Cloud-based IPC between C DLL and new data serving process

I have a Windows UI, written in C#, that calls a DLL, written in C.

Data is exchanged between the C# UI and C DLL using the marshaling techniques available through pInvoke. Both the UI and the the DLL are legacy code.

All of the software runs on the cloud; specifically, on Amazon Web Services (AWS). But it is portable to any cloud service provider (Azure, Google, etc).

I need to write a new piece of C code ("NewCode"), that runs on a separate AWS (or other) instance, that does nothing except read data from a proprietary database and service data requests from the existing DLL.

For lots of reasons, this NewCode needs to run on its own instance, so that it has its own, exclusive access to memory, cpu, and disk. Newcode needs to service a variety of data requests: a single number, a char string, an array of numbers, array of strings, etc. NewCode will be portable C, so it can run under Linux, Unix, etc.

My question:

What are my options for having the existing C DLL communicate with NewCode? I know it is too broad a topic to ask for a list of options and their relative merits, so all I'm asking for here is to what should be on the list so I can begin my research. I am a complete newbie in this area, but so far I have determined that on the list should be sockets and pipes. What else should be on the list?