Pure projection from an AMQP delivery's metadata to the pair the detector actually needs: (detector_key, diagnostic_timestamp).

What this is NOT

Unlike PhiAccrualUdp.Packet, this module is not a wire codec. In AMQP there is no wire format to parse — the broker hands the :amqp library a structured envelope, which the library passes to the consumer as a meta map. This module's job is to project that map down to the two pieces of information PhiAccrualAmqp.Consumer needs to feed PhiAccrual.observe/2.

It does NOT look at the payload. The payload is opaque to phi_accrual_amqp. If your application encodes the heartbeat identity inside the message body, write a custom resolver in application code that decodes that body and pass it via :key_resolver — but the contract of this package is that the envelope alone is sufficient to identify the source.

detector_key resolution

The detector key is the identity passed to PhiAccrual.observe/2; see PhiAccrual.detector_key/0 for the value type. It is extracted from the delivery meta via a :key_resolver function — a (meta -> PhiAccrual.detector_key() | nil) mapping. Returning nil yields {:error, :no_detector_key} and the delivery is dropped with a telemetry event.

The default resolver returns the :routing_key when it is a non-empty binary. This fits the common topic-exchange topology (heartbeat.<node> bound with # to a single queue). For other topologies — header-based, queue-name-based, app_id-based — supply a custom resolver.

diagnostic timestamp

AMQP messages carry an optional :timestamp property (BasicProperties.timestamp). It is not used to feed the detector — PhiAccrual requires receiver-local monotonic time (see PhiAccrualAmqp.Consumer moduledoc on clock discipline). The timestamp is extracted here only for diagnostic telemetry (e.g., publisher-to-consumer latency when both sides are NTP-synced and the publisher uses a consistent unit).

Note that AMQP 0-9-1 does not specify a unit for this field; the convention is POSIX seconds, but the RabbitMQ message-timestamp plugin and some publishers use milliseconds. This module passes the integer through untouched.