Code cleaning on sdx_controller/messaging subfolder

[italovalcy]

As discussed on PR #223 (#223 (comment)) there is opportunity to review the code under sdx_controller/messaging subfolder and remove unused code. I'm opening this issue here so we can discuss this and see if it makes sense or not to keep the code there.

Cc'ing @congwang09, @YufengXin and @sajith to provide comments.

[sajith]

@italovalcy Thank you for raising this. I agree, that code should be refactored and simplified. I think these are the steps we should take:

• Removing unused code would be a good first step.
• Reviewing, updating, documenting, and adding some tests for the remaining code would be the the next step.
• There's some tight coupling of functionality, such as message handling and processing is done in a single method. Removing the coupling, and refactoring it into smaller, simpler methods would be useful.
• We also create a thread within a thread (here). I'm not sure if we need it anymore (it was added before sdx-controller became a "proper" WSGI app). We should figure out the correct way.

Broadly, I think we just need a subscribe() method (which receives messages and pushes them to a queue for subsequent processing) and a publish() method (which sends messages to LCs). We could hash out the details during refactoring.

If done right, we can reuse the final subscribe() and publish methods in sdx-lc as well. I was thinking we could pull the code out into a library ("sdx-base" perhaps, along with other assorted utility methods that both sdx-controller and sdx-lc can share), but perhaps that might be overkill, if the amount of final code is pretty small.

[YufengXin]

into the 'datamodel' repo? transformed to be the 'sdx-base'? just a thought.

[sajith]

Not sure. We should do the refactoring first, and then evaluate if we need a library at all.