Dangers of mutual dependencies

2009-03-05 11:11 am 1 Comment , and

Much like most people, I try work out my class dependencies through a top-down 'waterfall'-ish approach. By attempting this, I think allows me to keep the structure very clear and understandable.

Example of top-down dependency approach
excuse my non-existent UML skills

In this example the Ingredient class (and subclasses) is Never aware of any Recipe classes, but only the other way round.

I try to apply the same model to instantiated objects and packages (groups of classes). When an object encapsulates another object, I attempt to make sure the sub-object object is not aware of the parent. When I design packages, I attempt to make sure 2 packages don't require 'each other'.

An example where this could be a problem is the following. Say, I have a 'Database' package. I want to log every database error to a 'Log' package. The 'Log' package has a couple of implementations, such as 'Log_File', 'Log_Syslog', but now I added 'Log_Database' to log any problems to the database.

Mutual dependency

Strictly speaking these two packages can no longer be separated and will always have to be used/downloaded together. As a bonus a database-error could occur while logging, resulting in an endless loop (or segmentation fault if you're using PHP).

Another example of a mutual dependency:

  1. file1.php:
  2.  
  3. <?php
  4.  
  5. include 'file2.php';
  6.  
  7. ?>
  8.  
  9. file2.php:
  10.  
  11. <?php
  12.  
  13. include 'file1.php';
  14.  
  15. ?>

You get the idea ;)

However, these types of situations are sometimes simply unavoidable (that's why we have include_once). When they are needed, they should be implemented with care and consideration. The problem with the Log class could be fixed if the Log_Database is aware of errors thrown by itself, and this could be repackaged by creating by separating the Log_Database in a new package, depending on both the 'Connection' and 'Log' classes.

Editors note: this post was part of a much larger article around designing plug-in systems, but I lost inspiration and decided to post just this part.

PHP and circular dependencies

2007-08-14 10:44 pm 3 Comments , and

Today I noticed the strangest bug in PHP. Apparently there's a problem with circular dependencies and interfaces.

This is how the problem occurs:

  1. <?php 
  2.  
  3.   // this is class1.php
  4.  
  5.   require_once 'iclass.php';
  6.  
  7.   class class1 implements iclass {
  8.  
  9.   }
  10.  
  11. ?>
  1. <?php
  2.  
  3.   // this is class2.php
  4.  
  5.   require_once 'class1.php';
  6.  
  7.   class class2 extends class1 {
  8.  
  9.   }
  10.  
  11. ?>
  1. <?php 
  2.  
  3.   // this is iclass.php
  4.  
  5.   require_once 'class2.php';
  6.  
  7.   interface iclass {
  8.  
  9.      function setSomething(class2 $object);
  10.  
  11.   }
  12.  
  13. ?>

Now, if you run class1.php, you will get the following error:

  1. Fatal error: Class 'class1' not found in /home/filemobile/testscript/class2.php on line 5

I know this is not proper OOP, but its still a strange error. Normally PHP is pretty good at these weird structures ;). Weirdest thing was that it happened for me on one server, but not the other.

There is a bug marked bogus for this, which is understandable. It would be great though if the PHP could somehow throw an error thats a bit more helpful =P. Its a strange error, especially since I could only reproduce it with this structure, and not with for example just 2 classes with circular dependencies.

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My name is Evert, and I've been writing semi-regularly on this blog since 2006.

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