process it will also increment the version field

If so that means the business logic is keeping track of the "row locking" but in theory someone still could just go Where(x => x.Id == id) and grab that row and update at will.

If someone else updates the row via NHibernate, it will increment the version automatically, so when your user tries to save it with the wrong version you will get an exception which you need to decide how to handle (ie. try show some merge screen, or tell the user to try again with the new data)

What happens when the row gets updated? Do you set null to the timestamp?

It updates the version or timestamp (timestamp will get updated to the current time) automatically

What happens if the user never actually finishes updating and leaves. How does the row every become unlocked again?

The row is not locked per se, it is instead using optimistic concurrency, where you assume that no-one will change the same row at the same time, and if someone does, then you need to retry the update.

Is there still a race condition what happens or is this next to impossible in happening? I am just concerned 2 ppl try to get edit the same row and both of them see it in their gui for editing but one is actually going to get denied in the end because they lost the race condition.

If 2 people try to edit the same row at the same time, one of them will lose if you're using optimistic concurrency. The benefit is that they will KNOW that there was a conflict, as opposed to either losing their changes and thinking that it updated, or overwriting someone else's changes without knowing about it.

So I did something like this

var test = session.Query.Where(x => x.Id == id).FirstOrDefault(); // send to user for editing. Has versioning on it. user edits and send back the data 30mins later.

Codes does

test.Id = vm.Id; test.ColumnA = vm.ColumnA; test.Version = vm.Version;

session.Update(test); session.Commit(); So the above will work right?

The above will throw an exception if someone else has gone in and changed the row. That's the point of it, so you know that a concurrency issue has arisen. Typically you'd show the user a message saying "Someone else has changed this row" with the new row there and possibly their changes also so the user has to select which changes win.

but if I do this

test.Id = vm.Id; test.ColumnA = vm.ColumnA;

 session.Update(test); session.Commit(); it would not commit right? 

Correct as long as you haven't reloaded test (ie. you did test = new Xyz(), not test = session.Load() ) because the Timestamp on the row wouldn't match

If someone else updates the row via NHibernate, it will increment the version automatically, so when your user tries to save it with the wrong version you will get an exception which you need to decide how to handle (ie. try show some merge screen, or tell the user to try again with the new data)

Can I make it so when the record is grabbed this checked. I want to keep it simple at first that only one person can edit at a time. The other guy won't even be able to access the record to edit while something is editing it.

That's not optimistic concurrency. As a simple answer you could add a CheckOutDate property which you set when someone starts editing it, and set it to null when they finish. Then when they start to edit, or when you show them the rows to edit you could exclude all rows where that CheckOutDate is newer than say the last 10 minutes (then you wouldn't need a scheduled task to reset it periodically)

The row is not locked per se, it is instead using optimistic concurrency, where you assume that no-one will change the same row at the same time, and if someone does, then you need to retry the update.

I am not sure what your saying does this mean I can do

session.query.Where(x => x.id == id).FirstOrDefault(); all day long and it will keep getting me the record(thought it would keep incrementing the version).

The query will NOT increment the version, only an update to it will increment the version.

Answer 3

Yes, it is possible to lock a row with nhibernate but if I understand well, your scenario is in a web context and then it is not the best practice.

The best practive is to use optimistic locking with automatic versioning as you mentioned. Locking a row when page is opening and releasing it when page is unloading will quickly lead to dead lock the row (javascript issue, page not killed properly...). Optimistic locking will make NHibernate throws an exception when flushing a transaction which contains objects modified by another session. If you want to have true concurent modification of the same information you may try to think about a system which merge many users input inside a same document, but it is a system on its own, not managed by ORM.

You will have to choose a way to deal with session in a web environment. http://nhibernate.info/doc/nh/en/index.html#transactions-optimistic

The only approach that is consistent with high concurrency and high scalability is optimistic concurrency control with versioning. NHibernate provides for three possible approaches to writing application code that uses optimistic concurrency.

Answer 4

I don't know that much about nHibernate itself, but if you are prepared to create some stored procs on the database it can >sort of< be done.

You will need one extra data column and two fields in your object model to store information against each row:

• A 'hash' of all the field values (using SQL Server CHECKSUM 2008 and later or HASHBYTES for earlier editions) other than the hash field itself and the EditTimestamp field. This could be persisted to the table using INSERT/UPDATE triggers if needs be.
• An 'edit-timestamp' of type datetime.

Change your procedures to do the following:

• The 'select' procedure should include a where clause similar to 'edit-timestamp < (Now - 30 minutes)' and should update the 'edit-timestamp' to the current time. Run the select with appropriate locking BEFORE updating the row I'm thinking a stored procedure with hold locking such as this one here Use a persistent date/time rather than something like GETDATE().

Example (using fixed values):

BEGIN TRAN

DECLARE @now DATETIME 
SET @now = '2012-09-28 14:00:00'

SELECT *, @now AS NewEditTimestamp, CHECKSUM(ID, [Description]) AS RowChecksum
FROM TestLocks
WITH (HOLDLOCK, ROWLOCK)
WHERE ID = 3 AND EditTimestamp < DATEADD(mi, -30, @now)

/* Do all your stuff here while the record is locked */
UPDATE TestLocks
SET EditTimestamp = @now
WHERE ID = 3 AND EditTimestamp < DATEADD(mi, -30, @now)

COMMIT TRAN

If you get a row back from this procedure then you 'have' the 'lock', otherwise, no rows will be returned and there's nothing to edit.

• The 'update' procedure should add a where clause similar to 'hash = previously returned hash'

Example (using fixed values):

BEGIN TRAN

    DECLARE @RowChecksum INT
    SET @RowChecksum = -845335138

    UPDATE TestLocks
    SET [Description] = 'New Description'
    WHERE ID = 3 AND CHECKSUM(ID, [Description]) = @RowChecksum

    SELECT @@ROWCOUNT AS RowsUpdated

COMMIT TRAN

So in your scenarios:

1. User A edits a row. When you return this record from the database, the 'edit-timestamp' has been updated to the current time and you have a row so you know you can edit. User B would not get a row because the timestamp is still too recent.

2. User B edits the row after 30 minutes. They get a row back because the timestamp has passed more than 30 minutes ago. The hash of the fields will be the same as for user A 30 minutes ago as no updates have been written.

3. Now user B updates. The previously retrieved hash still matches the hash of the fields in the row, so the update statement succeeds, and we return the row-count to show that the row was updated. User A however, tries to update next. Because the value of the description field has changed, the hashvalue has changed, and so nothing is updated by the UPDATE statement. We get a result of 'zero rows updated' so we know that either the row has since been changed or the row was deleted.

There are probably some issues regarding scalability with all these locks going on and the above code could be optimised (might get problems with clocks going forward/back for example, use UTC), but I wrote these examples just to explain how it could work.

Outside of that I can't see how you can do this without utilising database level row-locking within the select transaction. It might be that you can request those locks via nHibernate, but that's beyond my knowledge of nHibernate I'm afraid.

Answer 5

Have you looked at the ISaveOrUpdateEventListener interface?

public class SaveListener : NHibernate.Event.ISaveOrUpdateEventListener
{

    public void OnSaveOrUpdate(NHibernate.Event.SaveOrUpdateEvent e)
    {
        NHibernate.Persister.Entity.IEntityPersister p = e.Session.GetEntityPersister(null, e.Entity);
        if (p.IsVersioned)
        {
            //TODO: check types etc...
            MyEntity m = (MyEntity) e.Entity;
            DateTime oldversion = (DateTime) p.GetVersion(m, e.Session.EntityMode);
            DateTime currversion = (DateTime) p.GetCurrentVersion(m.ID, e.Session);

            if (oldversion < currversion.AddMinutes(-30))
                throw new StaleObjectStateException("MyEntity", m.ID);
        }
    }

}

Then in your Configuration, register it.

    private static void Configure(NHibernate.Cfg.Configuration cfg)
    {
        cfg.EventListeners.SaveOrUpdateEventListeners = new NHibernate.Event.ISaveOrUpdateEventListener[] {new SaveListener()};

    }



    public static ISessionFactory CreateSessionFactory()
    {
        return Fluently.Configure().Database(...).
                    .Mappings(...)
                    .ExposeConfiguration(Configure)                       
                    .BuildSessionFactory();
    }

And version the Properties you want to version in your Mapping class.

public class MyEntityMap: ClassMap<MyENtity>
{
    public MyEntityMap()
    {
        Table("MyTable");

        Id(x => x.ID);
        Version(x => x.Timestamp);
        Map(x => x.PropA);
        Map(x => x.PropB);

    }
}

Answer 6

The short answer to your question is you can't/shouldn't do this in a simple web application with nhibernates optimistic (version) and pessimistic (row locks) locking. The fact that your transactions are only as long as a request are your limiting factor.

What you CAN do is create another table and entity class, and mappings that manages these "locks". At the lowest level you need an Id of the object being edited and the Id of the user performing the editing, and a datetime of when the lock was acquired. I would make the Id of the object being edited the primary key since you want it to be exclusive...

When a user clicks on a row to edit, you can try to acquire a lock (create a new record in that table with the ids and current datetime). If the lock already exists for another user, than it will fail because you are trying to violate a primary key constraint.

If a lock is acquired, when the user clicks save you need to check that they still have a valid "lock" before performing the actual save. Then, perform the actual save and remove the lock record.

I would also recommend a background service/process that sweeps these locks periodically and removes the ones that have expired or are older than your time limit.

This is my prescribed way of dealing with "locks" in a web environment. Good luck!

Answer 7

Hey you can try these sites

http://thesenilecoder.blogspot.ca/2012/02/nhibernate-samples-row-versioning-with.html

http://stackingcode.com/blog/2010/12/09/optimistic-concurrency-and-nhibernate