RPC - Remote Procedure Calls

RPC means Remote Procedure Call. Mongoose OS RPC (MG-RPC) service is simply a C or JavaScript function that:

  • Has a name, for example GPIO.Toggle,
  • Takes a JSON object with function arguments,
  • Gives back a JSON object with results.

These JSON messages could be carried out by many different channels: serial (UART), HTTP/Restful, WebSocket, MQTT, Bluetooth. RPC API allows to add support for other channels. mos tool provides an easy way to call device's RPC services over the serial connection or over the network. Alternatively, RPC services can be invoked programmatically.

That means, you can write a function in C or JavaSciprt, and call it remotely via multiple protocols. Example: an RPC service Sum that adds two numbers a and b:

RPC.addHandler('Sum', function(args) {
  return args.a + args.b;

Here is how you can call it:

Via serial connection, if a device is attached to a serial port:

mos --port /dev/ttyUSB0 call Sum '{"a":1, "b": 2}'

Via WebSocket, if a device is in the local network:

mos --port ws:// call Sum '{"a":1, "b": 2}'

Via RESTful call, if a device is in the local network:

curl -d '{"a":1, "b": 2}'

Via an MQTT server, talking to an device idendified by its ID esp32_6732ac:

mos --port mqtt://my.mqtt.server:1883/esp32_6732ac call Sum '{"a":1, "b": 2}'

To see a list of all RPC services implemented by a device, call RPC.List:

mos call RPC.List

MG-RPC frame format

MG-RPC frame formats for request, successful and failed responses are documented below. Couple of notes to keep in mind:

  • The criteria of whether the call is successful or failed is the presense of the error in the response. If error is present, it is a failure. Otherwise, it is a success. Note that the result in the successful response is optional - thus an empty response is a success.
  • Requests and responses are not necessarily go in the same order. For example, a response to a call that takes long to compute, may arrive later. An id is used to associate requests with responses.
  • It is possible to attach an arbitrary tag string to a request. MG-RPC will repeat that string unchanged in the response.


  "method": "Math.Add",     // Required. Function name to call.
  "args": {                 // Optional. Function arguments
    "a": 1,
    "b": 2
  "src": "joe/32efc823aa",  // Optional. Used with MQTT: response will be sent
                            // to that topic followed by "/rpc", so in this
                            // case it'll be "joe/32efc823aa/rpc".
  "tag": "hey!",            // Optional. Any arbitrary string. Will be repeated in the response
  "id": 1772                // Optional. Numeric frame ID.

Response - success

  "result": { ... },        // Optional. Call result
  "tag": "hey!"             // Optional. Present if a request contained "tag"

Response - failure

  "error": {
    "code": 123,
    "message": "oops"


For the quick introduction, see Mongoose OS quick start guide on how to add RPC service in JavaScript or C/C++.

See API reference for the C/C++ and JavaScript API spec.

Remote management

Mongoose OS has many built-in RPC services: for managing files, accessing hardware peripherals, performing over-the-air updates, etc. In fact, the majority of mos command talks to a device via an RPC call.

See Secure remote device management with Mongoose OS and AWS IoT for an example of remote management over the secure AWS IoT cloud. In that case, Mongoose OS uses secure MQTT RPC channel, authenticated via the mutual TLS - per AWS IoT standards.

An ability to call RPC services via network add an incredible level of control, implementing remote management capabilities at the highest levels.

Example: scan I2C bus and return addresses of I2C peripherals from a device at IP address using mos tool in a command line mode:

mos --port ws:// call I2C.Scan

Example: connect to a remote device via the MQTT server and manage it using mos tool Web UI: