Sony Arouje Blog

a programmer's log

Transparent Persistence with db4o

with 2 comments

I was working on a small Invoicing application for my brother and I uses db4o for data persistence. Again I used db4o here because of it’s simplicity and ease of doing future updates.

I started with POCO entities and db4o handles the insertions and deletion quiet well. I started experiencing trouble when I try to update object graphs. For e.g. I created a SaleOrder with two Items then I retrieve the order and added one more item and save the changes. db4o will never save the newly added Item, it always shows the items that added initially. Because db4o couldn’t make out the changes in the object graph. There are two ways to let db4o handle this scenario.

UpdateDepth

There is a problem with UpdateDepth, we have to define a depth say 5, 10, etc and db4o will traverse the graph to identify the changes, that means we should forecast our object graph depth. This also introduce performance issues. So I decided not to use it.

Transparent Persistence

With Transparent Persistence (TP) the object will tell which all the fields got modified and db4o will update the changed fields efficiently.

To enable Transparent Persistence we have to do some changes in

  • repository level
  • entity level.
  •  

    Let’s start with repository, I used the same kind of repository I created for iTraveller with some changes in creating the db4o context/container. The change is as shown below.

    public IObjectContainer Context
    {
        get
        {
            if (_context == null)
                this.CreateContext();
            return _context;
        }
    }
    
    public void CreateContext()
    {
        if (_context == null)
        {
            IEmbeddedConfiguration config = Db4oEmbedded.NewConfiguration();
            config.Common.Add(new TransparentActivationSupport());
            config.Common.Add(new TransparentPersistenceSupport());
            _context = Db4oEmbedded.OpenFile(config, DBNAME);
        }
    }
    

     

    It’s the time to do some modification in Entity level.

    using System;
    usingDb4objects.Db4o.Activation;
    usingDb4objects.Db4o.Collections;

    usingDb4objects.Db4o.TA;
    namespaceNet.Sales.Manager.Domain
    {
        public classSaleOrderHeader : IActivatable
      
    {
            [System.NonSerialized]
            privateIActivator _activator;

            publicSaleOrderHeader ()
            {
               
                this.OrderDate = DateTime.Now;
                this.SaleOrderItems = newArrayList4<SaleOrderItem>();
            }

            public int OrderId { get; set; }

            private IList<SaleOrderItem> _salesOrderItems;
            public IList<SaleOrderItem> SaleOrderItems 
            {
                get 
                {
                    this.Read();
                    return this._salesOrderItems;
                }
                private set
                {
                    this._salesOrderItems = value;
                    this.Write();
                }
            }
    
            private double _discount;
            public double Discount
            {
                get
                {
                    this.Read();
                    return _discount;
                }
                set
                {
                    this.Write();
                    this._discount = value;
                }
            }
            public void With(SaleOrderItem saleOrderItem)
            {
                if(this.SaleOrderItems==null)
                    this.SaleOrderItems = new ArrayList4<SaleOrderItem>();
                this.Read();
                this.SaleOrderItems.Add(saleOrderItem);
            }
    
    
            public void Activate(ActivationPurpose purpose)
            {
                if (this._activator != null)
                    _activator.Activate(purpose);
                
            }
    
            public void Bind(IActivator activator)
            {
                if (_activator == activator)
                    return;
                
                if (activator != null && null != _activator)
                    throw new System.InvalidOperationException();
                
                _activator = activator;
            }
    
            public void Read()
            {
                this.Activate(ActivationPurpose.Read);
            }
    
            public void Write()
            {
                this.Activate(ActivationPurpose.Write);
            }
    
        }
    }
    
    

    For simplicity I removed some functions and properties from the above class. As you can see the SalesOrderHeader is implemented IActivatable. All classes that implements IActivatable is a candidate for Transparent Activation and Transparent Persistence. IActivatable will add two methods Activate and Bind. Make sure you implement the functions as shown above. I also added two helper methods Read and Write to ease the use of Activate function. Have a look at the property Discount and see how we are using it.

    private double _discount;
    public double Discount
    {
        get
        {
            this.Read();
            return _discount;
        }
        set
        {
            this.Write();
            this._discount = value;
        }
    }
    

     

    It is as simple as that.

    If we a have collection and that needs TA and TP enabled then we have to use ArrayList4. It’s inherited from IList. In the above e.g. SaleOrderItem is a collection, see how we implemented it.

    private IList<SaleOrderItem> _salesOrderItems;
    public IList<SaleOrderItem> SaleOrderItems 
    {
        get 
        {
            this.Read();
            return this._salesOrderItems;
        }
        private set
        {
            this._salesOrderItems = value;
            this.Write();
        }
    }
    

     

    Now we have to implement IActivatable in SaleOrderItem entity as well, the approach is same as mentioned above so skipping the code.

    We are done, now db4o can handle Transparent Activation and Transparent Persistence and no need to provide any graph depth. Even if we provide Update or Activate depth db4o will ignore it.

    Note

    We don’t need to implement IActivatable in top level class if it’s not dealing with a collection. Say for e.g. we have Customer and Address class and Customer has an instance of Address. Then implement IActivatable in Address no need to implement it in Customer as by default db4o will update top level object.

    Query TA/TP enabled objects

    You can see that in my Generic Repository I query data using Lambda expression, and the query is created as shown below.

    public IList<TEntity> Find<TEntity>(Func<TEntity, bool> criteria) where TEntity : class
    {
        return this.Query<TEntity>().Where<TEntity>(criteria).ToList<TEntity>();
    }
    
    
    private IList<TEntity> Query<TEntity>() where TEntity : class
    {
        return  this.Context.Query<TEntity>();
    }
    

     

    _repository.Find<SaleOrderHeader>(soh => soh.OrderId == salesOrderNumber);
    

     

    This Query model is not working if we enable TA and TP. First or Find of SaleOrder always return null. I think I need to check with db4o devs.

    The alternative is use Query by Example (QBE) as shown below.

    public IList<TEntity> Find<TEntity>(TEntity criteria) where TEntity : class
    {
        return this.Context.QueryByExample(criteria).Cast<TEntity>().ToList<TEntity>();
    }
    
    public SaleOrderHeader getSalesOrder(int salesOrderNumber)
    {
        return _repository.First<SaleOrderHeader>(new SaleOrderHeader(salesOrderNumber));
    }
    

    You can find more details from db4o online help.

    I will be releasing the entire source to Github soon.

    About these ads

    Written by Sony Arouje

    August 8, 2012 at 12:44 pm

    Posted in .NET

    Tagged with , ,

    2 Responses

    Subscribe to comments with RSS.

    1. Hi,

      I Could not reproduce the query behavior you described.

      If you can create a simple example reproducing it I can check (you can send it to my email).

      BTW: Adding TrasnparentPersistence automatically enables TransparentActivation also.

      Best

      Adriano

      August 17, 2012 at 9:30 pm

      • Hi Adriano,
        Thanks for your comment Adriano, I will create a tracer and email you. From the db4o’s help I came to know that TP implicitly enables TA. db4o’s help docs are really helpful.

        Thanks again for going thru my post and giving me the feedback.

        Regards,
        Sony Arouje

        Sony Arouje

        August 19, 2012 at 1:02 pm


    Leave a Reply

    Fill in your details below or click an icon to log in:

    WordPress.com Logo

    You are commenting using your WordPress.com account. Log Out / Change )

    Twitter picture

    You are commenting using your Twitter account. Log Out / Change )

    Facebook photo

    You are commenting using your Facebook account. Log Out / Change )

    Google+ photo

    You are commenting using your Google+ account. Log Out / Change )

    Connecting to %s

    Follow

    Get every new post delivered to your Inbox.

    Join 161 other followers

    %d bloggers like this: