Relationships clarification

[KieranFleckney]

In the docs on Relationships it says:

Adding, removing or updating a link does not cause archetype fragmentation.
In flecs every relationship between two entities creates an individual archetype only containing a single entity / component.
So each entity relationship allocates ~ 1000 bytes required by the archetype stored in the heap for each link.
The more significant performance penalty is the side effect in queries. Many archetypes need to be iterated if they are query matches.

Changing an entity link does not cause a structural change.
In flecs a new archetype needs to be created and the old archetype gets empty.

When adding a 'ILinkComponent' it does seems to change the the archetype but when you add a 'ILinkRelation' it does not. Now from a my point of view this makes sense to me but a little contradiction from what the docs say. As I know from the Relation docs it says this:

Info
Relations are not components but both have similar interfaces too add, remove or access them.
Relations are not stored within archetypes to avoid archetype fragmentation.

So I guess I just want to clarification on what is the real truth and maybe have the docs amended if so.

[friflo]

hi @KieranFleckney,

The "hard" stuff about this topic is the terminology.
I tried to be conform to the term relation used for relational databases.
A relation - my understanding - is a tuple of two things.

Relationship - is a directed link between two entities. So relationship is more specific.
Both things of a relationship (a relation) are entities.

If only one thing of a relation is an entity. It is not a relationship.

In this ECS relationships are represented by either ILinkRelation or ILinkComponent.

1. ILinkComponent An entity can have only one link component per type at a time.
2. ILinkRelation An entity can have multiple link relations - one per target entity.

ILinkComponent are components and can cause fragmentation as all other components.
ILinkRelation are not components. They are using a separate data structure and cause no fragmentation.

Not sure if this clarify your post.

[KieranFleckney]

Hi @friflo,

Thank you for the response. Yeah that makes sense and I think I already understand so I am glad it clarified for me. But also glad you confirmed that ILinkComponent can cause fragmentation which was my assumption anyway but just little miss leading by the docs and the comparison vs Flecs.

appreciate the reply and the clarification

[friflo]

Thx for your input!
I updated the docs:
friflo/ECS-wiki@622b301