deadlock detection problem mysql/php problem

[yeahman45]

here my problem:

i am developing a web application allowing you to create modules(a piece of code to execute) and execute them. One particularity about the application is that you can make compositions of modules. i.e. a module can be composed of several other modules. I have designed a scenario below so that you can understand more easily:

i have 8 modules: 1,2,3, 4,5,6,7 and 8

I have my initial setup (Scenario 1)



Module 1 is composed of module 2 and 3.
Module 2 is composed of module 4 and 6.
Module 5 is composed of module 1 and 7.
Module 8 is composed of module 5

Execute module 1:
Now,e.g., if i execute module 1... it will execute module 2 + module 3.

As module 2 is composed of module 4 and 6, the latters will also be executed.

This is the execution chain. In the diagram, the left hand side shows all the modules that are actually executed


The problem
In scenario 1, everything is ok and executes smoothly... but in scenario 2, i add module 5 to module 1.

When you execute module 1, it will execute module 2, 3 and 5. As module 2 is a composition, it will also executes module 4 and 6 found in module 2. And as module 5 is a composition too, it will execute module 1 and 7 found in 5.

The problem is that the module 1 found in module 5. It will also execute.... you can see that this will result in an infinte loop!!! kind of a deadlock!

same thing happens in scenario 3 where i add module 8 to module 1. Module 8 has module 5 as child and the latter has module 1 as child. This will also result in a deadlock!!

What is the solution??

My problem is how to detect such deadlocks?? When i am composing module 1, e.g., i should not be given the option to add module 5 or module 8 as they would cause a deadlock....

In my interface, when i am composing/editing a module, i am giving a list of all modules available (1,2,3,4,5,6,7,8) which i can click and add to the module i am composing/editing. I need to filter/hide all the modules that will cause a deadlock.

database structure
Below is my database structure:



The values in the tables are for scenario 1.

and i am using javascript/html, php and mysql. I have idea how to efficiently detect these deadlocks... can it be done all in a query? (e.g. when i am editing module 1, i write a query to fetch only modules that will not cause deadlocks)

Your advices and tips are most welcomed.. thx for reading this lengthy topic.

[yeahman45]

just realised i haven't attached the pics properly; here they are:

1. scenarios
http://postimage.org/image/5cspmguc/

2.database
http://imageshack.us/photo/my-images/840/tablesq.png

[nuzzle]

My problem is how to detect such deadlocks?? When i am composing module 1, e.g., i should not be given the option to add module 5 or module 8 as they would cause a deadlock....

Strictly you don't have a deadlock. That terms is used for a certain concurrency problem.

To avoid cycles you must ensure that compositions always stay a DAG (Directed Acyclic Graph). So when a composition is defined it should be a DAG. And when it's changed you need to establish that it's still a DAG otherwise you must reject the change.

http://en.wikipedia.org/wiki/Directed_acyclic_graph

http://allisons.org/ll/AlgDS/Graph/DAG/