1. Supported Downlink Events

Kafka downlink supports mapping messages to:

• WritePoint
• CommandReceived
• RpcResponseReceived

Downlink overview see: Downlink Overview

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2. Topic and Consumer Group

2.1 Exact Topic Restriction

Downlink topic must be Exact String, does not support:

• {{template}}
• * wildcard
• re: / regex:

2.2 Consumer Group

Consumer group id is fixed as:

• ng-gateway-plugin-{app_id}

And:

• enable.auto.commit=false (Commit controlled by AckPolicy)
• auto.offset.reset=latest

Meaning of auto.offset.reset=latest

When this consumer group has no offset, it will start consuming from latest (Will not backtrack history). If you expect "Consume from earliest", current version does not provide this option (Needs separate handling on platform/ops side).

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3. AckPolicy / FailurePolicy

Kafka downlink "Confirmation" corresponds to commit:

• ack_policy=never: Do not commit
• ack_policy=always: Commit regardless of success/failure (Discard bad message)
• ack_policy=on_success:
  • Parse success or ignored by filter → commit
  • Parse failure:
    • failure_policy=drop → commit (Discard)
    • failure_policy=error → do not commit (Wait for redelivery/fix)

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4. Payload Selection

Recommend prioritizing EnvelopeJson (Stable, mixed topic routable):

• Downlink EnvelopeJson

If platform side shape is inconsistent, then use MappedJson, and be sure to add filter in mixed topic scenarios:

• Downlink MappedJson + Filter

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5. Common Pitfalls

• Topic configured with template: Will be rejected (Downlink must be exact topic)
• Same topic mixes multiple routes, but ack_policy inconsistent: Will error when building routing table
• Headers non-UTF-8: filter.mode=property cannot match (Will be ignored)