Build a durable real-time API with AsyncAPI 3.1.0 and NATS JetStream using validated contracts, streams, and pull consumers. Full breakdown.
Why contracts and durable messaging belong together
A real-time API that only works while both sides are online is fragile. Clients drop, workers restart, and network partitions happen. Durable messaging keeps events available until they are acknowledged, while an AsyncAPI contract describes channels, messages, and operations so producers and consumers agree on shape before any code ships. AsyncAPI 3.1.0 formalizes that agreement for event-driven systems the way OpenAPI does for request/response HTTP.
NATS JetStream adds persistence on top of NATS subjects: streams capture published messages, retain them according to policy, and hand them to consumers that pull or receive work at their own pace. Pairing a validated AsyncAPI document with JetStream means the protocol is documented, the payload is checkable, and delivery survives restarts instead of vanishing into fire-and-forget pub/sub.
Model the API with AsyncAPI 3.1.0
Start from the contract, not from ad hoc subject names. Define channels that map cleanly to NATS subjects, message schemas for payloads and headers, and operations that mark who publishes and who consumes. Use the same schemas for client libraries, server stubs, and CI validation so a breaking field change fails the pipeline instead of failing in production. Treat subject tokens and reply subjects as part of the public surface: if consumers depend on them, they belong in the document.
Validation should run on both ends. Producers check outbound events against the schema before publish; consumers reject or quarantine messages that do not match. That keeps bad data out of the stream and makes the AsyncAPI file an executable source of truth rather than a stale wiki page.
Streams, subjects, and retention
In JetStream, a stream binds one or more subjects and stores matching messages. Choose retention and discard policy for your use case: time or size limits for high-volume telemetry, interest or work-queue style retention when every message must be processed by workers. Keep stream subject filters aligned with the channels in your AsyncAPI document so operators can map a channel to a concrete stream without guesswork.
- Name subjects by domain and event type so routing stays readable as the API grows.
- Prefer explicit stream limits over unbounded storage; durability is not infinite disk.
- Document which subjects are command-style versus event-style so consumers know whether replay is expected.
Pull consumers and operational discipline
Pull consumers suit backends that want control over batch size, concurrency, and backpressure. A worker fetches a batch, processes it, then acknowledges success or negative-acknowledges for redelivery. Design handlers to be idempotent: the same message may arrive more than once after crashes or timeouts. Ack only after the side effect is committed, and use short, clear ack wait windows so stuck work becomes visible rather than silent.
Operationally, treat the AsyncAPI contract, stream config, and consumer config as one deployment unit. Version channels when you must break compatibility, stand up a new consumer or stream path for the new shape, and drain the old path once clients migrate. That combination—validated AsyncAPI 3.1.0 contracts, JetStream streams for durability, and pull consumers for controlled processing—gives you a real-time API that stays correct under load and recoverable when parts of the system fail.