Synchronizing Worlds

Synchronizing Worlds#

This document explains how worlds are synchronized across multiple instances, threads, or processes. It answers the question:

How can I synchronize worlds when I do not have access to the concrete memory pointer?

For all synchronizations, ROS2 topics are used to communicate changes in a peer-2-peer like network. The following ROS messages are relevant to understand the synchronization:

  • MetaData.msg

  • WorldState.msg

  • WorldModelModificationBlock.msg

  • WorldModelReload.msg

You can build the messages for ROS2 by linking it to your ROS2 workspace.

In the semantic world package, the following classes and modules are needed to understand this document:

Modules:

Classes:

How it works#

The world state is synchronized whenever the state_change_callbacks of a World are called by publishing the changed free variables. The details are found in semantic_world.adapters.ros.world_synchronizer.StateSynchronizer.

The changes to the world model are a bit more complicated. Conceptually, every instance of the World keeps track of atomic modifications done to it in World._atomic_modifications. Atomic modifications are changes to the world, that, if replayed, produce the same world structure. Atomic modifications cannot be split further and hence must not call other atomic modifications. When the model_change_callbacks are triggered, the latest changes to the world are published and repeated by the other subscribers. The details are found in semantic_world.adapters.ros.world_synchronizer.ModelSynchronizer.

If you ever have the case that you make changes to a world that are not repeatable via this mechanism or just want every process to load a new world, you can use the semantic_world.adapters.ros.world_synchronizer.ModelReloadSynchronizer to force all worlds subscribed to that to do so. The details are found in semantic_world.adapters.ros.world_synchronizer.ModelReloadSynchronizer.

Expanding Modifications#

If you want to expand the capability to communicate changes to the world’s model via ROS2 topics, you have to check out the semantic_world.world_modification module. In there you find different ways of communicating different changes to the world via data structures. This is not trivial, since ROS topics cannot communicate data structures that have many-to-one relationships easily. For instance, when a semantic_world.world_entity.Body is removed from the world, this must not be communicated by sending the entire body data around. Instead, every process needs some way to identify this body in their memory and remove it. Hence, the semantic_world.world_modification.RemoveBodyModification just takes the name of the body and publishes a call to remove the body with this name.

Why JSON?#

Due to the limited capabilities of ROS2 communication, it is not trivial to reflect the definitions and mechanisms of the classes of semantic world in ROS2 messages. If you choose a dedicated message for each class, you get issues with polymorphism, many-to-one references and back-references. Furthermore, maintaining the ROS2 messages when the datastructures change is complicated. JSON provides an easy fix to some of these problems.

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