Welcome to The Specs Book, an introduction to ECS and the Specs API. This book is targeted at beginners; guiding you through all the difficulties of setting up, building, and structuring a game with an ECS. You can find the API docs on

Specs is an ECS library that allows parallel system execution, with both low overhead and high flexibility, different storage types and a type-level system data model. It is mainly used for games and simulations, where it allows to structure code using composition over inheritance.

Additional documentation is available on

You don't yet know what an ECS is all about? The next section is for you! In case you already know what an ECS is, just skip it.

What's an ECS?

The term ECS is a shorthand for Entity-component system. These are the three core concepts. Each entity is associated with some components. Those entities and components are processed by systems. This way, you have your data (components) completely separated from the behaviour (systems). An entity just logically groups components; so a Velocity component can be applied to the Position component of the same entity.

ECS is sometimes seen as a counterpart to Object-Oriented Programming. I wouldn't say that's one hundred percent true, but let me give you some comparisons.

In OOP, your player might look like this (I've used Java for the example):

public class Player extends Character {
    private final Transform transform;
    private final Inventory inventory;

There are several limitations here:

  • There is either no multiple inheritance or it brings other problems with it, like the diamond problem; moreover, you have to think about "is the player a collider or does it have a collider?"
  • You cannot easily extend the player with modding; all the attributes are hardcoded.
  • Imagine you want to add a NPC, which looks like this:
public class Npc extends Character {
    private final Transform transform;
    private final Inventory inventory;
    private final boolean isFriendly;

Now you have stuff duplicated; you would have to write mostly identical code for your player and the NPC, even though e.g. they both share a transform.

This is where the ECS comes into play: Components are associated with entities; you just insert some component, whenever you like. Also, you only define the functionality once: One System, taking a Force and a Mass - it produces a Velocity.

In fact, an entity does not even own the components; it's just

struct Entity(u32, Generation);

where the first field is the id and the second one is the generation, used to validate if the entity hasn't been deleted. The components are stored separately, which also has the advantage of being more cache efficient.

Another way you can think of an entity is that it just groups together a group of components. We'll see how this works in practice later - it allows us to just pick sets of components which include both A and B.

Where to use an ECS?

In case you were looking for a general-purpose library for doing it the data-oriented way, I have to disappoint you; there is none. ECS libraries are best-suited for creating games; in fact, I've never heard of anybody using an ECS for something else.

Okay, now that you were given a rough overview, let's continue to Chapter 2 where we'll build our first actual application with Specs.