Boundary event

A Boundary event is the typed value an Accumulator emits and a Reactor reacts to. It is coupled to both — a boundary only exists between an accumulator and a reactor; it has no standalone lifecycle.

Mental model

• An accumulator’s process() produces a boundary output, tagged with a SourceName and sent to the reactor.
• The reactor slots it into its input cache under that source name; the cache is what the Computation Graph reads when it fires.
• The boundary value is serialized (bytes) as it crosses the channel.

Key facts

• Names must line up. The boundary’s SourceName must match the reactor’s accumulators = [...] / mode sources and the graph’s entry-node source name. This is the most common wiring mistake.
• One per emit. Each process() that returns a value produces one boundary; returning None produces none.

Typed inject/fire interface (opt-in)

The boundary type is also what an operator supplies when manually injecting into an accumulator or firing a reactor with inputs (POST .../inject, fire_with). The declared interface — GET /v1/health/{reactors|accumulators}/{name}/interface — derives each slot’s JSON Schema from the boundary type, but only when that type derives schemars::JsonSchema:

// Opt in → the slot exposes a rich {best_bid, best_ask} schema and the web UI
// renders a typed inject/fire form. Without JsonSchema the slot schema is `{}`
// (permissive) and the UI falls back to a raw-JSON field.
#[derive(Debug, Clone, Serialize, Deserialize, schemars::JsonSchema)]
pub struct OrderBookUpdate {
    pub best_bid: f64,
    pub best_ask: f64,
}

Add schemars = "1" to the package’s dependencies. Schema derivation is the only thing the derive affects — boundary serialization on the wire is unchanged.

Python parity

Python boundaries are plain dicts, so there’s no type to derive from. Declare the shape explicitly with @cloaca.boundary_schema(field=type, …) on the accumulator (CLOACI-T-0770) — the compiler parses it from source at build time into the same typed slot; at runtime it’s a no-op:

@cloaca.boundary_schema(bid=float, ask=float)
@cloaca.passthrough_accumulator
def py_alpha(event):
    return event

Supported scalar types: str, int, float, bool, list, dict. Without the decorator the slot stays untyped and inject/fire fall back to a raw-JSON field — exactly like a Rust boundary that doesn’t derive JsonSchema.

See also

• Accumulator — emits boundaries.
• Reactor — reacts to them.