how is http call different from lets say RPC call in terms of operating system interaction
They are not, since both are simply TCP/IP connections, as far as the OS is concerned.
Is connection reuse a property of operating system or application code. Let's say there are 2 clients running on 1 client machine, will they be able to reuse existing connection?
Application code controls reuse.
How is application thread notified once response from server is available for consumption.
The OS method call returns. Unless the OS support asynchronous calls. Answer depends on OS and what API method the application called.
Is there in difference b/w how programming languages handle this? lets say java vs python vs go.
Yes. I mean, they pretty much do the same thing, i.e. implement the protocol according to the specification, but they have their own code for doing it.
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