ZMP : PUB SUB pair for feedback loop

[asmodehn]

A zmq (x)pub/(x)sub pair can make a feedback loop. This is the complementary concept of service that we need, with inverted control flow, and possible multicast + high rate + lossy comm. With this we should be able to interface topics from ROS without much problems... We need to do a first implementation of it...

Two tests of this can be :

• a node that is null element : like a service provider can call another service and return the answer ( and therefore have no effect, a node when connecting with pub/sub should be able to do another pub/sub with another node and virtually have no effect.
• A node that throttle the caller to the speed of the listener... Communication optimization ( similar to FPS when displaying on screen / talking to graphic layer )

[asmodehn]

Other things to keep in mind : service is a simple concept. We probably needs something simpler than pub sub... there also might some complement concept to service.

We also need to provide a solution if possible for traditional distributed computing issues :

• https://en.m.wikipedia.org/wiki/Virtual_synchrony
• partial order of events ( interval vector clock )

Remember we need total order for implementation of global state ( ros params ) but probably not for topics. However we want consistent messaging order and maybe even prediction.

Promise Theory is also something to keep in mind.