Hello Ellen, port 80 - i.e. http - is used by PiCtory. This means that you have to configure the Apache webserver to free this port. Alternatively, if you don't need to use PiCtory, you can simply shut down the Apache web server completely. Have a look at this discussion:
How to change the WEB LOGIN page to the page provided by third-party software?
Parallel access to resources is the task of the operating system. It controls the parallel access of processes to resources based on priorities. There are both real time and non-real time priorities - piControl, the driver for IO communication via the process image, for example, is a process with real time priority. Have a look at the command "nice".
The question if you implement HTTP access within logi.cad or for example with a script is up to you. In the background the operating system finally controls parallel accesses. But if you have to exchange data between two processes you should be aware of the fact that the data is not modified while reading. The professionals do this with semaphores or so called triple buffers.
https://man7.org/linux/man-pages/man7/s ... iew.7.html