Logging in the Application Server

Thank you for your answer. I try to write like this:
class MySecuredDataServer : SecuredDataServer
    {
        public MySecuredDataServer(IDataLayer dataLayer, QueryRequestSecurityStrategyHandler
                                                           querySecurityEnvironmentHandler, ILogger logger,
                                                           EventHandler<DataServiceOperationEventArgs> committingDelegate) :
        base(dataLayer, querySecurityEnvironmentHandler, logger, committingDelegate)
        {
            var securityStrategyProvider = new RequestSecurityStrategyProvider(dataLayer, querySecurityEnvironmentHandler);
            var serverSecurity = CreateDefaultServerSecurity(securityStrategyProvider);
            var objectLayer = CreateDefaultSecuredSerializableObjectLayer(dataLayer, securityStrategyProvider, committingDelegate);

if (logger != null)
            {
                serverSecurity = new ServerSecurityLogger(serverSecurity, logger);
                objectLayer = new SecuredSerializableObjectLayerLogger(objectLayer, logger);
            }

Initialize(serverSecurity, objectLayer);
        }
    }
but Initialize method is private and I cann't use it.
I am using standart SecuredDataServer class:
        var dataServer = new SecuredDataServer(Session.DataLayer, securityProviderHandler, logger, MyCommitHandler);
        …
}
private static void MyCommitHandler(object sender, DataServiceOperationEventArgs e)
{
          var objects = e.ObjectSpace.ModifiedObjects;
}
It works, but problem is that my MyCommitHandler method does not contains information about user that perform this changes.

I apologize for the delay. We are overloaded with support queries. We will answer you as soon as we can.

Thanks for your update.
I have extended the SecuredDataServer class to provide the capability to substitute the standard logging objects with custom objects.
The Initialize method of the SecuredDataServer class is not designed to be used in descendants. Instead, the ServerSecurity and ObjectLayer objects should be created and configured manually and passed to the 'SecuredDataServer(IServerSecurity serverSecurity, ISecuredSerializableObjectLayer securedSerializableObjectLayer)' constructor. I have incorrectly advised this approach. I apologize for this inconvenience.
Access the SecuritySystem.CurrentUser property to obtain an object that represents the current user. A value of this property is thread specific and initialized before each method is executed.