Messages and Envelopes

Messages and envelopes describe the finite carriers used to move values and interaction metadata across system-graph boundaries.

A message is not a new domain-semantic primitive. It is a structural carrier participating in an interaction. Message emission, admission, delivery, and ingress are occurrences. At a receiving boundary, ingress is an exogenous event carrying a value whose shape, contract, addressing, and context provide evidence for semantic interpretation.

Intent and Interpretation

A message contract may make an intended interpretation explicit:

A command message carries singular intent that an understood observer attempt a transition.
An event message carries a value reporting an occurrence.
A document message carries an observation, state transfer, artifact, or other shaped value.
A request message carries a request effect whose emission establishes an emitter-side response or terminal-failure obligation.
A reply message carries a terminal result that discharges one admitted request when accepted under the request protocol.

These correspondences are strong without being identities. A command message makes command interpretation expected, but the receiving observer still admits and interprets the exogenous event relative to its boundary, authority, policies, state, and target subject. An event message may report one domain occurrence while its delivery creates distinct messaging occurrences.

Envelope Structure

An envelope separates interaction metadata from a carried payload. Depending on the contract, it may include:

Stable message, emission, request, process, subject, and contract identities.
Target, channel, route, partition, return address, and reply-path information.
Correlation and causation links.
Schema, semantic, payload, and format versions.
Ordering key, sequence position, deadline, expiration, and retention demands.
Idempotency basis, attempt identity, and expected version.
Tenant, authority, classification, trace, and provenance context.

Transport-assigned identifiers do not automatically replace these semantic and protocol identities. An envelope wrapper may add the metadata required by a channel without changing the carried value's domain meaning. A claim-check arrangement may replace a large carried value with a stable reference into storage, but the reference's authority, lifetime, consistency, and access rules then become part of the interaction contract.

Structural Boundary

Messages and envelopes state what is carried and how its intended correspondence is declared. Interaction channels state where it can move. Routing models select paths or recipients. Flow operators transform or compose carried values. Delivery semantics and other operational concerns supply guarantees. Brokers, files, networks, databases, and runtimes realize the carrier and its movement.

External References

Gregor Hohpe and Bobby Woolf, Message, Command Message, Document Message, and Event Message, Enterprise Integration Patterns, 2003.
Gregor Hohpe and Bobby Woolf, Envelope Wrapper, Enterprise Integration Patterns, 2003.