Entities, components & the law.

BCS inherits the Entity-Component-System split — identity apart from data apart from behaviour — and sharpens it on the branded id. An entity is just a 14-byte id; it carries no record of its own. Its components are plain data, property bundles in the tables of the last dive, and crucially they are only data — there are no behaviour modules attached to an entity, no class hierarchy of code. This is what lets an archetype be data too: an archetype is an ARC entity whose value is a property bundle, optionally extending one parent via a single :extends link, and an instance carries its archetype’s id plus an overrides map. The composed view is not a class lookup; it is a fold computed at read time — base bundle first, each descendant merged after, the instance’s overrides merged last, right-most wins, at most one :extends deep. Inheritance becomes a `Map.merge` reduction over data, with the resolver taking a fetch function so the boundary stays wherever the definitions live. The payoff is that everything about an entity — its identity, its current state, its template lineage — is a value you can store in a table, ship in a message, and replay from a log. There is nothing else to serialize, because there is nothing else.

The single rule that makes the whole thing compose is about boundaries. A system is a process that owns its private state, gated on a branded namespace, and the only values that cross its boundary are identities and messages about identities — never a copy of another system’s data. So a Broadcast does not hold the recipient list; it holds the RGP id and resolves the audience from the group that owns it. A relation like “portfolio holds asset” is not an id list embedded in the portfolio; it is a row keyed by the pair of names {subject, object} in an owning EdgeStore, which maintains its own forward and reverse indexes and exports neither. The figure shows why the rule is not pedantry: reference by id and there is exactly one copy of the audience, so an edit to the group is seen by everyone who holds its id, instantly and for free; embed a copy and you have created a second source of truth that goes stale the moment the original changes — the dual-write divergence of Chapter 12, reborn at the data-model layer. The law is the same instinct the whole platform runs on: a branded id is the unit that crosses every boundary — between runtimes (Chapter 10), between replicas (Chapter 12), and now between systems — and the data stays home with its owner. That is BCS, and it is the shape the durable floor was built to carry.