Life without mediators

In a small program, the permissiveness that makes Ruby and other dynamic languages famous acts as a catalyst that hugely accelerates its development, but as the program grows, this same permissiveness becomes a downside as every module is allowed to call every other module, and without careful curation a sane model for organization won’t emerge naturally. In the worst case, every module will be calling every other module, and the program’s flow becomes a rat’s nest of spaghetti code that can only be made sense of by stepping through it with a debugger.

I’ve previously suggested the use of mediators to overcome this problem, a pattern that encourages all major program flows to be encapsulated in PORO classes that read like lists of instructions, and control the interaction between primitive types like database models.

It’s an interesting experience now developing in the largest Ruby codebase I’ve ever seen (~1M lines of code), and one where the use of a pattern like mediators never developed. Domain logic can be, and therefore is, implemented in any number of different places:

  • In models.
  • In API endpoints.
  • In base classes (on either models, endpoints, or other).
  • In mix-ins (on either models, endpoints, or other).
  • In various static utility classes.
  • In internal gems added as dependencies.

But the larger problem is that it’s rarely just one of these; instead, logic gets strewn amongst all of them in dozens or hundreds of small complex interactions across countless small modules.

Another side effect is that because so much domain logic is locked up in API endpoints, it’s common to rely heavily on functional tests that use rack-test to make mock HTTP calls to adequately test it. It’s not the end of the world, but has the effect of making debugging problems inordinately difficult (because exceptions come back as status 500 responses), and makes the development cycle very slow due to the extra overhead of the HTTP stack in every test case.

Possibly the most important thing you can do when starting a new Ruby project is to get a handle on the mediator pattern (or one like it) right away because it’s very work intensive to get backported. It’s also a somewhat unnatural direction given that the idea is not present in major frameworks like Rails, and more generally, is also missing from MVC.

Life without mediators

April 27, 2016

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