Choosing a sequence communication path

Choose the narrowest path that matches delivery needs:

Need	Use	Contract
Transform a bounded or continuous payload	input/output stream	Backpressure and content type are part of the stream contract.
Ask the Hub or Space for current state	hubClient() or spaceClient()	Request/response; Hub and Space scopes remain distinct.
Notify observers	emit() or emitToSpace()	Transient event; not durable and not proof of delivery.
Decouple producers and consumers	Hub/Space topic	Named live route; no persistence or replay.

Streams

Streams carry data through the sequence pipeline and can be streamed, paged, or represented by a configured artifact/reference when the result is large. Preserve the declared content type. A consumer disconnect can end the stream; the platform does not provide exactly-once delivery.

API calls and events

Use the v2 fluent hubClient() for current-Hub operations and spaceClient() for Manager/Space operations. The legacy this.hub and this.space properties remain for compatibility. Events are notifications over the monitoring/control path. They are scoped to an instance or Space, transient, and may be lost on disconnect; do not use them as a checkpoint or command queue.

The sequence owns its payload and route policy. The Hub/Manager owns transport and routing. Authentication, authorization, TLS, and public ingress remain deployment responsibilities.

Request/response and RPC

For a typed request/response call, use the fluent client and inspect the returned envelope:

import type { AppConfig, SequenceAppContext } from "@scramjet/sequence-types";
import type { HubClient, SpaceClient } from "@scramjet/rest-api2";

type Context = SequenceAppContext<AppConfig, unknown, HubClient, SpaceClient>;

export async function request(this: Context) {
  const health = await this.hubClient().health.get();
  if (!health.body.healthy) throw new Error("Hub is not ready");
  const inventory = await this.spaceClient().instances.get();
  return { hub: health.body, instances: inventory.body.items.length };
}

An exposed instance endpoint is reached through the instance RPC route. The v2 RpcRequest shape contains (method, path, optional headers and body) and the RpcResponse shape returns status, headers, and an optional body; use the Manager/Space-prefixed route when the instance belongs to another Hub:

curl --fail --request POST \
  'http://manager.example/api/v2/spaces/space-1/hubs/hub-2/instances/instance-7/rpc/health' \
  -H 'content-type: application/json' \
  --data '{"method":"GET","path":"/health"}'

This is ordinary request/response traffic. It is not event delivery, topic publication, or a durable command queue.

For installed execution, use the Filtering local object data for a consumer Process Adapter workflow, then refer to the canonical installed Process Adapter example baseline. Maintainers may use npm run test:sequence-appcontext as optional local AppContext evidence; it does not replace the installed workflow or prove adapter visibility and consumer durability.