In the beginning, there were mainframes. Every program and piece of data was stored in a single almighty machine. Users could access this centralized computer only by means of dumb terminals. (See Figure 1.)
In the 1980s, the arrival of inexpensive network-connected PCs produced the popular two-tier client-server architecture. In this architecture, there is an application running in the client machine which interacts with the server—most commonly, a database management system (see Figure 2). Typically, the client application, also known as a fat client, contained some or all of the presentation logic (user interface), the application navigation, the business rules and the database access. Every time the business rules were modified, the client application had to be changed, tested and redistributed, even when the user interface remained intact. In order to minimize the impact of business logic alteration within client applications, the presentation logic must be separated from the business rules. This separation becomes the fundamental principle in the three-tier architecture.
In a three-tier architecture (also known as a multi-tier architecture), there are three or more interacting tiers, each with its own specific responsibilities (see Figure 3):
Tier 1: the client contains the presentation logic, including simple control and user input validation. This application is also known as a thin client.
Tier 2: the middle tier is also known as the application server, which provides the business processes logic and the data access.
Tier 3: the data server provides the business data.
These are some of the advantages of a three-tier architecture:
It is easier to modify or replace any tier without affecting the other tiers.
Separating the application and database functionality means better load balancing.
Adequate security policies can be enforced within the server tiers without hindering the clients.
In order to demonstrate these design concepts, the general outline of a simple three-tier “Hangman” game will be presented (check the source code in the archive file). The purpose of this game, just in case the reader isn't familiar with it, is to try to guess a mystery word, one letter at a time, before making a certain number of mistakes.
The data server is a Linux box running the MiniSQL database management system. The database is used to store the mystery words. At the beginning of each game, one of these words is randomly selected.
At the client side, a Java applet contained in a web page (originally obtained from a web server) is responsible for the application's graphical user interface (see Figure 4). The client platform may be any computer with a web browser that supports applets. The game's logic is not controlled by the applet; that's the middle tier's job. The client only takes care of the presentation logic: getting the user's input, performing some simple checking and drawing the resulting output.
The server in the middle tier is a Java application, also running within a Linux box. The rules of the “Hangman” game (the business rules) are coded in this tier. Sockets and JDBC, respectively, are used to communicate with the client and the data server through TCP/IP.
Figure 5 presents a UML (Unified Modeling Language) deployment diagram that shows the physical relationship among the hardware nodes of the system.
Even though the design described gives the impression of requiring a different machine for each tier, all tiers (each one running on a different process) can be run in the same computer. This means the complete application is able to run in a single Linux system with a graphical desktop, and it doesn't even have to be connected to the Net!
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
Join Linux Journal's Mike Diehl and Pat Cameron of Help Systems.
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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