Sync dependencies

[lippserd]

At the moment dependencies are not synchronized in Redis and the database and there is no idea for the storage scheme yet.

Since dependencies represent trees (or even graphs?), They can be difficult to store in the database.

At the moment I can think of the following ways to store the dependencies in the database:

• Associate parent
• Nested set
• Materialized path
• Adjacency list / matrix
• Closure table

I'm sure there's more.

All schemes must be compared in terms of complexity and performance when writing and reading.

I think it also makes sense to store the parent dependencies directly for the objects. This would allow us to display these in the detail areas in Web without much effort.

[julianbrost]

Another idea: don't sync dependencies to the relational database at all but instead only keep them in Redis. Whenever Web wants to display something about dependencies, the required graph algorithms are implemented as Lua scripts and executed on the Redis server.

Only downside with this: you couldn't use the dependency information in filters then (trying to support this would probably require some nasty work merging result sets from Redis and SQL), but things like showing all (reverse) dependencies of a checkable or rendering nice dependency graphs starting from a checkable should work nicely.

There could also be some combined solution where we think about what filtering on dependencies should be supported, write information specifically for these queries to the relational database and use Lua for everything else.