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 = "0.8" 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.

Boundary event

Boundary event

Mental model

Key facts

Typed inject/fire interface (opt-in)

Python parity

See also