Which db4o feature provides you with the ability to ensure that specific constraints are not violated whenever objects are to be committed to a database file?

Which db4o feature allows you to keep your client-side object cache in sync with whatever changes are committed to a server in a client-server setting?

Yeah, you got that right, commit time callbacks.

Two new events are being added to our event interface:

• Committing: event subscribers are notified before the container starts any meaningful commit work and are allowed to cancel the entire operation by throwing an exception; the ObjectContainer instance is completely blocked while subscribers are being notified which is both a blessing because subscribers can count on a stable and safe environment and a curse because it prevents any parallelismwith the container;
• Committed: event subscribers are notified in a separate thread after the container has completely finished the commit operation; exceptions if any will be ignored; still a working in progress to hit a development version near you soon;
  

The following snippet shows how to subscribe to the Committing event and add some application specific validation:

41:        private IObjectContainer _container;
42:
43:        public Program()
44:        {
45:            _container = Db4oFactory.OpenFile("c:/test.db4o");
46:            EventRegistry().Committing += new CommitEventHandler(Container_Committing);
47:        }
48:        
49:        // CommitEventArgs includes information about
50:        // the objects being added, updated and deleted in the 
51:        // current transaction
52:        void Container_Committing(object sender, CommitEventArgs args)
53:        {
54:            CheckSSN(args.Added);
55:            CheckSSN(args.Updated);
56:        }
57:
58:        private void CheckSSN(IObjectInfoCollection collection)
59:        {
60:            foreach (IObjectInfo info in collection)
61:            {
62:                Person p = info.GetObject() as Person;
63:                if (p == null) continue;
64:                
65:                IQuery query = _container.Query();
66:                query.Constrain(typeof(Person));
67:                query.Descend("_ssn").Constrain(p.SSN);
68:                IObjectSet found = query.Execute();
69:                if (1 != found.Count)
70:                {
71:                    throw new Db4oException("SSN must be unique: " + p.SSN);
72:                }
73:            }
74:        }
75:
76:        private IEventRegistry EventRegistry()
77:        {
78:            return EventRegistryFactory.ForObjectContainer(_container);
79:        }

This feature landed in our repository pretty recently but it is already available in the 6.2 release.

And by the way, you don't have to write unique constraint code yourself!