in order to understand the data flow between sensors - consoles - data logger application software (like CumulusMX) and other 3rd parties (e.g. weather networks), please read
https://www.wxforum.net/index.php?topic=40730.0 chapter 8.
CMX only receives
permanently data from a console/gateway it is connected to.
There are two ways to connect an Ecowitt console to CMX (depending on the console's capabilities):
- CMX retrieves data from the local Ecowitt Gateway API (GW1x00, GW2000, WH2650, WN19x0 consoles)
- CMX receives data via the Customized server function every (newer) Ecowitt console has; i.e. the console posts data to CMX
(in CMX terms the http(Ecowitt) "station")
Independent of that, if registered with Ecowitt.net and the data posting activated, the cosnole sends data into the Ecowitt cloud, exactly speaking so-to-say to your account in the cloud. This data can be seen in the Ecowitt dashboard at ecowitt.net and is stored there on a 5-minute interval basis for 90 days (longer time with be longer intervals - see also the above link chapter 14 - retention times and intervals).
Data in the Ecowitt cloud can be retrieved via some software using the Ecowitt cloud http API. Details see also the above link chapter 14.
CumulusMX uses this functionality in case you do not run CMX permanently (or an interruption has occurred). If the console was still able to post data to ecowitt.net during this interruption, CumulusMX can retrieve this data at the next restart provided the option has been activated in CMX and the APP key, API key and the MAC address of the console in question have been provided to CMX (and been saved to Cumulus.ini).
Then CMX checks against the last time stamp in the today.ini file (first section) and tries to retrieve the data which were posted between the last shutdown and the new restart and updates its database files (e.g. MMMYYYYlog.txt, and, if applicable, also the Extra Sensor file) and if necessary also the dayfile.txt file.
That's the so-called "backfill option". It's not a permanent download. Once the download is completed, CMX continues processing the data from its connection to the console (local API or custom server http post).
If you want to retrieve your Ecowitt cloud data yourself, under the above link chapter 14 this Ecowitt cloud API is documented, directly or indirectly via further links.
One can also retrieve data from the console via the local http console API. However, CMX does not do this.