Boundary Taxonomy

Brokered execution belongs under runtime boundary policy, not under a global pipeline transport switch.

Translation Table

Use TPF terms when discussing brokered designs:

Broker term	TPF meaning
Topic/queue/channel	Step inbox, await request/completion channel, checkpoint publication channel, or transition dispatch queue
Consumer/worker group	Scaled step-host or provider group for one boundary
Partition key/message group	Item key, lineage key, document id, fan-in key, or await correlation key
Offset/visibility timeout	Delivery cursor or redelivery lease, not the TPF replay pointer
Retention/redrive window	Transport-level redelivery window, not business persistence
Lag/queue depth	Boundary pressure signal
DLQ topic/queue	Item reject sink or failed execution lane
Record/message	TPF envelope carrying typed payload, serialized payload, payload reference, or controlled loose payload

Kafka replay means re-reading records from a log. SQS redrive means making queued messages visible again from a queue or DLQ. TPF replay means re-running or reusing a known pipeline boundary with step identity, lineage, cache/persistence policy, and replay metadata.

Boundary Types

TPF boundary	Meaning	Broker fit
Local step boundary	Same process, direct invocation	Usually no
Remote operator boundary	Immediate request/response external step host	Usually REST/gRPC/LOCAL transport first
Await boundary	Execution parks and resumes after correlated completion	Strong fit for request/completion channels
Checkpoint handoff	One pipeline publishes work to another owner	Strong fit for publication/subscription
Transition-worker boundary	Durable coordinator dispatches one bounded continuation	Possible dispatcher provider
Brokered step-host boundary	A step is executed by a scaled step-host group	Future design candidate
Envelope compatibility boundary	Step host receives a generic envelope rather than a strong DTO	Possible across any dispatch substrate

Serialization choices such as protobuf, JSON, bytes, or payload references belong to payload policy, not broker fit. See Envelope And Data Policy.

Await completion and checkpoint handoff both admit transport messages into TPF-owned boundaries. The immutable queue-async facts for those admissions are described in Immutable Segment And Boundary Model.

Ownership

TPF should own:

1. pipeline definition and release identity,
2. step identity and step order,
3. cardinality and fan-out/fan-in semantics,
4. lineage, item identity, and parent-child relationships,
5. retry, reject, and replay metadata,
6. declared DTO/envelope contracts,
7. replay topology and viewer interpretation.

The dispatch substrate should own:

1. durable delivery,
2. queue depth, lag, and message age,
3. partition-local ordering or queue visibility/redrive behavior,
4. consumer/worker distribution,
5. broker-level redelivery,
6. retention and substrate ACLs.

The substrate must not own step meaning, fan-in correctness, replay meaning, mapper selection, or pipeline topology.