MQTT Event Bridge

The @purista/mqttbridge package integrates MQTT v5 brokers with PURISTA routing semantics.

Delivery semantics

Delivery behavior depends on your MQTT QoS and retention/session settings:

• QoS 0: at-most-once
• QoS 1: at-least-once
• QoS 2: exactly-once on broker/client protocol level (application side effects still must be idempotent)

Durability and replay are broker- and topic-config dependent.

For command invocation, PURISTA applies both:

• caller-side pending invocation timeout tracking
• MQTT messageExpiryInterval derived from command timeout for non-event command messages

This keeps timeout handling predictable while still preferring broker-native expiry where available. Command handlers are single-shot request/response: failures are returned as CommandErrorResponse (UnhandledError) and are not retried by subscription-style delivery loops.

Stream support

PURISTA stream runtime (openStream) is currently not implemented for MQTT bridge.

Topic strategy

PURISTA composes topics from message metadata (type, sender, receiver, tenant/principal, event). Keep prefixes/QoS consistent across all instances.

Reliability recommendations

• choose QoS per traffic class (command vs event/subscription)
• align expiry and timeout values intentionally
• use shared subscriptions carefully for scale-out consumers
• test reconnect and retained-session behavior in integration tests