Terminology

This is a list of definitions used throughout Gazebo. Some of them are important for downstream users, while some of them are only interesting to developers touching the source code.

• World: The complete description of a simulation, including all robots, movable and static objects, plugins, scenes and GUIs. It corresponds to an SDF <world> tag.
• Entity: Every "object" in the world, such as models, links, collisions, visuals, lights, joints, etc. An entity is just a numeric ID and may have several components attached to it. Entity IDs are assigned at runtime.
• Component: Adds a certain functionality or characteristic (e.g., pose, name, material, etc.) to an entity. Gazebo comes with various components ready to be used, such as Pose and Inertial, and downstream developers can also create their own by inheriting from the BaseComponent class or instantiating a template of Component.
• System: Logic that operates on all entities that have a given set of components. Systems are plugins that can be loaded at runtime. Gazebo ships with various systems, and downstream develpers can create their own systems.
• Entity-component manager (ECM): Provides functions for querying, creating, removing and updating entities and components. See the whole API here.
• Level: Part of a world, defined by a box volume and the static entities inside it. An entity can be present in more than one level, or in none of them. Levels may overlap in their volumes and may be far from each other.
• Buffer zone: Each level has a buffer zone, which is an inflation of the level's volume outside its boundaries used to detect when a performer is about to come into the level, or has left and is far enough away to exclude the entity from the level.
• Performer: All simulation entities which may change levels during the simulation, such as robots, actors and dynamic models. A performer only has meaning if there are levels.
• Global entities: Entities which are present on all levels, such as the sun, ground plane, heightmaps, etc. These entities will be duplicated across all simulation runners.
• Default level: Level which handles all entities that are not within any other levels.
• Network manager: Controls the flow of information in a simulation distributed across processes.
• Primary / secondary network manager: For each world that is split across multiple managers, there is exactly one primary network and one or more secondary runners. The secondary runners are running a set of levels of the world, while the primary runner is keeping all secondaries in sync. Worlds that are not split across runners don't have a primary runner.
• Event manager: Manages events that can be sent across systems and the server. Plugins can create and emit custom Events and / or emit / listen to events from Gazebo.
• Simulation runner: Runs a whole world or some levels of a world, but no more than 1 world.
  • It has a single ECM with all the entities and components relevant to the levels / world / performer being simulated.
    • It's still an open task to support multiple <worlds> in parallel.
  • It has an event manager.
  • It has a network manager, if simulation is distributed.
  • It loads up a set of systems.
• Server: Gazebo Sim's entry point. It's responsible for loading an SDF file and instantiating a simulation runner per world.