I have written code (mosquitto_publish()) using Mosquitto to publish data to AWS.
My problem is the sequence with which data is arriving on the MQTT broker. In the Paho client, I see waitForCompletion(), but nothing similar in Mosquitto. Would anyone please help me in dealing with this problem ?
Based on the mosquitto_publich documentation, the function returns when sending has been "successful". MQTT does not guarantee the order in which messages arrive, so you should arguably watch for the arrival rather than the sending, and avoid having two messages race each other to the broker. With QoS 0, the client never knows if a message arrived; that requires QoS 1 or 2, for which additional communications are exchanged. Raise the quality of service, and you can use mosquitto_max_inflight_messages_set(mosq, 1) so that the client queues any additional messages until it receives confirmation from the server. This may be even more efficient than "waiting" for completion, since non-MQTT operations can continue. The queue might pile up if you send bursts of many messages.
The more complex alternative is to send messages unrestricted, but include an index with each, so that the subscriber can sort them upon receipt (for which it would need its own queue and delay). Not recommended if this burden is going to fall on multiple subscribers.
