This Integration provides a degree of persistence for transient MQTT messages that are of interest.
It is particulary useful in the scenario where a MQTT source sends data with zero retention and does not provide a method for querying the current data.
For example, some MQTT thermometers might send an update only every minute, or even only when the temperature changes; using this Integration you can retain and query the last update on demand.
Unlike most of the AGHAST system, this Integration maintains a live state. If you are making heavy use of this, then you might want to consider carefully when to restart AGHAST.
An example should be self-explanatory...
# Example MqttCache configuration
[[Buffer]]
Topic = "pizero01/gpio/sensor/dht22_temperature"
RetainSecs = 600
[[Buffer]]
Topic = "pizero02/gpio/sensor/dht22_humidity"
RetainSecs = 600
You may add as many caches as you wish.
The MqttCache can be used within AGHAST and by any other MQTT clients on the local network.
To get the latest data first subscribe to the topic aghast/mqttcache/<original-topic>,
then when you want the data send a get request to aghast/mqttcache/get/<original-topic>.
If the data you are interested in originally come from pizero02/gpio/sensor/dht22_humidity
then ensure that you have the second Buffer configured as above.
In your code or front-end do not subscribe to the original source, but rather aghast/mqttcache/pizero02/gpio/sensor/dht22_humidity.
When you want the data send a message to aghast/mqttcache/get/pizero02/gpio/sensor/dht22_humidity - the payload is ignored.
MqttCache will respond on the topic aghast/mqttcache/pizero02/gpio/sensor/dht22_humidity with the exact payload that was originally sent from the source.
There are two possible error conditions:
- No data have yet been received in a cache
- The data in a cache has expired
In case 1 you will receive a payload containing {"Error": "No data collected yet"}.
In case 2 you will receive a payload containing {"Error": "Data expired"}.