Interaction Protocols

An interaction protocol describes the allowed temporal structure of interactions through an interface. It constrains not only the values that may be exchanged, but also which roles may initiate, which occurrences may follow, how concurrent attempts relate, and how a conversation completes, fails, is cancelled, or remains live.

An interface answers what roles and obligations are available at a boundary. A protocol answers how participation in those roles may unfold over time. An interaction channel supplies logical exchanges and directions over which protocol occurrences can move. An interaction binding assigns the interface and protocol roles to exact endpoints and channel directions.

Protocol Contents

A protocol may declare:

initiator, responder, producer, consumer, subscriber, or peer roles;
request, reply, notification, acknowledgment, settlement, demand, and streaming sequences;
conversational states and allowed transitions;
correlation, causation, and response-obligation rules;
ordering, concurrency, and arbitration constraints;
retry, duplication, deduplication, and idempotency expectations;
timeout, cancellation, compensation, and failure behavior;
half-close, terminal completion, session, or connection lifetime; and
negotiation, compatibility, versioning, and evolution rules.

These rules may be explicit in a specification or merely assumed by implementations. Unstated protocols are still protocols; they are simply harder to validate, evolve, and operate.

Interaction Shape and Full Protocol

A protocol-neutral interaction shape describes only the coarse morphology that a channel realization must support: fire-and-forget, publication, unary invocation, request stream, response stream, bidirectional stream, datagram, or correlated request/reply.

A full protocol adds the legal trace and meanings omitted by that shape. Two systems may both support unary invocation while disagreeing about admissible retries, cancellation, terminal errors, correlation, authority, or whether a response proves admission, commitment, or only protocol completion. Likewise, two bidirectional streams can expose different half-close, demand, resumption, and failure rules.

The interaction shape therefore belongs with channel topology as a provider-neutral realization demand, while the complete protocol remains the authority for legal conversations.

Layered Protocols

Protocols compose across abstraction layers:

Protocol layer	Representative responsibility
semantic or service protocol	Meaningful operations, participant obligations, admissible outcomes, and business-relative completion
interface protocol	Legal use of the roles exposed at one boundary
channel protocol	Observable delivery, demand, credit, settlement, cancellation, stream, and continuity behavior at one channel boundary
provider or application protocol	HTTP, RPC, AMQP, MQTT, WebSocket, broker, or custom protocol exchanges
transport and network protocol	Framing, addressing, streams, datagrams, retransmission, transport flow control, and connection lifecycle

These layers can share an operation name or edge shape without sharing its meaning. Asynchronous send appears at messaging, runtime, and network boundaries, but completion may mean local buffer admission, broker acceptance, remote protocol receipt, or application commitment. One higher-level occurrence may lower into several request, acknowledgment, control, and retry occurrences at lower layers.

Network-channel protocols can realize messaging-channel behavior only when their composition with codecs, brokers, storage, adapters, and application logic preserves the required upper-layer trace. A transport connection can disappear and be replaced while a durable subscription protocol continues; a single connection can multiplex many application protocols; and one messaging interaction can use separate network channels for payload, reply, flow control, and recovery.

Specification	What it constrains
interface	Available interaction roles and semantic obligations
shape or schema	Structure of exchanged values
semantic contract	Meaning of interactions, outcomes, and failures
interaction protocol	Legal traces, conversational state, ordering, concurrency, and completion
session type	Formally typed participant-local or projected communication behavior
interaction shape	Coarse exchange morphology required from a channel
operational envelope	Quantitative expectations such as capacity, throughput, latency, and queue limits
channel	Logical exchange and directions through which occurrences move
binding	Exact association of contracts and protocol roles with endpoints and channel directions
realization binding	Concrete encoding, addressing, transport, provider configuration, and runtime mechanisms

Operational Envelope

A protocol can expose backpressure, admission, acknowledgment, settlement, or maximum-in-flight rules, but throughput and queueing behavior are not properties of a schema alone. They arise from the protocol together with arrival patterns, service time, resource capacity, scheduling, retries, and downstream dependencies.

The operational envelope therefore qualifies an interface-protocol-channel binding with measurable operational concerns, including:

supported concurrency and serialization scopes;
expected and maximum throughput;
latency and deadline expectations;
boundedness and queue discipline;
flow-control and overload signals; and
fairness and priority expectations.

See queueing theory, scheduling, and flow control for the corresponding operational models.

Opaque Protocols

A service may perform an operation whose domain meaning belongs to an external or intentionally hidden model. The local graph can still specify the interface, protocol, observable outcomes, channel binding, and guarantees without pretending to own the external semantics.

Relationships

Services realize capabilities under their interface protocols.
Interaction channels carry protocol occurrences but do not define their complete meanings or legal traces.
Network channels and network protocols can realize parts of a service or channel protocol without being identical to it.
Process structures can describe longer-lived conversations that span several interface operations or protocol sessions.
Session types can formally constrain participant-local traces, duality, branching, recursion, and endpoint use without automatically supplying the protocol's domain meaning or operational guarantees.
Traces provide evidence that observed conversations conform to their protocols.

Formal relations

constrains: Interaction — An interaction protocol restricts the legal boundary-relative traces in which participants may take interaction roles.