Entity Component Systems - component cross-lookup vs denormalization

Question

This is my first project using ECS design and I come from a RDMS background for reference.

I'm building a ecology simulation that spans multiple planets, with the following ECS structure:

Entities & Components

• Planet (Components: Year)
• Biome (Components: Planet (Link to planet this biome is in))
• Species (Components: Population, Growth Rate, Biome (Link to biome they reside in))

Systems

• TimeSystem (Adds +1 to the year component every N ticks)
• PopulationGrowthSystem (Updates the Population component for each species if a year has passed on the planet)

Question:

I'm struggling with how the PopulationGrowthSystem should filter which Species Populations to update on each iteration.

Iterating through each Population component requires me to do a Species Entity->Biome Component->Planet Component lookup to check the year, which is terribly inefficient.

I've considered de-normalizing by putting something like year_last_updated in the Population Component, but it doesn't feel clean.

I've also considered just updating Population components every N-ticks, but Year needs to always be updated before the population is updated, and to my reading on proper ECS design, I shouldn't depend on the Systems being run in order.

I feel like I'm missing something obvious in how I'm approaching this.

Answer 1

If I were to do this I would consider two approaches. It sounds like you considered the second option already but maybe the first one suits your design better.

1. Time Manager takes responsibility

I would add EndYear() method to the PopulationGrowthSystem and update like this.

TimeManager::update()
{
  for (component : Components) 
    component.iterate();
  if (endYear) 
    for (populationComponent: PopulationComponents) 
      populationComponent.endYear();
}

2. Components take responsibility

In this case the component would take a parameter with the world state. I would probably use a struct with all the data in the game and pass a parameter by reference.

TimeManager::update()
{
  g_worldData.endYear = endYear;
  for (component : Components) 
    component.iterate(g_worldData);
}

PopulationComponent::iterate(WorldData& data)
{
  if (data.endYear) ...
}