以下来自Mastering Delphi 6，第17章《多层数据库应用程序》
The Connection Protocol
DataSnap defines only the higher-level architecture and can use different technologies for
moving the data from the middle tier to the client side. DataSnap supports most of the leading
standards, including the following:
Distributed COM (DCOM) and Stateless COM (MTS or COM+) DCOM is directly available
in Windows NT/2000 and 98/Me, and it requires no additional run-time applications on
the server. You still have to install it on Windows 95 machines. DCOM is basically an extension
of COM technology (discussed in Chapter 19, “COM Programming,” and Chapter 20)
that allows a client application to use server objects that exist and execute on a separate computer.
The DCOM infrastructure allows you to use stateless COM objects, available in the
COM+ and in the older MTS (Microsoft Transaction Server) architectures. Both COM+ and
MTS provide features such as security, component management, and database transactions,
and are available in Windows NT/2000 and in Windows 98/Me.
Due to the complexity of DCOM configuration and of its problems in passing through
firewalls, even Microsoft is abandoning DCOM in favor of SOAP-based solutions.
TCP/IP sockets These are available on most systems. Using TCP/IP you might distribute
clients over the Web, where DCOM cannot be taken for granted, and have many fewer configuration
headaches. To use sockets, the middle-tier computer must run the ScktSrvr.exe
application provided by Borland, a single program that can run either as an application or as
a service. This program receives the client requests and forwards them to the remote data
module (executing on the same server) using COM. Sockets provide no protection against
failure on the client side, as the server is not informed and might not release resources when
a client unexpectedly shuts down.
HTTP and SOAP The use of HTTP as a transport protocol over the Internet simplifies
connections through firewalls or proxy servers (which generally don’t like custom TCP/IP
sockets). You need a specific Web server application, httpsrvr.dll, which accepts client
requests and creates the proper remote data modules using COM. These Web connections
can use SSL security but must register themselves by adding a call to EnableWebTransport
in the UpdateRegistry method. Finally, Web connections based on HTTP transport can
use DataSnap object-pooling support.
CORBA Common Object Request Broker Architecture is an official standard for object
management available on most operating systems. Compared to DCOM, the advantage is
that your client and server applications can be also written with Java and other products.
The Borland implementation of CORBA, VisiBroker, is available with Delphi Enterprise.
CORBA provides many benefits, including location transparency, load balancing, and failover
from the ORB run-time software. (An in-depth discussion of CORBA is certainly
beyond the scope of this book, and in practice only a limited number of Delphi programmers
use CORBA.)
Internet Express As an extension to this architecture, you can transform the data packets
into XML and deliver them to a Web browser. In this case, you basically have one extra
tier: the Web server gets the data from the middle tier and delivers it to the client. I’ll discuss
this new architecture, called Internet Express, in Chapter 23. The DLL can also be
folded into the executable file by using the MidasLib unit.
