Remote Procedure Calls (RPC)

spooky actions at a distance

Do something remotely and send back the answer.

RPC has been a design pattern for decades, and, no surprise, we have RPC in the atSDK also. This demo code is in another directory named at_rpc_demo. You can get to it by using VS Code and open folder and select that folder. Next, once again, open two windowpanes.

In the right-hand pane enter (again using your Atsigns!)

dart pub get

This pulls in needed libraries, and then this starts the RPC listener.

dart pub get
dart run .\bin\arithmetic_server.dart -a "@energetic22" -n "atsign" --allow-list "@7capricorn"

Then in the left window, you can run up the client

dart run .\bin\arithmetic_client.dart -a "@7capricorn" -n "atsign" --server-atsign "@energetic22"

You will get a prompt after a second or two and you can put in a math expression and hit enter. The expression will be sent to the other Atsign, the answer calculated, and then returned.

See our session in action:

RPC

Something to notice here is that the RPC server will only respond to RPCs from the designated Atsign with the --allow-list argument.

The RPC can, of course, do anything you want it to, and the Atsigns can be running anywhere.

The code

An AtRpc object is set up pretty simply with:

    var rpc = AtRpc(
      atClient: atClient,
      baseNameSpace: atClient.getPreferences()!.namespace!,
      domainNameSpace: 'at_rpc_arithmetic_demo',
      callbacks: DemoRpcServer(),
      allowList: allowList,
    );

    rpc.start();
  } catch (e) {
    print(e);
    print(CLIBase.argsParser.usage);
  }

The Callbacks are sent to the DemoRpcServerclass and handled appropriately.

On the sending side the RPC client is initiated:

    var rpc = AtRpcClient(
        atClient: atClient,
        baseNameSpace: atClient.getPreferences()!.namespace!,
        domainNameSpace: 'at_rpc_arithmetic_demo',
        serverAtsign: serverAtsign);

Then the RPC simple sent and awaiting a reply

var response = await rpc.call({'expr': expr}).timeout(Duration(seconds: 5));