Important
Before the following points make sure, that 127.0.0.1:9999 is not in use.
cargo run -- -s
cargo run -- -c
{ "jsonrpc": "2.0", "method": "add", "params": [42, 23], "id": 1 }
{ "jsonrpc": "2.0", "method": "exit", "params": null, "id": 1 }