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channel

Since v1.6.0 · Top-level declaration

Grammar​

channel <Name> {
    message: <TypeRef>                          # required — the typed payload
    qos: <at_least_once|at_most_once|exactly_once|broadcast|queue>   # optional (default at_least_once)
    lifetime: <affine|persistent>               # optional (default affine)
    persistence: <ephemeral|persistent_axonstore>  # optional (default ephemeral)
    shield: <ShieldRef>                         # optional — gate for publish/discover
}

channel declares a typed communication channel in the π-calculus sense: a first-class, mobile, affine-by-default conduit Channel<τ, q, ℓ, π> carrying values of the declared message: type. Producers write with emit; daemon listeners consume; publish/discover extrude and import the channel as a shield-gated capability.

Surface​

channel is a top-level declaration.

type SkillResult { task_id: String  status: String }

shield WebhookEgress { sign: hmac_sha256  on_breach: halt }

channel SkillCompleted {
    message: SkillResult
    qos: at_least_once
    lifetime: affine
    persistence: persistent_axonstore
    shield: WebhookEgress
}

Fields​

message: (required)​

The declared payload type. Every emit onto the channel is checked against it.

qos: (optional, default at_least_once)​

Delivery discipline, enforced by the typed event bus at runtime:

ValueBehaviour
at_least_onceRedelivered until acknowledged (default).
at_most_onceDelivered once; dropped silently on failure.
exactly_onceDeduplicated by event id.
broadcastFan-out to every subscriber queue.
queueFIFO work-queue semantics.

lifetime: (optional, default affine)​

affine — a handle may be dropped but never used after consumption (use-after-consume is a typed runtime error). persistent — no upper bound on uses.

persistence: (optional, default ephemeral)​

ephemeral — in-process delivery only; events die with the process. persistent_axonstore — every emit is appended to the durable event outbox (v2.31.0): claimed with SKIP-LOCKED, redelivered until acked (at-least-once), it survives restarts and is replayable. Signed egress requires it (axon-T848).

shield: (optional)​

The shield gating publish/discover capability extrusion. When the shield declares sign:, a publish of this channel marks it for signed external egress (v2.34.0): the enterprise runtime delivers each durable event as an HTTP POST to every registered subscription, signed X-Axon-Signature: sha256=<hex(HMAC-SHA256(secret, body))>.

Delivery model (what the runtime actually does)​

  1. emit Channel(value) — routes by persistence: durable → outbox append; ephemeral → in-process typed bus.
  2. daemon … { listen Channel as ev { … } } — the v2.31.0 receive driver claims outbox events and runs the listener body, acking on success.
  3. publish Channel within Shield — capability extrusion; with a signing shield it additionally declares the egress surface (see axon://primitives/publish).

What this primitive is NOT​

  • Not a message broker config. No URLs, no destinations — binding to external subscribers is enterprise configuration (the webhook registry), never source code.
  • Not a stream. For token/SSE streaming use stream: tool effects; a channel carries discrete typed events.

See also​

  • axon://primitives/emit — the output prefix.
  • axon://primitives/publish — capability extrusion + egress.
  • axon://primitives/discover — the dual import.
  • axon://primitives/listen — the consumer side.
  • axon://primitives/shield — the gate (+ sign: egress).