Creating a Client-Server Database System with Windows 95 and Linux

Here's the way to develop a dial-on-demand database cluster in Linux.
Database Server Consideration

When this article was being written, Informix-SE for Linux was just becoming available, and Oracle had started to port their database server to Linux. We did not consider these two popular databases. We considered only database servers with these basic properties:

  • It follows the relational model and supports SQL (or a subset of SQL). The relational model has become a standard for modern database servers. Our client software can communicate with the database in the standard way using SQL. Therefore, even if we change the database server software in the future, most of our client code will not need to be changed.

  • It is free or low cost. We believe one can find good software for free or low cost in the Linux world so we considered the free database servers first. If none had satisfied our requirements, we could then have considered a commercial one.

  • It is open source, if possible. We wanted the source code of our whole system including operating system, client-server software and database server, so that our system would not be affected by any standard or format changes by third parties. Of course, we cannot have the code to Windows 95, which is why we want to port the client software to another platform.

The database servers were compared in four aspects: available C API, available ODBC driver, Big5 code support and concurrency control method. C API is important for the client software running on Linux. The ODBC driver is for client software running on Windows 95. Big5 code support, as mentioned above, is one of the basic requirements of our system. Since it is a multi-client system, the method for preventing concurrent client data access from interfering with each other is also important. We carried out a survey of four popular database systems: PostgreSQL, Beagle SQL, mSQL and MySQL. All source code to these systems is available. Both PostgreSQL and Beagle SQL are free of charge. mSQL is free if you use it in academic and registered charity organizations, otherwise it costs $250 US for a single license. MySQL is free if you don't sell it; otherwise, it costs $200 US per copy. The results are shown in Table1.

Table 1.

MySQL was chosen as our database server. The most important reasons were that it has an ODBC driver for Windows 95, and it supports Big5 characters. MySQL is a multi-threaded process, with one thread for each connection. Moreover, many support utilities such as table repairing tools are provided. We recompiled it for Big5 support. During beta testing, we found the system to be stable, efficient and reliable on Red Hat 4.2. However, we found it could not successfully compile and run on Red Hat 5.0, even when we strictly followed the manual. We think the main reason is library incompatibility.

Client-Side Consideration

We considered different C++ (or C) compilers for Windows 95, and finally chose Borland (Inprise) C++Builder as our client-side software development environment. Some visual objects are in C++Builder (similar to Delphi and Visual Basic) to access the content of a table in the database directly. They are supposed to be simple and easy to use. However, when the program becomes large, maintenance of the code with these kinds of objects is not easy, because the behavior of each database widget is almost independent. We decided to develop a layer of database objects to act as a bridge between the visual objects and the database content. The SQL statements are embedded in the database objects. In this way, the clinical objects can be defined as database objects naturally and consistently. Moreover, security checks can be implemented in this layer of objects to protect the data being displayed on the visual objects. We expect the portability of the software will be improved as well.

System Architecture

Figure 1.

The proposed system architecture of NORA is shown in Figure 1. In order to simplify the discussion, we assume the connection is between two servers. The configuration can be generalized to connections among several servers. For each clinic, there is a Linux server for the database and diald. The Windows 95 clients are connected to the local or remote database server through ODBC drivers. diald starts the connection to another server, if needed.

Configuring a Dial-Up Server

Now we come to connection and configuration. Basically, we will follow the FAQs and man pages on these topics. Since an agent is needed to receive the dial-up call from another computer, we installed mgetty to answer the call from the modem. If the call is normal data communication, login will be executed to prompt the user on the other side. One of the nice features of mgetty is that it can also act as a fax receiver if the call is a fax, forwarding it to e-mail or printing it. mgetty should be started by init and specified in inittab.

The user on the other side can start pppd after logging in to the Linux server. The options file of pppd should be kept in the simplest form, i.e., IP address, netmask, etc., should not be specified. This is necessary because this configuration is used by any execution instance of pppd, even one started by diald. For example, if an IP address is specified in the options file, the dial-out connection (by diald) IP address will be fixed to the same address. It is incorrect, since this address should be assigned by the target server when the server dials out. A suggested PPP option file, called options, is shown here:


Listing 1.

We leave the other configuration options to the connection script. An example of a script for the dial-in PPP startup, called startppp, is shown in Listing 1. It should be noted that defaultroute may not be necessary for dial-up from a stand-alone PC, but it is necessary for a LAN connection. Otherwise, there is no way for the dial-up connection to route to all machines in the LAN. Moreover, we skip the connect script for pppd since it can be found in the FAQs and man pages.

You may check the completion of this phase by dialing in to the server with a Windows 95 or another Linux PC using PPP. Start PPP by executing startppp after login. The appearance should be similar to dialing in to an ISP.