These concepts clarify the API's messaging resources:
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Topic. An ordered collection of messages.
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Message block. A message or group of messages sent from a single topic.
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Message. An individual object stored in a topic.
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Consumer group. Multiple subscribers sharing a single subscription. All group members receive alternating messages, with each message sent only to one group member. It's most commonly used for load balancing.
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Retained message. It's an MQTT message automatically sent to the subscriber at the moment of subscribing. It is most commonly used for configuration or status updates.