Kevin’s right, and that’s half of it. We hit the same thing on a wind site.
Hermetry’s MQTT is 3.1.1 and a CONNECT with clean session false is served as clean. Nothing is queued for a device while it’s offline, apart from commands that were sent as persistent. So no session state to resume, in either direction for telemetry.
What Hermetry does promise is on the way in: the PUBACK goes out only after the message is safely queued on their side. If it can’t be queued you get no PUBACK and the session closes, and the client is expected to resend after reconnecting. Duplicates of the same device, key and timestamp are stored once, so resending is safe.
So the buffer is yours, and the pattern is:
- Write every reading to a local store (we use a SQLite file) with its own ts in ms.
- Publish from the store, delete a row only when its PUBACK arrives.
- After reconnect: subscribe, ask for shared attributes, then send the backlog before live data.
Send the backlog as arrays of {ts, values} so it lands at the right times. The profile has a separate Replay messages per second limit for messages whose records are all older than a minute, so a long backlog drains over a while. Don’t panic when it doesn’t all appear at once.
The MQTT page has the acknowledgement section, worth reading once.