Event contract design

Event Contract Design

Concept of the skill

Use when designing or reviewing asynchronous event contracts: producer/consumer ownership, event envelope, schema, topic/channel naming, ordering, idempotency, versioning, compatibility, replay, dead-letter behavior, and AsyncAPI/CloudEvents-style documentation.

Coverage

Design asynchronous event contracts for producers and consumers. Covers event envelope, schema, event type, topic/channel naming, producer ownership, consumer expectations, required and optional fields, idempotency keys, ordering, causation and correlation IDs, schema evolution, replay, dead-letter behavior, compatibility, observability, and machine-readable documentation such as AsyncAPI or CloudEvents-style metadata.

Philosophy of the skill

An event is a public promise once another consumer depends on it. If the payload, ordering, retry, or compatibility rules are implicit, every consumer invents its own interpretation and the event stream becomes shared folklore.

Do not confuse event discovery with event contracts. Discovery asks what happened in the domain. Contract design asks what exactly will be published, consumed, replayed, and evolved.

Method

1. Name the producer, owner, intended consumers, and event purpose.
2. Separate business event type from transport topic or queue name.
3. Define envelope fields: id, type, source, time, subject, schema version, correlation, causation, tenant, and idempotency key.
4. Define payload schema with required, optional, nullable, and deprecated fields.
5. State ordering, delivery, retry, replay, and dead-letter expectations.
6. Define compatibility rules: additive fields, breaking changes, versioning, deprecation, and consumer migration.
7. Add observability fields needed to reconstruct publishing and consumption failures.
8. Provide at least one positive and one negative contract fixture.

Evals

This skill ships a comprehension-eval artifact at examples/evals/event-contract-design.json. The checklist below is the authoring gate for async event contracts; the eval file is the grader surface.

Verification

• Producer, owner, and consumers are named
• Event type, topic/channel, envelope, and payload are distinct
• Required, optional, nullable, and deprecated fields are explicit
• Idempotency, ordering, retry, replay, and dead-letter behavior are stated
• Compatibility rules distinguish additive from breaking changes
• Correlation and causation IDs cross async boundaries
• Positive and negative fixtures exist for contract testing

Do NOT Use When

Use instead	When
event-storming	You are still discovering domain events, commands, policies, aggregates, or timelines.
system-interface-contracts	The boundary is not specifically asynchronous events or messages.
webhook-integration	You are implementing inbound provider webhooks, signatures, retries, and raw payload handling.
api-design	You are designing HTTP endpoints, status codes, pagination, filtering, or error envelopes.
observability-modeling	The event contract is settled and the task is telemetry design.

Skill Graph context

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Classification

• Subject: software-architecture
• Public: true
• Domain: architecture/events
• Scope: Use when designing or reviewing asynchronous event contracts: producer/consumer ownership, event envelope, schema, topic/channel naming, ordering, idempotency, versioning, compatibility, replay, dead-letter behavior, and AsyncAPI/CloudEvents-style documentation. Do NOT use for domain-event discovery (use event-storming), broad interface contracts (use system-interface-contracts), inbound provider webhook mechanics (use webhook-integration), or HTTP endpoint design (use api-design).

When to use

• design the event contract for publishing OrderPaid to downstream consumers
• define topic names, payload schema, idempotency, and versioning for this event stream
• review this outbound webhook event schema before customers integrate with it
• write the compatibility rules for consumers of these async messages

Not for

• discover the domain events, commands, and policies in this business process
• define every boundary contract between services, jobs, and APIs
• verify inbound provider webhook signatures and retry behavior
• design REST endpoints, status codes, and pagination

Related skills

• Verify with: system-interface-contracts, observability-modeling, real-time-updates
• Related: api-design, state-machine-modeling, entity-relationship-modeling, event-storming, system-interface-contracts, observability-modeling, webhook-integration

Keywords

• event-contract, async-api, cloudevents, event envelope, topic naming, async event schema, event compatibility, replay contract, dead-letter behavior, consumer fixtures

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