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Télécharger Turbo Pascal PT. Télécharger Turbo Pascal FR. Rewriting an application from scratch, in contrast, is not a matter of days, but requires months or even years.
In addition, it is obvious that nobody can come to a decision about rewriting an application within a couple of hours. A decision of such importance requires careful planning and analysis. Rewriting Program Code from Scratch We already know that program code ages over time. When code is being changed and extended, the original structure and architecture slowly blurs.
Code must be regularly cleaned up by refactoring to counterbalance the aging effects. If you have no good source code and development documentation, you are in trouble. In that situation, the question is whether somebody should spend time on familiarizing themselves with the code and to whip it into shape, or if it is better to rewrite from scratch.
Writing new software is always risky, as many software projects fail. There are different approaches to rewriting a system.
Besides having the option of using new tools and methods, developers will try not to repeat the same mistakes they made in the first place. If you rewrite software, you can use the existing system as specification, so there is a clear vision of how the software should look like. The danger is that developers focus too much on making everything better. Frederick Brooks wrote about this phenomenon in his legendary book The Mythical Man Month and called it the second system effect1.
Brooks realized that many teams that had successfully completed a software project together were bound to fail in the follow-up project, because they tried to build too much functionality into the software to make up for everything they felt they had forgotten in the first project. When working on existing software, you can be sure that certain functionality is present, unless you break it while modifying the software.
When starting from scratch, there is no guarantee that the result is a working piece of software. Many software projects, especially big ones, go through great troubles until the result is satisfactory.
In theory, it is just a matter of calculating the cost for a new development and compare these with the estimated costs for maintenance and extension of existing code. In reality, this calculation it is virtually impossible. Generally, at short term maintenance of existing software is cheaper, while in the long run a newly developed software will probably have lower maintenance costs. In any case, a good code basis makes maintaining and extending software easier and thus more cost-effective.
Rewriting software means new possibilities. You can get rid of old code that is hard to maintain, and can start using existing third-party components and classes.
In addition, you can use new PHP features. If there is no software that completely fulfills all your needs, you can still possibly use an existing product as the starting point for your own development. It is one of the great advantages of open source software that you can reuse existing products. To reuse existing components, you do not have rewrite your software, though. When migrating the software or by code refactoring you can replace existing code by another implementation, thus introducing third-party code into your application.
Compared to the extremely sumptuous task of rewriting software, rebuilding a system seems like a walk in the park. But since most production systems are used to earn money, directly or indirectly, downtime is expensive. While the decision to rewrite software usually is reached because existing software is too tedious or expensive to maintain, it is very helpful to rebuild a system from scratch instead of making modifications to an existing system.
While you are installing, the production system is still running, so there is no downtime. Once you are done with setting up, configuring, and testing the new system, you can switch the two systems, making the newly installed system the production system.
If anything goes wrong, you can switch back to the original system. By switching between the two systems, there is virtually no downtime. Of course, it does not always make sense to perform a full installation of a second system, just to install one patch.
For major updates when migrating a system, the effort is definitely worthwhile. To make this approach of installing a new system actually work, you need a good and up-to-date documentation about how to set up the system, which software to install, and how to configure it.
Striking a Balance In the previous sections, we have learned about basic strategies and procedures for a migration. Like so often, in information technology it is not a good idea to fall from one extreme into the other. You should try and strike a good balance. It is much better to be anticipatory and proactive. This helps you plan ahead and lets you avoid panic reactions when things go really wrong at some point.
Knowing the market trends will help you plan better and recommend a sensible course of action. In real IT environments, there are a lot of influencing factors, and not all of them are technical. Besides the business environment, IT decisions are often influenced by corporate politics.
A fixed budget is an important precondition for a successful ongoing development of an IT environment. You would probably not want to enter a plane that has not been serviced for months, just to cut the costs. Similarly, nobody should entrust their data and business processes to a badly-maintained IT system.
Information technology means accepting changes as an integral part of daily life.
Not only the technology changes, but also the business requirements and, last but not least, the persons involved. Only if you have a solid understanding of the IT systems involved, you can accept permanent changes and adjust to them. You need to know which software runs on the system, how it is configured, and which dependencies there are between the various system modules. The better the documentation, the easier it becomes to make changes.
You can learn an important lesson for system administration - and therefore the migration of a system - from the agile development methods: work in little iterations or steps. This simplifies management and is more productive. Multiple small steps will take you as far as one big step, but should problems turn up, it is easier to isolate them.
Should you really screw something up on the way, it is easier to roll back one little step than to spend hours searching for the mistake. This gives you enough time for quality assurance and lets you make the necessary changes to your software, if need be. You do not always run the latest software in production, but can refine your testing and deployment process over time.
Should a critical update become necessary, you have a proven process that you can rely on. Instead of pushing the necessary migration of your environment farther and farther ahead, until you finally are forced to act, you constantly proceed in small Download from Wow!
This usually gives you enough time to sort out any problems without putting the production system into danger. Regardless of when you migrate, you will run your application in a new environment. This usually requires some modifications of the existing PHP code, to adapt it to the new environment. In the following chapters we will explain the course of action. Chapter 7 introduces you to tools that will help you migrate your code, while Chapter 8 describes in detail the potential problems that can arise when migrating PHP code.
This book only deals with forward migration, which means adapting an existing PHP application to work in a software environment of newer versions. In a backwards migration, the PHP application is adapted to working with environment of older software versions. Though the basic procedure in both cases is similar, we will not deal with backwards migration.
It does not usually make much sense to adapt code that was written for a recent PHP version to work under an older PHP version. There are a lot of aspects to keep in mind when planning and acting out a migration. This chapter deals with the most important aspects that you have to keep in mind when migrating a PHP environment.
These aspects are the platform, operating system, web server, database, PHP itself, the PHP source code and some other aspects that might seem unimportant at first glance, like system configuration, external programs, or interfaces to third-party systems. Figure 3.
The dependencies are shown by arrows. The starting point of our consideration is the platform, meaning the computer architecture of the application we are planning to migrate runs on.
The platformspecific operating system hides the platform details from the applications running on it. There are some important differences between the main operating system families.
When migrating, you should know them, even if you are not planning to switch the operating system family while migrating.
The operating system executes the various applications. For us, the two most important applications are the database and the web server.
The system programs are Download from Wow! Still, there are some differences in behavior between the platforms with these applications, especially the web server. The web server runs PHP, which can access system programs and the database. Strictly speaking, this is not always correct, but depends on how PHP is integrated with the web server. In fact, the most features of the language are provided by extensions. The PHP core itself offers a rather small set of user visible features.
In addition to your own PHP code, you might also use third-party PHP code in the form of libraries, components or frameworks. Usually, you will not have to migrate third-party code, since the manufacturer should have taken care of making their code work under recent PHP versions.