Question

There is a multithreads application, that works with large DB file (>600 Mb). “Database is locked” problem started when I added blob data, and started operate with >30 Kb of BLOB data per request. I think problem related to small HDD speed. It looks like SQLite deletes -journal file, one thread of my application got out of lock (because -journal file was applied and deleted), and other my thread want to do smth with DB, but SQLite still updates DB file… Sure, I can do a minute delays after each DB call, but this is not a solution, because I need more speed.

Now I use session per conversation (per thread) implementation. So there is one ISessionFactory per application object and many ISession objects.

There are my helper classes (as you can see I use IsolationLevel.Serializable and CurrentSessionContext = ThreadStaticSessionContext):

public abstract class nHibernateHelper
{
    private static FluentConfiguration _configuration;
    private static IPersistenceContext _persistenceContext;

    static nHibernateHelper() {}

    private static FluentConfiguration ConfigurePersistenceLayer()
    {
        return Fluently.Configure().Database(FluentNHibernate.Cfg.Db.SQLiteConfiguration.Standard.ShowSql().UsingFile(_fileName).IsolationLevel(IsolationLevel.Serializable).MaxFetchDepth(2)).
                Mappings(m => m.FluentMappings.AddFromAssemblyOf<Foo>()).CurrentSessionContext(typeof(ThreadStaticSessionContext).FullName);
    }

    public static ISession CurrentSession
    {
        get { return _persistenceContext.CurrentSession; }
    }

    public static IDisposable OpenConnection()
    {
        return new DbSession(_persistenceContext);
    }
}

public class PersistenceContext : IPersistenceContext, IDisposable
{
    private readonly FluentConfiguration _configuration;
    private readonly ISessionFactory _sessionFactory;

    public PersistenceContext(FluentConfiguration configuration)
    {
        _configuration = configuration;
        _sessionFactory = _configuration.BuildSessionFactory();
    }

    public FluentConfiguration Configuration { get { return _configuration; } }
    public ISessionFactory SessionFactory { get { return _sessionFactory; } }

    public ISession CurrentSession
    {
        get
        {
            if (!CurrentSessionContext.HasBind(SessionFactory))
            {
                OnContextualSessionIsNotFound();
            }
            var contextualSession = SessionFactory.GetCurrentSession();
            if (contextualSession == null)
            {
                OnContextualSessionIsNotFound();
            }
            return contextualSession;
        }
    }

    public void Dispose()
    {
        SessionFactory.Dispose();
    }

    private static void OnContextualSessionIsNotFound()
    {
        throw new InvalidOperationException("Ambient instance of contextual session is not found. Open the db session before.");
    }

}

public class DbSession : IDisposable
{
    private readonly ISessionFactory _sessionFactory;

    public DbSession(IPersistenceContext persistentContext)
    {
        _sessionFactory = persistentContext.SessionFactory;
        CurrentSessionContext.Bind(_sessionFactory.OpenSession());
    }

    public void Dispose()
    {
        var session = CurrentSessionContext.Unbind(_sessionFactory);
        if (session != null && session.IsOpen)
        {
            try
            {
                if (session.Transaction != null && session.Transaction.IsActive)
                {
                    session.Transaction.Rollback();
                }
            }
            finally
            {
                session.Dispose();
            }
        }
    }
}

And there is repository helper class. As you can see there are locks by every DB call, so concurrence DB call can’t appear, for different threads too, because _locker object is static.

public abstract class BaseEntityRepository<T, TId> : IBaseEntityRepository<T, TId> where T : BaseEntity<TId>
{
    private ITransaction _transaction;
    protected static readonly object _locker = new object();

    public bool Save(T item)
    {
        bool result = false;

        if ((item != null) && (item.IsTransient()))
        {
            lock (_locker)
            {
                try
                {
                    _transaction = session.BeginTransaction();
                    nHibernateHelper.CurrentSession.Save(item);
                    nHibernateHelper.Flush();
                    _transaction.Commit();          
                    result = true;
                } catch 
                {
                    _transaction.Rollback();
                    throw;
                }
                //DelayAfterProcess();
            }
        }
        return result;
    }

    //same for delete and update 

    public T Get(TId itemId)
    {
        T result = default(T);

        lock (_locker)
        {
            try
            {
                result = nHibernateHelper.CurrentSession.Get<T>(itemId);
            }
            catch 
            {
                throw;
            }
        }
        return result;
    }

    public IList<T> Find(Expression<Func<T, bool>> predicate)
    {
        IList<T> result = new List<T>();
        lock (_locker)
        {
            try
            {
                result = nHibernateHelper.CurrentSession.Query<T>().Where(predicate).ToList();
            }
            catch 
            {
                throw;
            }
        }
        return result;
    }


}

I use previous classes like this (I call nHibernateHelper.OpenConnection() once per thread). Repository is instantiated by singletone:

using (nHibernateHelper.OpenConnection())
{
    Foo foo = new Foo();
    FooRepository.Instance.Save(foo);
}    

I tried to change IsolationLevel to ReadCommited, but this not changes a problem. Also I tried to solve this problem by change SQLite journal mode from journal to WAL:

using (nHibernateHelper.OpenConnection()) 
{
    using (IDbCommand command = nHibernateHelper.CurrentSession.Connection.CreateCommand())
    {
        command.CommandText = "PRAGMA journal_mode=WAL";
        command.ExecuteNonQuery();
    }
}

This helped on computers with fast HDD, but on some I got same error. Then I tried to add “DB update file exist” check to repository, and delay after each save/update/delete procedure:

    protected static int _delayAfterInSeconds = 1;
    protected void DelayAfterProcess()
    {
        bool dbUpdateInProcess = false;
        do
        {
            string fileMask = "*-wal*";
            string[] files = Directory.GetFiles(Directory.GetCurrentDirectory(), fileMask);
            if ((files != null) && (files.Length > 0))
            {
                dbUpdateInProcess = true;
                Thread.Sleep(1000);
            }
            else
            {
                dbUpdateInProcess = false;
            }
        } while (dbUpdateInProcess);
        if (_delayAfterInSeconds > 0)
        {
            Thread.Sleep(_delayAfterInSeconds * 1000);
        }
    }

Same solution (check for DB update file) not worked for -journal file. It reported, that -journal file was deleted, but I still got errors. For -wal file it works (as I think. I need more time to test it). But this solution seriously brake program.

Maybe you can help me?

Answer 1

Answering to myself.
Problem was related to .IsolationLevel(IsolationLevel.Serializable). When I changed this line to .IsolationLevel(IsolationLevel.ReadCommitted) problem disappeared.