I have a TCP server written in SwiftNIO, based on this documentation.
I want my client to be written in python from which I can send multiple JSON strings & can receive similar/different multiple JSON string(s) as a response periodically for a few minutes.
In which format do I need to convert those JSON strings from the python client & how do I get the same JSON string on the SwiftNIO server (and vice versa)?
If I were you, I'd use HTTP using the Vapor web server and any Python HTTP library such as requests. If you do that, then your job will be pretty straightforward. The Vapor community is also super helpful in their Discord chat.
If you really want to do this in a low-level library like SwiftNIO then that's of course possible but you'll need to design a so called "wire protocol" for the framing (ie. when does one JSON message start and end). SwiftNIO is very well equipped for these things but you'll likely need to learn a bunch of things.
You could for example use NIO Extras' LineBasedFrameDecoder and send each JSON (make sure it doesn't contain newlines) followed by a \n. Or you could say that you prepend the JSON by say a 32 bit length field (which you could decode using the LengthFieldBasedFrameDecoder. There are many options...
You could also implement JSON-RPC and you could get some inside in this example which is also explained in this talk.