Subscription

In one line: engine state replicated outward to remote clients, each session seeing what lies around it, and their typed commands drained back into the tick.

When the consumer of engine state is remote — a game client, a browser, a bot, another process — Subscriptions carry it there and keep it current. A client mirrors "the characters near my player" without querying, and the application writes no change detection, encoding or networking.

It is one engine feature with four moving parts:

• What is visible — a projection per archetype: which fields travel, quantized how, grouped how, and which are private to the entity's owner. Declared once, by attributes on the data or by a builder call.
• Who sees what — a profile per kind of viewer: the whole world, a sphere around a point or an entity, a footprint the client sends, plus optional per-tile counts. Beyond geometry, a profile may hold whole groups — entities sharing a key, such as a town's buildings or a player's listings — and a session never receives an entity its visibility mask excludes. A session is bound to one profile.
• What changed — push replication: after a system writes a replicated field it calls Replicate(cluster, slot). The engine pushes spawns, destroys, moves and migrations on its own, compares each pushed entity's quantized values with what it last sent, and encodes a change once, whatever the number of sessions.
• What comes back — typed commands from clients, rate-limited and role-checked, delivered to systems in the next tick; and events the server emits to the sessions they concern.

Replication runs as an engine track after the tick fence and publishes after the tick's durability flush, so a client never sees a state the database could lose. Clients decode against a catalog sent at connection, never against C# layouts, so a browser is as much a client as a .NET one.

⚠️ Not database replication. Subscriptions send a view of engine state to clients. They are not a way to replicate a database to another server: Typhon has no multi-node replication or clustering.

How it relates

• System — systems say what they changed (Replicate), read commands and emit events through TickContext.Subscriptions.
• Spatial index — a replicated archetype has a position, and a session's view is found through the spatial grid.
• Tick fence — the fence records the tick's pushes; replication computes after it and publishes after the flush.
• View — a view is an in-process delta stream; a subscription is its remote counterpart, built from projections rather than from a view.

In the API

• TyphonRuntime.Subscriptions — the SubscriptionsRegistry you declare everything on, before Start().
• SubscriptionsCommands — what a system reaches as ctx.Subscriptions: Replicate, Commands<T>(), Emit, SessionEvents, Session(…), TryResolve.
• SubscriptionsOptions — capacity, budgets and the required replication cell.

Learn & use

• Narrative: Guide ch.7 — serving remote clients
• Feature detail: Subscriptions
• Internals: Technical overview 15 — Subscriptions
• The algorithm: who receives what, and why it scales