Building a Distributed Spreadsheet in Modula-3
The netobj daemon acts like a bulletin board. First, the server posts a note saying, “I've got a spreadsheet object for sale.” Then the client comes along and says, “I'll buy that.” The server exports; the client imports; the daemon mediates. In the nomenclature of CORBA, the daemon is an object request broker. Once the sale is complete, the client and server talk to each other directly. Code details are found in Listing 4.
Listing 4 will work when the server and client are located on the same machine. Suppose instead that the server runs on some Linux box—eggnog.cmu.edu—and that the clients are elsewhere. Ensure that netobjd is running on eggnog and change one line in the client program.
address := NetObj.Locate( "eggnog.cmu.edu" );
With that, our programs now talk over the Net.
Because Modula-3 comes ready-made with thread support, it also provides mutexes (mutual exclusion semaphores) so that parallel operations on the same datum are serialized. In our discussion so far, the Money.T type has been left unspecified. It might actually be something like this:
INTERFACE Money; TYPE T = MUTEX OBJECT cents: INTEGER; END; END Money.
Mutexes protect data so that client B does not modify values before client A is finished. Granted, protecting each cell separately is overkill. A more elegant approach is to protect ranges of cells, with the lock initiated by user action.
Figure 2 shows a spreadsheet from the point of view of user A (Alice). She is working on the cell range tinted red. User B (Bob) cannot modify these cells. He is working on the blue cells, indicating to Alice that to her they are read only.
To port our user interface program from Linux to Windows NT, do the following:
Archive the client source code by using the tar command.
Copy the tar file to your Windows machine.
Unarchive the file using tar. Convert end-of-line markers.
At the command line, type m3build.
Assuming there are no stunts of low-level programming, all the Modula-3 code in this example—including the GUI—is transparently portable. Differing path name conventions, for example, are hidden behind OS-independent interfaces. There's not an #ifdef in sight.
In this article I've highlighted the creation of a multi-platform, distributed spreadsheet using Modula-3. The key step is to wrap the spreadsheet into a network object. In this way, remote objects may be invoked with exactly the same syntax as local objects. Most of the hard work is done for you.
Modula-3 is not the only means for creating distributed applications, but in my mind it strikes an optimal balance between simplicity and power. By its very intent, it is a language for building large, solid systems in order for you to get your work done.
Clearly, my discussion has omitted many details. To help fill this gap, a companion tutorial is available on the Web (see the sidebar “Getting Started”.) Full source code is available for experimentation and invention.
John Kominek holds a master's degree in Computer Science from the University of Waterloo, and is currently a graduate student at CMU. When pressed, he admits to pronouncing Linux to rhyme with Linus. He can be reached via e-mail at email@example.com.
Fast/Flexible Linux OS Recovery
On Demand Now
In this live one-hour webinar, learn how to enhance your existing backup strategies for complete disaster recovery preparedness using Storix System Backup Administrator (SBAdmin), a highly flexible full-system recovery solution for UNIX and Linux systems.
Join Linux Journal's Shawn Powers and David Huffman, President/CEO, Storix, Inc.
Free to Linux Journal readers.Register Now!
- Download "Linux Management with Red Hat Satellite: Measuring Business Impact and ROI"
- Client-Side Performance
- Tibbo Technology's Tibbo Project System
- Sony Settles in Linux Battle
- July 2016 Issue of Linux Journal
- Peppermint 7 Released
- Libarchive Security Flaw Discovered
- The Giant Zero, Part 0.x
- Profiles and RC Files
- Maru OS Brings Debian to Your Phone
With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide