Php – Constant Interface antipattern – PHP

It can be quite tricky to handle numbers with PHP/MySQL. If you use decimal(10,2) and your number is longer or has higher precision it will be truncated without error (unless you set proper mode for your db server).

To handle large values or high precision values you can use library like BCMath it will allow you to make basic operation on the large numbers and keep required precision.

I'm not sure what exactly calculations you will make but you have to also keep in mind that (0.22 * 0.4576) + (0.78 * 0.4576) will not equal 0.4576 if you will not use proper precision through the process.

Maximum size of DECIMAL in MySQL is 65 so it should be more than enough for any purpose. If you use DECIMAL field type it will be returned as string regardless of the use of an ORM or just plain PDO/mysql(i).

How should the data be stored in the database? column type? size?

DECIMAL with precision you need. If you are using exchange rates then you will need at least four decimal places

How should the data be handling in normal addition, subtraction. multiplication or division?

Use BCMath to be on save side and why using float may not be a good idea

When should I round the values? How much rounding is acceptable if any?

For monetary values normal two decimal places are acceptable but you may need more if for example you are using exchange rates.

Is there a difference between handling large monetary values and low ones?

Depends on what you mean by large. There is definitely a difference between handling numbers with high precision.

First, lets understand what was the issue: The Staple job knows about many more methods which it doesn't use today, (e.g., the print method) but which are available for it to use, should it want to. However, the Job class "thinks" that the Staple class will be "a good citizen" and never use the print method at all.

There are many potential big issues here - For some reason, the Staple job may start using the print method - by accident or intentionally.

Then down the road, either any changes to the print method may go untested,
OR, any changes to the print method will trigger a regression test in the Staple job also,
AND, any impact analysis for changes to the print job will necessarily involve impact analysis of the staple job too.

This is just the issue of Staple knowing about the print functions. Then there's the case of the Print job knowing all about stapling functions. Same problems.

Very soon, this system would reach a point where any change will require a full blown impact analysis of each module and a full blown regression test.

Another problem is that today, all jobs which can be printed can be stapled, and vice versa on this particular printer.

However, tomorrow, there could be a need to install the same firmware on a device that only prints or only staples. What then? The code already assumes that all Jobs are printable and stapleable. So any further granular breakdown / simplification of responsibilities is impossible.

In more recent terms, imagine a class called "AppleDevice" which has functions for MakePhoneCall as well as PlayMusic. Now your problem is while you can easily use this on an iPhone, you cannot use it for an iPod since the iPod cannot make phone calls.

So, the issue is not that the Job class is all-powerful. In fact, that's how it should be, so that it can act as a common link in the entire "workflow" where someone may scan a job, then print it, then staple it etc. The problem is that the usage of all its methods is not restricted. Anyone and everyone could use and abuse any method whenever they want to, thus making the maintenance difficult.

Hence, the Dependency Injection approach of only telling users "exactly what they need to know, and nothing more" ensures that calling modules only use the code that they are meant to. A sample implementation would look like:

interface IStapleableJob { void stapleYourself(); }
interface IPrintableJob { void printYourself(); }
class Job implements IStapleableJob, IPrintableJob {
....
}

class Staple {
  public static void stapleAllJobs(ArrayList<IStapleableJob> jobs) {
    for(IStapleableJob job : jobs) job.stapleYourself();
  }
}

class Print {
  public static void stapleAllJobs(ArrayList<IPrintableJob> jobs) {
    for(IPrintableJob job : jobs) job.printYourself();
  }
}

Here, even if you pass a Job object to the Staple and Print methods, they dont know that its a Job, so they cannot use any methods that they are not supposed to. Thus, when you make any changes to a module, your scope of impact analysis and regression testing is restricted. That's the problem that ISP solves.