Feature flags, publish/subscribe options, metrics, and responsibility boundaries for A3S Event.

Event Operations

A3S Event is a library, not a standalone daemon. Operational behavior comes from the selected provider plus the optional application-level modules you enable.

Feature Flags

Feature	Default	Adds
`nats`	Yes	NATS JetStream provider.
`encryption`	Yes	AES-256-GCM payload encryptor and key rotation support.
`cloudevents`	Yes	CloudEvents v1.0 conversion.
`routing`	Yes	Broker, Trigger, EventSink, and sink-backed DLQ.
`full`	No	Enables all optional feature modules.

Minimal core still includes event types, EventBus, memory provider, schema registry, DLQ abstraction, metrics, and custom provider traits.

Publish Options

PublishOptions exposes provider-native publishing controls. Providers that do not support an option may ignore it.

Option	Meaning
`msg_id`	Deduplication id, mapped to NATS message id when supported.
`expected_sequence`	Optimistic concurrency guard.
`timeout_secs`	Per-publish timeout override.

SubscribeOptions exposes provider-native consumer controls:

Option	Meaning
`max_deliver`	Maximum delivery attempts before drop or DLQ handling.
`backoff_secs`	Redelivery backoff intervals.
`max_ack_pending`	Backpressure cap for unacknowledged messages.
`deliver_policy`	Where a consumer starts: all, last, new, by sequence, by time, or last per subject.
`ack_wait_secs`	Redelivery wait after an unacked message.

NATS maps these to JetStream consumer settings where available. MemoryProvider is lightweight and does not provide durable delivery semantics.

Metrics

EventMetrics uses lock-free atomic counters. EventBus::metrics() returns the live metrics object, and snapshot() gives a point-in-time view:

Metric	Meaning
`publish_count` / `publish_errors`	Successful and failed publishes.
`subscribe_count` / `unsubscribe_count`	Subscription lifecycle operations.
`dlq_count`	Events routed to DLQ.
`validation_errors`	Schema validation failures.
`encrypt_count` / `decrypt_count`	Payload crypto operations.
`avg_publish_latency_us` / `max_publish_latency_us`	Publish latency summary.

The crate does not expose a metrics server; scrape or export snapshots from the host application.

Responsibility Boundary

Capability	Owner
Retry and backoff	Provider-native consumer settings.
Backpressure	Provider-native flow control and `max_ack_pending`.
Connection resilience	Provider client, such as async-nats reconnect behavior.
Partitioning or sharding	Provider-native routing model.
Transport encryption	Provider configuration such as TLS.
Event versioning and schema	A3S Event.
Payload encryption	A3S Event, before publish and after receive.
Dead letter representation and routing	A3S Event.
Subscription filter persistence	A3S Event state-store trait and host application.
Metrics and tracing hooks	A3S Event plus host exporter.

This boundary is intentional: the crate avoids reimplementing the delivery mechanics that durable event providers already own.

Validation

Focused validation should run from crates/event, not the monorepo root:

cargo test --all-features
cargo test --no-default-features
cargo test --features nats

NATS integration tests require a JetStream-enabled NATS server.

Operations