System order

Lightyear provides several SystemSets that you can use to run your systems in the correct order.

The main things to keep in mind are:

• All packets are read during the PreUpdate schedule. This is also where all components that were received are replicated to the Client World.
• All packets are sent during the PostUpdate schedule. All messages that were buffered are then sent to the remote, and all replication messages (entity spawn, component updated, etc.) are also sent
• There are 2 SystemSets that you should interact with:
  • BufferInputs: this is where you should be running client.add_inputs() so that they are buffered and sent to the server correctly
  • Main: this is where all your FixedUpdate Schedule systems (physics, etc.) should be run, so that they interact correctly with client-side prediction, etc.

Here is a simplified version of the system order:

---
title: Simplified SystemSet order
---
stateDiagram-v2

   classDef flush font-style:italic;
   
   ReceiveFlush: Flush

   
   PreUpdate --> FixedUpdate
   FixedUpdate --> PostUpdate 
   state PreUpdate {
      Receive --> ReceiveFlush
      ReceiveFlush --> Prediction
      ReceiveFlush --> Interpolation
   }
   state FixedUpdate {
      TickUpdate --> BufferInputs
      BufferInputs --> Main
   }
   state PostUpdate {
       Send
   }

Full system order

---
title: SystemSet order
---
stateDiagram-v2

   classDef flush font-style:italic;
   
   SpawnPredictionHistory : SpawnHistory
   SpawnInterpolationHistory : SpawnHistory
   SpawnPredictionHistoryFlush : Flush
   SpawnInterpolationHistoryFlush : Flush
   SpawnPredictionFlush : Flush
   SpawnInterpolationFlush: Flush
   CheckRollbackFlush: Flush
   DespawnFlush: Flush
   ReceiveFlush: Flush
   FixedUpdatePrediction: Prediction
   
   PreUpdate --> FixedUpdate
   FixedUpdate --> PostUpdate 
   state PreUpdate {
      Receive --> ReceiveFlush
      ReceiveFlush --> Prediction
      ReceiveFlush --> Interpolation
   }
   state Prediction {
      SpawnPrediction --> SpawnPredictionFlush
      SpawnPredictionFlush --> SpawnPredictionHistory
      SpawnPredictionHistory --> SpawnPredictionHistoryFlush
      SpawnPredictionHistoryFlush --> CheckRollback
      CheckRollback --> CheckRollbackFlush
      CheckRollbackFlush --> Rollback
   }
   state Interpolation {
       SpawnInterpolation --> SpawnInterpolationFlush
       SpawnInterpolationFlush --> SpawnInterpolationHistory
       SpawnInterpolationHistory --> SpawnInterpolationHistoryFlush
       SpawnInterpolationHistoryFlush --> Despawn
       Despawn --> DespawnFlush
       DespawnFlush --> Interpolate
   }
   state FixedUpdate {
      TickUpdate --> BufferInputs
      BufferInputs --> WriteInputEvent
      WriteInputEvent --> Main
      Main --> ClearInputEvent
      Main --> FixedUpdatePrediction
   }
   state FixedUpdatePrediction {
      PredictionEntityDespawn --> PredictionEntityDespawnFlush
      PredictionEntityDespawnFlush --> UpdatePredictionHistory
      UpdatePredictionHistory --> IncrementRollbackTick : if rollback
   }
   state PostUpdate {
        state Send {
            SendEntityUpdates --> SendComponentUpdates
            SendComponentUpdates --> SendInputMessage
            SendInputMessage --> SendPackets
        }
        --
        Sync
   }