Handle lastchange over application restarts

[LeSpocky]

Today the docker containers running the AmpelController and the SpaceAPI-Service were restarted. Since then the lastchange attribute in the delivered SpaceAPI json is set to the value 1234 which as a Unix Timestamp translates to the obviously wrong 1st of January 1970 at 1:20 am. This is the same value we get as retained value from the MQTT broker in our current setup. Not sure how it got there, but the SpaceAPI service picks it up and happily puts it into the json output (see attached file). I suspect the problem to be the AmpelController, which has no lastchange value to publish until the first actual change happens.

Consumers of our spaceapi record display that value, e.g. like this:

Several options to solve this were discussed already:

1. persist the lastchange value over application restarts, maybe in a cache file?
2. publish certain values like lastchange with a retained flag to MQTT
3. (only publish those values as retained on app shutdown, otherwise normally)

Maybe some other solution is possible?

Attachment: spaceapi.json

[penguineer]

persist the lastchange value over application restarts, maybe in a cache file?

Adding a cache file would increase the deployment complexity. We have to manage a volume mount to contain that file and deal with potentially multiple instances trying to access this file in fail-over cases (basically turning this app from a deployment into a stateful set).

I propose we find a solution that relies on the existing MQTT mechanims.

[penguineer]

Note

The effect happens in the Space API service, however, the easiest fix can be applied here.

[penguineer]

Why is this so bad? 👀

[penguineer]

Time is up-to-date after service restart.

[LeSpocky]

Why is this so bad? 👀

Access to our public API endpoint is https only (missing redirect from http) for quite a while now, while the entry in the space API endpoint list was still on http. I proposed a pull-request to change this earlier this year, but that connectivity stat was already down to 40% back then.
See SpaceApi/directory#291 for the PR on SpaceAPI directory.