It's not a good idea to merge them per se -- problems in one part will affect the other, and this sort of tight coupling is a bad idea both because you can't run the two parts of the program on separate machines and because if one crashes, so does the other one. It's better to have them communicating over some sort of protocol.
If I were designing this, I would probably do the same thing as you did, except that instead of using an SQL database for this, I would use something like Redis which stores its data in memory. Redis allows you to subscribe to events rather than poll for updates, and polling for updates is cheaper because it's in memory.
We have an existing python application (let's call it control app) that does operation data logging as well as smaller controlling tasks on a machine. We want to extend this application with a web interface, which is based on flask (let's call it web app). Both parts, the control app as well as the web app, are already present, however, the setup feels somehow fishy. In the process of rethinking the setup, I'm somehow undecided on how to structure those two parts.
At the moment, the control app gathers machine data and stores it in a postgres database. Based on several machine states, additional operations are performed that provide new input for the PLCs that control the machine.
The web app currently polls the database to react to machine states to e.g. update visualisation data, change some (state representing) images and such things.
The web app polling the database is the part that somehow smells. So my idea was to unify both apps into one to have the web app tightly coupled to the control app to be able to react on machine state changes instead of polling the database for those state changes.
Based on that idea, I'm wondering how to add a flask app to an existing python app. When I'm not mistaken, the flask app consumes the application's main thread, which would break to already existing logic. Thus I would need to have one of the two parts running on another thread. Thinking about this problem, I'm further wondering whether this merging is a good idea at all.
So, the questions are: Is it a good idea to merge both applications? If yes, how to merge them without breaking one of them? If not, how else should I try to get rid of the database polling (how to synchronize and also move some data from the web app to the control app)?
Add flask based web interface to existing application