Brokered Runtime Boundaries

This implementation-facing guide explains where Kafka-style brokerage fits in TPF without turning Kafka into a new pipeline semantic.

Recommendation

Do not introduce a general "Kafka mode" for TPF.

Kafka, SQS, REST, gRPC, and local execution should be implementation choices under TPF-owned runtime boundaries. TPF must still own step identity, cardinality, lineage, replay metadata, retry/reject meaning, and declared DTO/envelope contracts.

The important separation is:

1. TPF semantics define what the pipeline means.
2. Boundary policy defines where execution crosses a runtime seam.
3. Payload policy defines how data is represented at that seam.
4. Substrate policy defines how bytes or envelopes move.

Guide Map

1. Boundary Taxonomy maps broker concepts into TPF runtime boundaries.
2. Dispatch Substrates explains why Kafka is a substrate, not a TPF mode.
3. Envelope And Data Policy separates loose payloads from strict TPF control metadata.
4. Adoption And Slices captures the value proposition and implementation ordering.

Naming

Use step host for an external process, service, pod, or function that executes a TPF step boundary.

Keep transition worker only for the existing durable coordinator concept described in Worker Protocols. Avoid using "worker" as a generic synonym in this guide because it clashes with existing runtime worker terminology.

Core Guardrail

Kafka can provide durable delivery, consumer scaling, and operational familiarity. It must not define the pipeline topology, replay meaning, mapper validation, fan-in correctness, or step contract.