Integrator here. A customer has about 300 pump controllers in the field, ESP32-S3 boards running our own firmware, all on MQTT to Hermetry. We need to change a few settings remotely, per site: the reporting interval, a deadband on the pressure reading, and the NTP server (some sites block the public pools).
Two approaches on the table internally:
- a command, say
cfg.set, with the settings as JSON params, answered with ok/error - shared attributes that the device applies
What I care about: knowing for certain that a given unit has applied a value, and not losing changes for units that are offline. Some rural sites drop off for two or three days at a time.
The firmware already reads a JSON config from flash at boot, so applying the values is not the hard part. The units report every 60 s and sit behind a mix of LTE routers and site Wi-Fi. Which way is Hermetry built for, and is there anything like a desired-vs-reported view?
Student here, curious: why would it matter? A command with the settings in it sounds simpler to me.
Commands to an offline unit only wait if you send them persistent, and that gives up after 7 days at most. Attributes are state, commands are actions. I’d go attributes.
We did it with commands on a smaller fleet and it was a mess every time a unit missed one. Nobody could tell which units had which interval.
Use cfg.* shared attributes, that’s exactly what they’re for, and you get the desired-vs-reported view for free.
Declare the keys on the profile’s Configuration keys tab (type, label, limits), then set them as shared attributes: cfg.telemetry.interval_s, cfg.pressure.deadband_bar, cfg.ntp.server. The device gets changes on v1/attributes/set, applies them, and reports the applied value back as a client attribute with the same key. Hermetry compares the two and shows In sync, Pending, Rejected or Drifted per key on the Attributes tab.
Device side, after every connect:
SUBSCRIBE v1/attributes/set
PUBLISH v1/attributes/req/1 {"sharedKeys": "cfg.*"}
# answer on v1/attributes/res/1 -> apply everything -> report back:
PUBLISH v1/attributes {"cfg.telemetry.interval_s": 300, "cfg.pressure.deadband_bar": 0.05}The request after connect is what covers your rural sites: a unit that was offline for three days asks for all cfg.* keys when it comes back and catches up. If a unit can’t apply something, report it in cfg_rejected, e.g. {"cfg_rejected": {"cfg.ntp.server": "apply_failed"}}, and the row shows Rejected with the reason. Apply idempotently, a message can arrive twice.
For 300 units, write the values with the API (POST …/attributes/shared, up to 100 keys per call) in a loop over the device list. The baseline and bulk-job screens are for devices that publish a settings manifest (licensed devices), so with your own firmware cfg.* is the route. Attributes page has the status table.
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