Is it OK to open a DB4o file for query, insert, update multiple times?

Question

This is the way I am thinking of using DB4o. When I need to query, I would open the file, read and close:

using (IObjectContainer db = Db4oFactory.OpenFile(Db4oFactory.NewConfiguration(), YapFileName))
{
    try
    {
        List<Pilot> pilots = db.Query<Pilot>().ToList<Pilot>();
    }
    finally
    {
       try { db.Close(); }
       catch (Exception) { };
    }
}

At some later time, when I need to insert, then

using (IObjectContainer db = Db4oFactory.OpenFile(Db4oFactory.NewConfiguration(), YapFileName))
{
    try
    {
        Pilot pilot1 = new Pilot("Michael Schumacher", 100);
        db.Store(pilot1);
    }
    finally
    {
       try { db.Close(); }
       catch (Exception) { };
    }
}

In this way, I thought I will keep the file more tidy by only having it open when needed, and have it closed most of the time. But I keep getting InvalidCastException

Unable to cast object of type 'Db4objects.Db4o.Reflect.Generic.GenericObject' to type 'Pilot'

What's the correct way to use DB4o?

Answer 1

Hi,

It is ok to reopen the database multiple times. The problem would be performance and loosing the "identity". Also you can't keep a reference to a result of a query and try to iterate it after closing the db (based on you code, looks like you want to do that).

GenericObjects are instantiated when the class cannot be found.

Can you provide a full, minimalist, sample that fails for you?

Also, which db4o version are you using?

Best

Answer 2

No, it's not a good idea to work this way. db4o ObjectContainers are intended to be kept open all the time your application runs. A couple of reasons:

• db4o maintains a reference system to identify persistent objects, so it can do updates when you call #store() on an object that is already stored (instead of storing new objects) . This reference system is closed when you close the ObjectContainer, so updates won't work.
• Class Metadata would have to be read from the database file every time you reopen it. db4o would also have to analyze the structure of all persistent classes again, when they are used. While both operations are quite fast, you probably don't want this overhead every time you store a single object.
• db4o has very efficient caches for class and field indexes and for the database file itself. If you close and reopen the file, you take no advantage of them.
• The way you have set up your code there could be failures when you work with multiple threads. What if two threads would want to open the database file at exactly the same time? db4o database files can be opened only once. It is possible to run multiple transactions and multiple threads against the same open instance and you can also use Client/Server mode if you need multiple transactions.
• Later on you may like to try Transparent Activation and Transparent Persistence. Transparent Activation lazily loads object members when they are first accessed. Transparent Persistence automatically stores all objects that were modified in a transaction. For Transparent Activation (TA) and Transparent Persistence (TP) to work you certainly have to keep the ObjectContainer open.

You don't need to worry about constantly having an open database file. One of the key targets of db4o is embedded use in (mobile) devices. That's why we have written db4o in such a way that you can turn your machine off at any time without risking database corruption, even if the file is still open.

Possible reasons why you are getting a GenericObject back instead of a Pilot object:

• This can happen when the assembly name of the assembly that contains the Pilot object has changed between two runs, either because you let VisualStudio autogenerate the name or because you changed it by hand.
• Maybe "db4o" is part of your assembly name? One of the recent builds was too agressive at filtering out internal classes. This has been fixed quite some time ago. You may like to download and try the latest release, "development" or "production" should both be fine.
• In a presentation I once did I have once seen really weird symptoms when db4o ObjectContainers were opened in a "using" block. You probably want to work without that anyway and keep the db4o ObjectContainer open all the time.