About the dependency mechanism in the rmf_traffic package

[acf986]

In the rmf_traffic package, we have this core class "Route", which have the following function:

  Route& add_dependency(
    CheckpointId dependent_checkpoint,
    Dependency dependency);

It says, it will make this Route's certain checkpoint to "depend on" the other participant's checkpoint.
What is the physical meaning of this "dependency"?

Edit:
I have checked the code in the Crawler::ignore function, it seems that the "dependency" means that, if the other agent have moved passed the dependent waypoint, we could ignore the collision checking? Is my understanding correct? A little bit confused now.

[mxgrey]

I think the best way to explain it is to share this slide.

For a short overview, a "traffic dependency" means that one robot intends to wait at a certain waypoint (let's say B2 to match the slide) along its route until another robot has reached a certain checkpoint (let's say A3 to match the slide). We encode that explicitly into the traffic schedule because if there is a significant delay in the robot that is being waited on, that could be interpreted as a traffic conflict. For example, if we had a purely time-based system and the schedule says that Robot A will reach A3 at t=10 and therefore Robot B will wait at B2 until t=10, that would be fine if Robot A has no delays.

However, what if in real life Robot A is only just reaching A2 at t=10? The schedule would see that as a conflict and demand that the robots re-negotiate their routes. That renegotiation is a waste of effort if we can just tell the schedule that Robot B will wait at B2 until Robot A reaches A3 regardless of the precise timing. Then the schedule knows that delays in Robot A prior to A3 will not create any conflicts with Robot B after B2.

Algorithmically this means that when detecting traffic conflicts we can have the conflict detector ignore any "collisions" it detects between

• Robot B's path that come after B2, and
• Robot A's path that come before A3

We would still check for collisions that involve Robot B's path before B2 or Robot A's path that come after A3.

[acf986]

Thanks very much for your elaboration!

[acf986]

And may I also check with you, the waiting logic is implemented in which part of the rmf? Just didn't find them in rmf_traffic.

[mxgrey]

The main role of rmf_traffic is to implement the planning algorithms that are used by the system. It doesn't take care of any details for executing the plan. However, the rmf_traffic::schedule::Viewer does provide a watch_dependency function which is used by downstream libraries to trigger a reaction when a robot reaches a checkpoint.

Since RMF is a highly distributed system there isn't a single point to refer to and say "Here is where waiting is implemented" but perhaps the most relevant module would be the WaitForTraffic event in rmf_fleet_adapter which uses the watch_dependency function to manage the waiting behavior.