Network synchronization intricacies

OnUserEnter & OnUserLeave, entities availability

Our user

All components (PBIdentityData, PBAvatarEquippedData, PBAvatarBase) related to the host identity are present on the scene on start-up.

The original necessity of it is described here.

Thus, these components are present throughout the whole lifecycle of the scene.

⚠️ It may lead to the unexpected behaviour from the JS perspective: Host Components will be always present, no matter if the user inside the scene or outside!

Remote Entities

In turn Remote Entities function completely differently:

• If the scene is not the current scene (the host is not standing there) remote entities can't be present on the scene (in CRDT state)
  • It might be confusing because visually they could be there:
    • In worlds as the same LiveKit room is used for the whole world
    • In Genesis City as remote entities are retrieved from an Island which is not related to the current room
• When the host enters the scene all remote participants standing within the bounds of that scenes are propagated as they were just entered that scene:
  • Corresponding PBIdentityData, PBAvatarEquippedData, PBAvatarBase added
  • As a result the initial state of the scene is broadcasted through the Scene LiveKit room
• When the host is within scene boundaries and other players enter and leave the scene it works as expected:
  • When the remote participant enters the corresponding components are added
  • When leaves - components are removed

⚠️ Entities themselves are never deleted explicitly: their range for remote participants is reserved.

Why it works this way?

• Scene Synchronization is driven through Scene LiveKit room
• Host can only have one connection to the Scene room at a time so the client connect to the room of the scene the host is currently located on
• So even if remote participants stand on non-current scenes we can't synchronize scene state that is caused by users' actions
• Apart from Scene LiveKit room there is an Island room
  • We don't use Island for scene synchronization as it leads to broadcasting of irrelevant data and it can consume all bandwidth
  • However, we use it in unity-renderer as there is not concept of Scene rooms. It can lead to additional confusion if two versions are compared face-to-face
  • But we use Island to display other participants (as otherwise the world would be empty and users would pop-up out of nowhere when host enters the scene)