Plan Management for Robotic Systems

Useful and useless tasks

So far, we have seen that the plan is basically a bunch of tasks and events that are related to each other.

What this page will detail is how Roby knows what is needed by the system and what is not. And what it does with the garbage …

Missions and permanent tasks

In any Roby plan, a certain, usually very limited, number of tasks are marked either as missions or as permanent tasks. The missions are the current robot’s main activities (i.e. what it should do). The permanent tasks are the tasks that the system’s controller wants to keep around, but which are not essential to its function. This marking is either done manually, or automatically by some of the Roby interfaces when the system is given a high-level command.

Garbage collection

At the end of each execution cycle, Roby looks at all the tasks in the system’s plan. Tasks that are needed by a mission or a permanent tasks are left alone. Everything else is stopped automatically and

Roby determines what is “needed” and what is “not needed” by looking at the task relations. Unless specified otherwise in the relation declaration, relations all express a “needed by” relationship, i.e. the source of a relation edge needs the target of that edge to function properly.

In other words, Roby follows all the relations, from the missions and permanent tasks, and marks all tasks it finds as “useful”. Everything else is garbage collected.

The stopping order is top-down, i.e. the toplevel tasks (that are not a child in one of the task relations) is stopped. Once that task is stopped, it is removed from the plan and the process continues until there is nothing to stop.

In general, the garbage collection mechanism allows to manage only the high-level actions (i.e. the missions), leaving the rest of the tasks in the system to Roby.


What we have seen so far:

  • the concept of reactors
  • the structuration of tasks and events in plans
  • how these plans get executed
  • and how, in Roby, one can only manage the high-level mission tasks, leaving the cleanup to automatic garbage collection handling

The next step is to understand how Roby represents and handles errors